| Summary: | 博士 === 國立臺灣大學 === 物理學系 === 85 === Abstract
Two dimensional angular correlation of electron positron
annihilation radiation (2D-ACAR) spectroscopy has been applied
to the studies of microstructures in porous silicon and
Y-Zeolites Br*nsted acidity. The lifetime measurement of
positron annihilation has been used as a complementary
method to confirm the physical concluded results from the 2D-
ACAR experiment.
The electron positron momentum spectrum of porous silicon can be
well resolved into a positronium peak and a wider component.
The sample is placed at 15 K in a vacuum of 10-6 torr to
let the air slowly condense and shield the pore surface. The
positronium peak is found to have a wider momentum spectrum and
a lower yield at 15 K. From the results, the narrow peak is
attributed to the positronium trapping in the etched pores and
not on the surface of pores. These results shows the possibility
of applying positron technique for the study of the morphology
of porous silicon.
By measuring electron positron momentum spectra of porous
silicon with different HF concentrations, a surprising result is
found the size of the etched pores decreases with decreasing HF
concentration in the etching solution. The trend is
different from the intuitive expectation that the pore size
increases with increasing porosity. We interpret this result in
terms of the formation mechanism of porous film.
By studying the correlation between photoluminescence (PL) and
positron annihilation spectra in porous silicon, we found that
the samples with PL peak energy below 1.8 eV exhibit a strong
positronium signal and a good correlation with PL spectra.
However, the samples with PL peak energy above 1.8 eV show
little amount of positronium signal and the width of positronium
peak remain almost constant. This behavior can be explained by
the formation of silicon compounds on the surface of pores. With
the results, we suggest that the emission arises from silicon
nano-crystal for PL energy peak under 1.8 eV and silicon
compounds on the surface are responsible for the emission
of the PL peak energy above 1.8 eV.
In the study of Y-Zeolites Bronsted acidity, 2D-ACAR spectra
were measured for different base adsorbed Y-zeolite under a
temperature controlled heating system in order to
investigate the acid strengths and concentrations of Bronsted
sites in the samples. These results demonstrated
that the intensity of positronium peak of the sample
increases with the strength of adsorbed base and decreases with
the acidity concentration of samples. The physical and
chemical adsorption molecules are found to be easily
identified from observing the correlation between Ps
intensity and the decomposing temperature.
To the molecules which are chemical bonding with Bronsted acid
site, the positronium intensity of the Y zeolite with weak
bonding water and alcohol is found to exhibits two stages
change with decomposing temperature. This behavior is
not found in ammonia. We suggest that the two stages change
is caused by two different Bronsted acid site with
different strength. Ammonia captures the hydrogen atom from
Bronsted acid site to form a free NH4+ ion and stays away
Bronsted acid site. Thus, decomposing ammonia shows no
sensitive to different Bronsted acid site.
From the results of porous silicon and zeolite, the 2D-ACAR
technique has been demonstrated to be a very useful tool in
studying the physical and chemical properties of porous
materials which can not be obtained by other experimental
methods.
|
|---|
