Single Quantum Dot Optical Spectroscopy
Submitted as a Course Project for ECE 580
By
Amrita Urdhwareshe
Rohini Vidya Shankar
1. Introduction
Quantum dots (QDs) signify a new class of materials, with size-dependent electronic,
optical and magnetic properties. These structures are predicted to have discrete atomiclike energy levels (Figure 1), and a spectrum of ultra-narrow transitions that is tunable
with the size of the quantum dot. These are interesting because of the possible promising
applications making use of these discrete energy levels, and the possibility to study
fundamental physical aspects in these man-made “artificial atoms”.
Figure 1 Density of states in a semiconductor as a function of dimension.
Ref [1]
The optical properties of semiconductors can arise either from the recombination of
electrons and holes or from the creation and annihilation/excitation of excitons. An
exciton is a bound state of an electron and a hole in a semiconductor, and can be modeled
on the lines of a hydrogen atom. It therefore, has a characteristic “Bohr radius” and
discrete energy levels.
According to the density of states and the energy levels in a quantum dot, optical
transitions of single excitons occupying the quantum dot ground states would be expected
to exhibit spectrally narrow emission characteristics. However, the conventional
spectroscopic measurements of these QDs revealed significantly broadened emission
spectra. This is attributed to the fact that these conventional studies measure an ensemble
of dots with size and shape fluctuations, which results in inhomogeneously broadened
spectral features (Figure 2). Hence, spectroscopy of single QDs becomes essential if one
is to understand and evaluate their intrinsic physical properties precisely, without ending
up getting ensemble averages.
Figure 2: Inhomogeneous broadening of spectra Ref [2]
In this report, we discuss some of the experimental techniques used for single quantum
dot optical spectroscopy, and summarize the key results obtained.
2. Experimental techniques
2.1 Sample preparation
1) Colloidal nanocrystals
This technique is usually used for preparing quantum dots of II-VI compounds (CdSe,
CdS, PbS etc.). The quantum dots are prepared via a chemical synthesis, and then
deposited onto a substrate.
The synthesis begins with the rapid injection of organometallic reagents into a hot
coordinating solvent to produce a temporally discrete homogeneous nucleation. Slow
growth and annealing in the coordinating solvent results in uniform surface derivatization
and regularity in core structure. Size selective precipitation provides powders of nearly
monodisperse nanocrystallites which can be dispersed in a variety of solvents. The
crystallites are slightly prolate with an aspect ratio of 1.1 to 1.3. The average crystallite
size, defined by its major axis, is tunable from ~12 to ~115 Ao, with a size distribution of
about 5 %. For example, a typical preparation of CdSe nanocrystallites would consist of
mixing a Me2Cd (dimethyl cadmium) solution and a TOPSe (tri-n-octylphosphine
selenide) solution in TOP (tri-n-octylphosphine) in suitable concentrations under suitable
reaction conditions depending upon the size of the particles to be synthesized.
Both the average size and the size distribution of crystallites in a sample are dependent on
the growth temperature, the growth temperature necessary to maintain steady growth
increasing with increasing average crystallite size.
Some core-shell quantum dots (eg. ZnS/CdSe or CdS/CdSe core-shell dots) have also
been used for spectroscopic studies. These dots are prepared by drop wise adding of the
shell material onto a solution of the core nanoparticles under appropriate reaction
conditions.
Single nanocrystal samples are then prepared by spin coating an extremely dilute solution
of nanocrystals, usually in poly (methyl methacrylate)/toluene onto a crystalline quartz
substrate. The concentration of nanocrystals in the initial solution (~nmol/L) is chosen to
produce an areal density of less than one nanocrystal per square micrometer in order to
allow spatial resolution of individual nanocrystals using far-field optics.
2) Epitaxially grown quantum dots
Molecular beam epitaxy is a commonly used technique for the fabrication of quantum
dots, especially for the III-V compounds (e.g. GaAs, InAs, etc.). The commonly
investigated samples under this category are InGaAs /AlGaAs self-assembled QDs grown
on (100) GaAs substrate fabricated by gas source molecular beam epitaxy. With the
Stranski-Krastanow growth mode process, a thin quantum well structure with thickness 4
multilayer (ML), so called wetting layer (WL), is formed under the QDs. The density of
the QDs is about 3X10-8/cm2 and the typical lateral size and height of the dots are 40nm
and 10 nm, respectively, measured by an atomic force microscope. The commonly used
technique for studying these samples is near-field spectroscopy.
