2011 Annual Report - Center for Integrated Nanotechnologies - Los ...

Integration Focus Activities
Integration Focus Activities
Integration Focus Activities (IFAs) are initiatives aimed at nanoscale integration challenges within the context of specific classes
of materials, systems, or phenomena that will have high scientific and technological impact. These topics are developed from a
“bottoms-up” process within the CINT community. They are cross-thrust activities, which provide canonical examples of integration
science and are intended to become strong attractors for users and collaborators. The IFAs both emerge out of a particular thrust effort
and build on areas of strength across thrusts. The studies center on particular classes of materials, systems, or phenomena and
derive their integration focus through a use-inspired science approach. These focused efforts bring an interdisciplinary approach by
CINT scientists and users to a particularly timely area of integration science. They enable rapid progress in specific areas of common
interest, attract and build user communities, and are variable term in nature. Each IFA is supported by two dedicated postdoc positions
whose research is targeted towards linking the work of CINT scientists and the user community in specific high-impact areas.
Nanowires for New Energy Concepts IFA
The novel properties of nanowires offer tremendous opportunities
for transformative energy applications. These multi-purpose
materials combine nanoscale and even quantum-confinement
effects with enhanced transport properties. CINT scientists
together with our user community are focusing to understand and
control the functionality and integration of heterogeneous semiconducting
nanowires for new energy harvesting and storage
concepts. We emphasize heterogeneous nanowires to realize
an unprecedented level of control over material performance by
tuning interface, strain, and materials-mixing effects. We combine
new synthesis strategies with structural, electrical, optical,
and thermal characterization, initially at the single-nanowire level
to answer critical science questions underlying new nanowire
materials concepts for photovoltaics, thermoelectrics, and energy
storage. We also drive toward functional integration of nanowires
into two- and three-dimensional architectures.
- Ultrafast optical spectroscopy. The ultrafast optical microscopy
(UOM) system uses a femtosecond pump-probe laser setup and
microscope objectives to measure carrier dynamics and carrier
velocities in single nanowires with high temporal and spatial
resolution in a non-contact, non-invasive manner. We showed
the first measurement of carrier-lifetime increase by surface passivation
on a single Si-SiO2 nanowire. By spatially separating the
pump and the probe along the nanowire we extracted the carrier
transport along the nanowire axis. We also used this system to
perform the first measurement of GaN/AlGaN core/shell nanowires
that revealed the influence of the AlGaN shell on carrier
relaxation.
Selected Recent Highlights:
- Flow-reactor solution-liquid-solid nanowire synthesis. We’ve
developed a flow reactor for solution phase growth of heterostructured
nanowires. In contrast to traditional “beaker” chemistry
the volume of reactants in the flow cell is very small. This
enables rapid and precisely controlled switching of the reactants
that makes growing axially heterostructured nanowires possible.
Using this technique we have shown the first example of
an eight-segment ZnSe-CdSe axially heterostructured nanowire.
By controlling the growth rate in the flow reactor we measured
the length-diameter dependence of CdSe and ZnSe nanowires
and determined for the first time that the solution-phase growth
mechanism involves both the Gibbs-Thomson effect and surface
diffusion.
- Toward Si colar cells with single crystal efficiency at thin film
cost. Using radial p-i-n nanowire arrays, we aim to decouple optical
absorption and carrier collection. We have gained new insight
into nanoscale radial epitaxial, low-temperature single crystal
growth and fabrication of large-area nanowire arrays. We have
measured external and internal quantum efficiencies by conducting
wavelength dependent photovoltaic and optical absorption
measurements in collaboration with our industrial partners.
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10The Center for Integrated Nanotechnologies | 2011 Annual Report