4. PL of the inhomogeneous PS film: Influence of
The acetone adsorption
Fig. 3 shows the PL spectra
of two PS patterns prepared at the constant and step-like
constant current when the PL was excited at
lex
= 405, 436 and 546 nm and registered in steady-state conditions.
At these excited illuminations the full absorption occurs
at 1.0, 1.7 and 8 mm depths, respectivelly [17]. Taking
into account the average thickness of PS sub-layers formed
at the step-like constant current, the 405 nm illumination
is absorbed by the top layer exceptionally, the 436 nm
one is absorbed by the upper and underneath layers, the
546 nm one - by whole PS film. For the first type of PS,
the ratio of PL yield for 405, 436 and 546 nm excitation
is 0.44:0.78:0.20 in the ambient atmosphere and 0.8:1.0:0.20
in the saturated acetone vapours. For the second type
of PS film the ratio is 0.36:0.87:0.44 in the ambient
atmosphere and 0.54:1.0:0.30 in the saturated acetone
vapours. The main experimental results can be summarised
as the following: i) For both type of PS samples the increase
of
ex
evokes the red shifting of the PL spectra from 680 nm
at
lex
= 405 nm up to 730 nm at lex
=546 nm. Simultaneously, the FWHM decreases from 135 to
108 nm. ii) The effect of the acetone adsorption
depends on
lex.
At 405 nm illumination the acetone adsorption results
in PL yield increasing and the red shifting of the PL
maximum, the 436 nm illumination results in the increase
of PL yield and blue shifting of the peak, the 546 nm
illumination reduces the PL intensity and leads to the
blue shifting of spectra.
Kinetics of PL yield of PS films formed at the constant
current strongly depends on lex<
At the beginning of measurement of the as-prepared PS
layer the intensity of PL in the acetone vapours is
less than in the ambient atmosphere for every lex
However, at the 405 nm excitation, the PL intensity
decreases in the ambient atmosphere and increases in
acetone vapours up to the saturation (Fig.4
a). After 6 min excitation the PL yield is higher
when the acetone molecules are adsorbed. On the other
hand, the excitation at the 546 nm causes no change
of PL intensity (Fig.4 b).
For the as-prepared PS layer formed at the step-like
constant current, the stronger quenching is observed
in the acetone atmosphere (Fig.4
a,b). However, the trend of the PL increase during
measurement in acetone atmosphere rests for 405 nm excitation.
The difference in PL kinetics among short and long
wavelength excitation can be explained by the photooxidation
of the silicon nanocrystallites of the uppermost sub-layers
which is caused by the short wavelength excitation [21
- 23]. The chemical or electrochemical derivatization
reactions can modify the ability of PS to gas sensing
[6, 24, 25]. For example, reactions which impart a degree
of hydrophilicity to the PS surface, change the ability
of H2O and ethanol to quench the PL. Figs.3,
4 show that the acetone adsorption affects differently
on the as-prepared nonoxidised and oxidised layers owing
to 405 nm illumination. The intensity of the as-prepared
PS is strongly quenched by acetone vapours (Fig.4).
Vice versa, the material that was made more hydrophilic
by the surface oxidation is quenched, to a lesser degree,
by the acetone molecules
(Fig.3) with good according to the hydrophobic-hydrophilic
PS modification. The acetone adsorption increases the
non-radiative recombination canal for the as-prepared
layers, whereas the adsorbed acetone molecules act as
a passivation coating for the oxidised PS that reduces
the non-radiative recombination. For the 546 nm excitation,
as the PL is not changed neither in the ambient atmosphere
nor in acetone vapours, the principal contribution to
the PL incoming is given by the inner sub-layers of
PS that was predicted in item 3 of this article (Fig.2).
For a more inhomogeneous PS film formed at the step-like
constant current, the PL of the oxidised upper layers
excited by 546 nm is noticeable.
abstract
1. Introduction
2. Experimental
3. PL of multilayer PS film:
simulation
4. PL of the inhomogeneous PS
film: Influence of The acetone adsorption
5. Conclusions
Captures for the figures
References
V.A.Skryshevsky
Radiophysics Department, Kiev Shevchenko
University,
64 Vladimirskaya, 01033, Kiev, Ukraine, fax.+380-44-2656744,
e-mail: skrysh@uninet.kiev.ua