2.2 Spectroscopy
1) Near-field optical spectroscopy
Near-field optical probing is a powerful technique to study the optical properties of
semiconductor quantum nano-structures with spatial resolutions well beyond the
diffraction limit of light. The high-spatial resolution of near-field optical fiber probe tips,
which can approach λ/40, where λ is the optical wavelength of the dot luminescence,
makes the imaging and spectroscopy of highly specialized nanostructures such as
quantum dots possible, even at a spatial density of the order of 100/µm2. Figure 3 shows
a schematic representation of a low temperature nano-probing system based on shearforce distance regulation.
The QD sample on the scanning stage is illuminated with He-Ne laser light through the
aperture of a high sensitive double-tapered fiber probe under shear force feedback
control. Most of the carriers are generated in GaAs and AlGaAs barrier layers. After
diffusing in the barrier layers, the carriers are captured in the confined states of the QDs.
The resultant PL signal from a single QD is collected by the same aperture to prevent the
reduction of spatial resolution due to carrier diffusion. The PL signal is detected by
means of a photon-counting avalanche photodiode through a bandpass filter. In the case
of collecting the PL spectra, the PL signal is sent into a monochromator and detected by a
cooled charged coupled device. The spectral resolution of this measurement is about
1meV.
Figure 3: Schematic representation of low temperature nanoprobing system used to measure the near field
luminescence properties of quantum dots Ref [3]
Figure 4: Near-field luminescence image of a single In0.4Ga0.6As/Al0.5Ga0.5As QD (T = 5 K)
Ref [3]
To image a single dot, the luminescence falling inside a single window of Δλ = 2nm
centered at 733nm is collected, and the tip is scanned across a 2μmX2μm area of the
sample. Figure 4 shows an image of a single quantum dot obtained using this technique.
The strong point of nano-probing is its ability to map the location, polarization state and
emission wavelength of every dot even if the areal density is as high as 100/μm2.
2) Far field Microscopy
Far-field epifluorescence confocal microscopy is another technique used to obtain images
as well as spectra of single quantum dots. A typical optical setup is shown in Figure 5.
Excitation light is transmitted through a high reflecting mirror (at an angle of 45o) and
focused by a long working distance microscope to a ~30 μm spot on the sample surface.
The fluorescent image is collected by the same objective lens, reflected off the high
reflecting mirror and passed through a wavelength specific filter to remove any excitation
light while allowing all of the fluorescence to pass. The image is then focused onto the
entrance slit of a spectrometer and detected with a cooled CCD camera (liquid nitrogen or
thermoelectrically cooled). Both images and spectra of the sample can be collected on the
same detector by switching between the diffraction grating and a mirror at 0o for spectra
and images, respectively.
Figure 5: Far-field epifluorescence microscope Ref [2]
In order to take spectra from single nanocrystals, the entrance slit of the spectrometer is
used to spatially isolate individual nanocrystals along a vertical region of the image. It is
usually possible to align several nanocrystals at different vertical positions within the
narrowed slit. The mirror in the spectrometer is then replaced with a diffraction grating
and light from each vertical position is dispersed onto the CCD. What results is an image
in which single nanocrystals are spectrally dispersed along the x-axis and are vertically
separated along the y-axis corresponding to their spatial position within the entrance slit
(Figure 6). In this way, it is possible to simultaneously obtain spectra from several
different nanocrystals. In Figure 6, four single nanocrystal spectra are collected.
However, careful manipulation of the sample concentration can allow the simultaneous
detection of up to 100 spectra.
Figure 6: A) Image of single 45 Ao nanocrystals at 10 K; B) Image of the same region as in (A)
with narrowed entrance slit; C) Spectrally dispersed image of the entrance slit in (B) Ref [2]
3. Observations
1) Single quantum dot spectra
Much lower linewidths are obtained in the spectra of single quantum dots. The ensemble
emission spectrum can be reproduced by a convolution of the average single nanocrystal
lineshape observed for individual quantum dots within the sample (Figure 7). The close
agreement between the ensemble average distribution and the average of the distribution
of single dot spectra confirms the proposition that the broadening of the ensemble spectra
is because of the size distribution of the ensemble.
