# Angular Dependence of Fluorescence Emission from Quantum Dots inside a Photonic Crystal

Authors: Kasper Baert; Branko Kolaric; Wim Libaers; Renaud A. L. Vallée; Marcel Di Vece; Peter Lievens; Koen Clays
Journal: Research Letters in Nanotechnology
 (2008)

---

## Abstract

The fluorescence of emitters embedded in a photonic crystal is known to be inhibited by the presence of an incomplete photonic band gap or pseudogap acting in their emission range. Here, we present a study of the angular dependence of the fluorescence emission of emitters embedded in a photonic crystal. Our results clearly show an angular dependence of the fluorescence emission, which is caused by the presence of an incomplete 3D band gap.


---

## Body

## 1. Introduction

Photonic crystals (PCs), often
also called photonic band gap materials, are the optical analogues of
electronic crystals, that is, semiconductor crystals, which form the basis of
all modern electronics applications [1, 2]. While the periodicity in the materials’ properties is on the ångström scale in electronic (ionic) crystals, it is on the optical wavelength scale in
photonic crystals. In electronic crystals, the periodic potential results in a band gap, that is, a forbidden range of energies for the electrons. In photonic crystals, the periodic refractive index results in an optical band gap, that is, a forbidden spectral
range for the photons. These periodic changes of refractive index radically alter the local density of optical states (LDOS), which governs the interaction between the emitter and the electric field in the structure. A simple way of producing three-dimensional (3D) photonic crystals consists of ordering, through convective self-assembly [3], Langmuir-Blodgett deposition [4], sedimentation [5], or spin coating [6] monodisperse, colloidal spheres. The self-assembly results in the thermodynamically most stable face-centered cubic (fcc) crystal structure or in the (random) hexagonal closed packing (Rhcp) crystal structure, both with a
packing efficiency of 74%. The type of packing, the size, and the refractive
index of the colloidal spheres determine the spectral position of the band gap.
The combination of these dense crystal structures (fcc or Rhcp) with the low-refractive index of most monodisperse spherical colloidal particles (latex, silica)
results in an incomplete band gap (pseudogap or stop band) [7], that is, a gap with properties
depending on the incidence angle of the impinging light. In this study, we
present the angular dependence of the fluorescence emission of emitters
embedded in PCs. Our investigations show that the angular dependence of the pseudogap causes a shift in both the maximum of fluorescence intensity as well as in the
reduced emission of the embedded emitters. These findings can be used to
optimize maximal tuning of the spontaneous emission from inside a PC.


## 2. Experimental Part

The Stöber method [8] was used
to obtain monodisperse silica spheres of both 185 and 260 nm for, respectively,
the reference (PC1) and the active (PC2) PCs. Silica colloids were chosen
because they allowed for monodispersity better than 3% which is essential to
produce good quality photonic crystals. Colloidal photonic crystals were then
fabricated from these particles using the self-assembly protocol developed by
Jiang et al. [3]. Figure 1 shows a transversal view of a colloidal crystal PC2
obtained by use of a scanning electron microscope (Philips XL30ESEM FEG). The figure allows one to judge the good
quality of the approximately 4.5 μm thick crystal consisting of 18 layers of
colloidal particles. CdSe/ZnS core/shell quantum dots of ~2.6 nm with an
excitation maximum at 540 nm and an emission maximum around 565 nm were purchased from Evident USA. The infiltration was performed by placing the
crystals in a 0.1 μM solution of quantum dots followed by overnight drying. Optical
extinction spectra were performed on large areas (millimeter sized) to ascertain
the quality and spectral features of the samples using a Perkin-Elmer Lambda 900
UV-VIS-NIR spectrophotometer. Fluorescence emission spectra of CdSe/ZnS in toluene were measured using a FluoroMax-3 (SPEX Instruments, Edison, NJ, USA).Figure 1
Transverse view obtained by scanning
electron microscopy of PC2, produced by convective self-assembly from a
colloidal suspension (size of the particles: 265 nm).Angle-dependent
photoluminescence spectra were recorded using a home built setup. The third
harmonic output of an Nd:YAG laser (GCR-11-2,Spectra Physics)  yields pulses at  a wavelength of 355 nm, a duration of 10 nanoseconds
and a repetition rate of 10 Hz. The fluorescence from the samples was directed to the entrance slit of a 60 cm focal length monochromator (HR 640, Jobin Yvon). For detection, at the
output slit a photomultiplier tube (C3531, Hamamatsu)
photocurrent was preamplified and recorded/digitized using a 500 MHz bandwidth
oscilloscope (TDS5052, Tektronix). All signals were averaged 20 times, all
spectra were recorded with a wavelength increment of 4 nm. In the front-face
excitation geometry, the light (50 mW) and the luminescence collection are both
oriented at the same angle with respect to the sample surface. The sample was
rotated at different angles which were measured with an accuracy of 3 degrees.


