th  - 1987 - 51st ENC Conference

• WF38
29SI DYNAMIC NUCLEAR POLARIZATION STUDIES OF
DEHYDROGENATED AMORPHOUS SILICON
H. Lock m, G. E. Maciel, R. A. Wind
Department of Chemistry, Colorado State University
and
N. Zumbulyadis
Corporate Research Laboratories
Eastman Kodak Co., Rochester, New York 14650
Dehydrogenated amorphous silicon, a-Si, contains a large number of
dangllng-bond paramagnetlc centers. This opens the possibility of
performing 29Si dynamic nuclear polarization (DNP) experiments,
where the 29SI polarization is enhanced by irradiating at or near
the electron larmor frequency. We performed 29Si DNP in a-Si
in an external magnetic field of 1.4 T, corresponding to 29Si
larmor frequency of 11.9 MHz, and an electron larmor frequency,
re, of ca. 40 GHz. The amplitude of the microwave field, B 1, was
0.06 mT (max.).
The main results are:
I. The 29Si DNP enhancement is antisymmetrical around re,
indicating that the dangling bonds are fixed in space.
2. The shape of the DNP enhancement curve as a function of
microwave frequency offset is independent of BI. This means that
at 40 GHz the ESR llne is mainly inhomogeneously broadened, which
is confirmed by ESR measurements at 9 and 40 GHz.
3. For a B1 of 0.06 mT the maximum 29Si DNP enhancement is 40.
This value is obtained using a microwave frequency ±29 MHz away
from the electron larmor frequency. The enhancement is probably
due to an unresolved solid state effect.
4. The integrated intensity of the 29Si spectrum, enhanced vi____aa
DNP is the same with and without magic angle spinning. Hence, the
DNP effect is mainly due to direct interactions between the 29Si
nuclei and the unpaired electrons.