Figure 7: Ensemble vs single nanocrystal spectra for three different sizes Ref [2]
All single nanocrystal spectra show qualitatively similar spectral characteristics,
including narrow peaks and a longitudinal optical (LO) phonon progression with peak
spacing comparable to the bulk LO phonon frequency. Differences in phonon coupling
are observed between different nanocrystals with an average value that is equal to the
phonon coupling measured in ensemble samples. The range of phonon couplings
measured in these experiments is something that can only be seen on the single
nanocrystal level and represents a clear example of ensemble averaging, in which
information about the individual members of a distribution is lost when averaged over the
ensemble.
Fore core-shell quantum dots, it has been observed that the quantum yield of the emission
is greatly enhanced as compared to the pure core quantum dots. For example, a ZnS
coating over CdSe has been reported to increase the fluorescence quantum yield to as
high as 50%. The coating also causes a slight red shift of the emission peak. This can be
qualitatively understood as follows: in spite of the larger bandgap of the coating, some of
the electron (hole) wavefunctions do extend into the shell, causing an increase in the
effective size of the quantum dot, hence lowering (raising) the energy of the electron
(hole), and resulting in a net red shift of the emission peak.
Some of the single QD spectra obtained using the above techniques have been illustrated
below. Figure 8 shows a low temperature (T = 5K) near-field luminescence spectrum of
In0.4Ga0.6As/Al0.5Ga0.5As quantum dots. The five sharp peaks correspond to excitonic
emission from five different quantum dots lying under the aperture tip. The linewidths of
the emissions are limited by the spectrometer spectral resolution of 0.5 nm. Far field
photoluminescence taken from the same sample with a much poorer spatial resolution of
5-10 µm does not show any sharp features, only a broad peak centered at 730nm.
Figure 8: Low temperature (T = 5K) near-field luminescence spectrum of
In0.4Ga0.6As/Al0.5Ga0.5As quantum dots Ref [3]
The emission wavelength is a sensitive indicator of each dot's structure and local
environment. By analyzing the polarization properties of the emission, it can be
Figure 9: (a) PL spectrum of a single QD at room temperature (b) QD spectra for different
excitation intensities Ref [4]
concluded that the five emission lines come from different dots. Since the direction of the
axis of the linearly polarized emission is different for each emission line in the set, it
indicates that the emission lines are uncoupled from each other. The lack of a preferred
polarization direction in the emission also indicates that the shape of even neighboring
dots is not preferentially stretched along a given crystal direction.
Figure 9(a) shows the PL spectrum of a single QD obtained in a high excitation condition
at room temperature. The PL signals from the WL and GaAs barrier layer were observed
at 1.34 and above 1.36 eV, respectively. The PL below 1.28 eV corresponds to that from
the confinement states of the QD. Figure 9(b) shows the PL spectra in single QD at
various excitation power densities. The peak intensity of each spectrum is normalized by
the excitation power density. In the weak excitation condition, a single PL peak is
observed at 1.158eV (E1). For excitation power density higher than 1.0kW/cm2, another
PL peak appears and grows at the higher energy of 1.205eV (E2).
To confirm the origin of emission lines, the plots of the integrated PL intensity for the
emission lines of both E1 and E2 are illustrated in figure 10 (a). The PL intensity of E1,
indicated by the closed circles, almost linearly increases up to 0.8 kW/cm2 and gradually
saturates. This fact strongly implies that the emission line of E1 originates from the
recombination of the ground state. At the power density of around 0.8W/cm 2, where the
ground state emission begins to saturate, the PL line of E2 indicated by the open circles
appears. From this threshold-like PL behavior as a function of excitation power density,
the PL line of E2 can be associated with the emission of the first excited state. The energy
separation between the ground state and the first excited state is about 45meV which is in
agreement with the prediction of the theoretical calculation by considering the
composition and size of the QDs.
.
