infillCdS

CdS infill

Let’s have a look on the effects opal based PXs have on the luminescence of guest materials. xInhibition of the spontaneous emission is a must when it comes to building lasers with a
low threshold and this can be reached by means of photonic crystals, taking advantage of
their particular photonic properties. Therefore, the synthesis of dye molecules inside
3D-photonic crystals operating in the visible and efficiently matching their
photoluminescence is required. Our recent work demonstrated that it is possible to fill
opal-like structures with CdS. Also, studies of the effect of the PX on CdS
photoluminescence have been performed. CdS can be embedded in the opal voids by a CBD
method.

FIG. 1. SEM image of internal {100}
facets obtained from a cleft edge of a CdS embedded opal. The inset shows a detail in the
structure.

Fig. 1 shows
an image of a (100) facet of a sample infilled with CdS to a 23 vol.-% of the pore. It can
be seen that CdS crystals are uniformly distributed inside the sample. There are not, at
least by this growth method, privileged zones in which the CdS grows more efficiently. The
pore volume is reduced due to the CdS formation therein, as can be readily seen in the
inset of the same figure.

FIG. 2. Transmission spectra at
normal incidence of both empty and CdS infilled opals of 390 nm diameter spheres. The
inset shows the peak positions vs. incidence angle for bare opal (squares) and for opal
with two different treatments.

Transmission
spectra at normal incidence of bare and CdS infilled opals of 390 nm diameter spheres can
be observed in Fig. 2. The dip in transmission due to (111) Bragg diffraction shifts from
about 846 nm to 890 nm under CdS loading. This effect is similar to that observed for
InP/opal samples and in good agreement with predictions. These allow us to estimate the
amount of infilled CdS. Concerning luminescence, none has been observed from bare opals.
However, when a bare opal is CdS loaded, a clear band centred at 530 nm, can be detected.
This emission is assigned to defect levels of interstitial sulphur. A judicious choice of
sphere size and CdS content allows to match the photonic pseudogap of the opal to the CdS
emission, so that PX effects of the host can be observed. For a sample of 260 nm spheres,
the forbidden band for normal incidence appears at 580 nm, well within the emission band.

FIG. 3. Photoluminescence spectra at
different collection angles for a CdS infilled opal. As the sample is tilted the dip in
the emission shifts following Bragg law. The PL of the grounded sample is shown as well.

The effect of
the host on the luminescence of the guest CdS can be seen in Fig. 3. A series of
luminescence spectra obtained as the sample is rocked are shown. A clear dip is detected
in each emission spectrum. When the sample is tilted the dip in the emission shifts
towards lower wavelengths, according to Bragg law, sweeping across the emission band.
These dips coincide with the transmission minima (forbidden bands) of the ordered
structure. So, for each direction a different region of the luminescence is not allowed to
propagate through the hybrid CdS/opal structure. When the sample is ground and the order
vanishes from the system, the luminescence recovers the original CdS line-shape and
photonic effects are lost. This demonstrates the partial inhibition of the emission
produced by the photonic structure of the host. In case a full PBG overlapping the whole
semiconductor emission band were present, total inhibition of CdS luminescence would
occur. However, a higher dielectric contrast than that here attained is required. At any
rate, this effect constitutes a first step in the construction of hybrid all-solid-state
semiconductor-PBG material in which the control of the spontaneous emission is achieved.