## 3. Results

As was mentioned above, the
CdSe/ZnS were infiltrated in photonic crystals with a pseudogap (i) outside the
spectral range of the fluorescence emission (PC1: reference,λmax⁡=406 nm) and (ii) in the spectral range of the fluorescence emission (PC2: active sample λmax⁡=573 nm) of the embedded
emitters, in order to investigate the alterations of their fluorescence
characteristics. Figure 2(a) shows the steady-state emission spectrum taken from
a solution of the CdSe/ZnS quantum dots in toluene and the extinction spectrum
of PC2 engineered to overlap with it.(a) Extinction spectrum of PC2 (black, left
axis); normalized emission spectrum of the CdSe/ZnS core/shell quantum dots
(0.1μM) in toluene (red, right axis), (b) normalized emission spectrum of
CdSe/ZnS quantum dots infiltrated in PC1 (black) and PC2 (red), (c) normalized
fluorescence spectrum of CdSe/ZnS (right) embedded in PC2 subtracted by the
normalized fluorescence spectrum of the respective emitters embedded in PC1.
(a)(b)(c)The
effect caused by the pseudogap on the steady state spectra of the quantum dots
infiltrated in the photonic crystals is presented in Figure2(c). These
(reduced) spectra are obtained by: (i) normalizing with respect to their maxima the steady-state emission spectra of CdSe/ZnS in
both the reference (PC1) and active (PC2)
photonic crystals these spectra are shown in Figure 2(b); (ii) subtracting the
normalized spectra obtained for emitters
embedded in PC1 from the normalized spectra pertaining to the corresponding emitters in PC2. This is done to allow for
clear observation of the shape modification of the fluorescence spectra. Figure 3(a) presents the reduced emission of the quantum dots embedded in a photonic crystal at different collection angles, as the effect became small for some angles these are shown as an inset. Figure 3(b) shows the angular dependence
of the spectral position of the fluorescence maximum of the quantum dots
embedded in the active sample (PC2).(a) Reduced
fluorescence intensity of CdSe/ZnS embedded in a PC at different collection
angles, the inset shows a zoom of the graph; (b) maxima of fluorescence
intensity as fitted with a lorentzian function, of CdSe/ZnS embedded in a PC as
a function of collection angle.
(a)(b)


## 4. Discussion

In our investigations, we used
CdSe/ZnS quantum dots due to their photophysical stability and because these
quantum dots are a very well-investigated system [9–12]. Figure 2
clearly shows that we succeeded in engineering a photonic crystal PC2 with a pseudogap
in the spectral range 561–615 nm that covers a substantial part of the
fluorescence emission of the chosen emitters. Because of the low-refractive
index contrast between silica particles and air, the photonic band gap is
incomplete and hence fluorescence emission was not inhibited completely and in
all directions in this spectral range. Accordingly only a decrease of
fluorescence emission in PC2 as compared to PC1 was observed [13, 14]. This
suppression of emission was expected due to the decrease of the number of
available photonic states in the pseudogap for PC2 [14–16]. Figure 3(a)
shows the pseudogap which can be seen in the reduced fluorescence spectra for
different incidence angles. In Figure 3(b), we show how the maximum of
fluorescence intensity shifts with collection angle. This is attributed to the
angular dependence of the pseudogap, which shifts to shorter wavelengths at
higher incidence angles. Note that the angular dependence of the intensity of
light scattering from a periodic structure or colloidal suspension was also
observed before and it is not a unique property of the photonic crystal [17]. A
unique property of the photonic crystal is a shift of the fluorescence maxima
as a function of the angle. The observed shift is a consequence of the shift of
the pseudogap at a different incidence angle [18] which changes the overlap of
the fluorescence emission and pseudogap. Since the fluorescence emission is not
completely covered by the pseudogap, at some angles more photonic states are available
for emission of the quantum dots and spontaneous emission is less suppressed.


## 5. Conclusion

We observed a shift of the
fluorescence maximum for CdSe/ZnS quantum dots infiltrated in a photonic
crystal engineered to have a pseudogap which covers a substantial part of the
CdSe/ZnS emission. We attribute this angular dependence to the angular
dependence of the spectral range covered by the pseudogap.



---

*Source: 974072-2008-04-17.xml*