Figure 10: (a) Excitation power dependence of integrated PL intensity at 1.158 eV (closed circles)
and 1.205 eV (open circles) (b) Excitation power dependence of the linewidth of the emission at
1.158 eV Ref [4]
In order to estimate the inhomogeneous width of a single QD at room temperature, the PL
line width (FWHM) from the ground state as a function of exciton power density is
investigated in Fig.10(b). In the weak excitation region, the linewidth remains constant at
the value of about 14meV. With an increasing excitation power density, the linewidth
also increases above 0.8kW/cm/cm, where the integrated PL intensity starts to saturate.
Such a broadening of the linewidth with increase of excitation power density has also
been reported at room temperature.
2) Intermittency/ blinking of the fluorescence
It has been observed that on the timescale of a single image (<0.5 s), the fluorescence
from individual nanocrystals flickers on and off (fluorescence intermittency) (Figure 11).
The pattern of on/off behavior is strongly dependent on the particular nanocrystal as well
as the excitation intensity. This binary fluorescent blinking, rather than a stepwise or
continuous dimming of the emission from a single point, is strong evidence that the spots
imaged and the spectra obtained originate from single nanocrystals. In the case of an
ensemble, this intermittency is averaged over all the particles, and hence a distinct on/off
pattern is not observed.
Figure 11: Fluorescence intensity vs time traces for a single ~21 Ao CdSe nanocrystal; sampling
time – 40 ms Ref [5]
It has been observed that the average on time is inversely proportional to the excitation
intensity, while the average off time is intensity independent (Figure 12a). Also, an
overcoating of a higher bandgap material on the quantum dot has been observed to
decrease this intermittency to the point where blinking occurs on the timescale of many
seconds to minutes (depending on the coated material and the coating thickness) (Figure
12b). In this case, images of single nanocrystals appear relatively stable over time.
Figure 12: (a) Comparison of fluorescence intensity versus time graphs for two different
excitation intensities (Sampling time = 10 ms) (b) Fluorescence intensity versus time graphs for a
bare CdSe and a ZnS coated (7 ML) CdSe nanoparticle Ref [5]
Several models have been proposed to explain the fluorescence intermittency of these
quantum dots, the most common one being the photo ionization of the quantum dot. This
model based on the Auger ionization of the nanocrystal, is consistent with the excitation
intensity dependence of the on/off times as well as the prolonging of the on/off cycle in
the presence of a capping layer. According to this model, in quantum dots with two or
more excitons, the energy released from the annihilation of an exciton may be transferred
to other carriers, one of which is then preferentially ejected into the surrounding matrix
depending on the conduction (valence) band offset at the nanocrystal interface and the
electron (hole) confinement energy inside the nanocrystal. Subsequent photogenerated
excitons then combine non-radiatively, transferring their energy to the carrier. A charged
nanocrystal thus becomes “dark”. The luminescence is restored when the ejected carrier
(or some other carrier) returns to neutralize the nanocrystal.
It has also been reported that the number of on/off cycles is considerably reduced at
lower temperatures, and that the on times follow an Arrhenius activated behavior down to
50 K. Therefore, a combination of thermally activated charge trapping processes and
Auger ionization has been proposed to explain this.
3) Spectral diffusion
Observation of many single quantum dot spectra revealed a wide range of linewidths and
lineshapes. The emission energies and linewidths obtained were also dependent on the
excitation intensity and the integration time for a given quantum dot. This is similar to
what is seen in single molecule spectroscopy, and the reason is thought to be similar in
both cases: the dynamic changes in the environment of the quantum dot/ single molecule.
Evidence of this can be seen from Figure 13. Figure 13A shows a high-resolution
spectrum of the zero phonon line of a single nanocrystal with a 10 s integration time.
Spectral features include a linewidth of ~2 meV as well as a second peak, shifted by ~3
meV. However, figure 13B, taken for the same sample with a 0.1 s integration time, and
figure 13C, showing a histogram of peak positions from 150 different spectra, clearly
imply that this linewidth and second peak cannot be attributed to the intrinsic physical
properties of the crystal, but is the result of averaging over time, the dynamic spectra of
the nanocrystal.
