FY2010 - Oak Ridge National Laboratory

Director’s R&D Fund—
Science for Extreme Environment: Advanced Materials and Interfacial Processes for Energy
funding from the DOE BES. Since other federal agencies such as NASA and the military branches also
have a tremendous interest in seeing efficient solar energy conversion come to fruition, we anticipate that
the information gained from our research will benefit the relevant research and development programs
sponsored by these agencies.
Results and Accomplishments
Two ultrafast optical spectrometers, namely, femtosecond transient absorption (TA) and picosecond timecorrelated
single-photon-counting (TCSPC), have been built and tested. The former employs a highprecision
DC motor-driven optical delay stage with a 0.3 fs resolution and dual lock-in amplifiers and
therefore enables weak signal detection with an absolute change of the sample optical density on the order
of 10 -5 (limited by the laser stability) and a time resolution of 50 fs. By using either single or dual optical
parametric amplifiers, or in combination with a broadband white-light continuum (460 to 1100 nm),
measurements at various combinations of pump and probe wavelengths can be performed. The TCSPC
system is based on a tunable femtosecond laser excitation source, an actively quenched single-photon
avalanche diode, and a TCSPC module. It has a typical instrumental response of 40 ps (full-width at half
maximum) and can be applied to resolve fluorescence emission decay with a time resolution of 10 ps.
These two spectroscopic tools enable elucidation of electronic relaxation processes from both bright and
dark excited states that occur within timescales ranging from 50 fs to tens of nanoseconds and are ready to
study various fundamentally important and technologically relevant materials.
A detailed femtosecond TA study was performed on copper-phthalocyanine (CuPc) single-crystal
nanowires, which have been considered to be perfect building blocks for molecular electronics and
photovoltaics. As the CuPc nanowires with different diameters/lengths were grown successively on an
opaque silicon substrate (provided by Dr. Kai Xiao, Center for Nanophase Materials Sciences), the
measurements were carried out with a reflective pump-probe configuration. We found that the exciton
relaxation is very sensitive to the growth temperature (from 192 to 204C), which affects not only the
molecular structure (- or -phase) but also the nanowire diameter (from 90 to 110 nm) and length (from
19 to 24 m). Measurements at different excitation intensities for the wires grown at selected
temperatures further showed that the exciton relaxation accelerates markedly with increasing excitation
intensity. The observed intensity dependence arises from an exciton-exciton annihilation process, and
quantitative analysis of this nonlinear phenomenon enabled us to elucidate exciton diffusive motion in
this quasi-one-dimensional material. Currently, our research is focused on time-resolved fluorescence
study of the optical properties of selected conjugated polymers, which are chosen in view of their
potential for high-efficiency photovoltaic applications.
05837
Cryogenic Development for a Measurement of the Neutron Electric
Dipole Moment at the Spallation Neutron Source
Weijun Yao
Project Description
A neutron electric dipole moment (EDM) can be detected by measuring the difference in the precession
frequency of a population of polarized neutrons in a magnetic field when subjected to a strong electric
field aligned parallel or anti-parallel to the magnetic field. To minimize systematic effects associated with
stray magnetic fields, a simultaneous measurement can be made on a “co-magnetometer,” a species that is
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