Figure 13: Contribution of spectral diffusion to single nanocrystal lineshapes Ref [2]
It has been suggested, especially after comparing spectral diffusion with the Stark effect on
the single nanocrystal spectra and noticing similarities between them, that spectral diffusion
is a result of the changing local electric fields in the vicinity of the nanocrystal. The presence
of an electric field causes a change in the overlap of the electron and hole wavefunctions in
an exciton. The phonon coupling changes as a result of this change in the overlap, and hence
the spectrum shifts to lower energies with a corresponding increase in phonon coupling. This
corresponds to the Stark effect when the electric field is externally applied, and the spectral
diffusion when the electric fields are local. In fact, fluorescence intermittency and spectral
diffusion of quantum dots have been correlated in that the photoexcitation or neutralization
(dark and light states) of a quantum dot correspond to changes in local electric fields, and
hence on/off intermittency is often accompanied by a spectral shift.
4) Polarized PL spectroscopy
Polarized photoluminescence spectroscopy of single quantum dots provides another probe for
the physical properties of the dots. A narrower linewidth allows for precise measurements of
luminescence character because of the absence of the inhomogeneous broadening effect in a
single dot. In addition the application of a magnetic field provides information about the
spin-related effects such as Zeeman splittings.
Figure 14: Typical unpolarized photoluminescence spectra from a single dot at various magnetic fields
Ref [6]
Relaxation processes in single GaAs/InAs quantum dots are considered using polarized
photoluminescence (PL) spectroscopy in an external magnetic field. The GaAs/InAs SQD
structures are grown by molecular beam epitaxy.
A typical magneto-optical luminescence spectrum from a single dot is shown in Figure 14. In this
measurement the luminescence is collected with fields B=0-8T and dispersed by the 1m double
monochomator. The luminescence lineshape at 0 T is fitted well as a Lorentzian function with a
linewidth less than 100μeV. The application of the magnetic field makes it possible to observe
splittings due to the different spin states of the carriers. The splitting changes linearly as a function
of the applied magnetic field. The average value of the energies of the two split peaks shows a
diamagnetic shift. The shift of the energy is proportional to B2. Quantitative analysis of the energy
shift shows the parabolic coefficient to be 3.7μeV/T2. This value is consistent with observed in farfield magneto-PL spectra. The observed diamagnetic shift indicates that strong confinement in the
lateral direction is achieved in the SQD structures.
Figure 15 shows luminescence spectra for different polarization geometries at 8T. The carriers are
excited with circularly or linearly polarised light and the resulting luminescence polarizations are
measured by tuning a variable wave plate. The excitation energy is set to be 1.4089 eV and the
detection energy is about 80meV lower. When carriers are excited with linearly polarized light, the
opposite polarities of the split lines are clearly shown. The excitation energy of 1.409 eV results in
the carriers being excited in the dot states. When excitation and detection polarizations are
crossed, the intensities of the split peaks are small compared to when polarizations are the same.
There still remain small peaks in crossed polarization due to imperfect polarization selectivity of
the experimental setup or to spin relaxation within the recombination.
Figure 15: Luminescence spectra for all polarization geometries at 8 T Ref [6]
4. Conclusion
This review would be incomplete without a mention of the possible applications of this
kind of spectroscopy. Due to its narrow, symmetric and easily tunable spectrum and
stability against photobleaching, a single quantum dot can be considered as a
monochromatic source, with the advantage of being easily tunable, as well as easily
incorporated into a variety of systems, giving rise to a myriad of interesting applications.
Highly luminescent single quantum dots potentially can overcome the functional
limitations encountered with chemical and organic dyes, and hence can prove to be a
valuable tool for biological applications like DNA and protein labelling. The novel
optical properties of quantum dots can render them ideal fluorophores for sensitive,
multicolor, and multiplexing applications in molecular biology and bioengineering.
Quantum-dot-based super-luminescent light-emitting diodes are being developed to serve
as high-bandwidth high-power light sources for application in optical coherence
tomography. These devices consist of self-assembled indium-arsenide quantum dots
within indium-gallium-arsenide quantum wells, with barrier layers made of gallium
arsenide. The spectra of these devices can be largely tuned by varying factors such as
indium composition of the well layer, size of dots, etc.
A possible application might exploit the intermittent character on the fluorescence,
however, that would involve, as a first step, the exact explanation and the ability of
tailoring this effect. Also, the single dot spectra still show some inhomogeneous
broadening at higher temperatures, which is due to the phonon coupling, and the
temperatures of ~10 K are not suited for wide range practical applications. So a way has
to be found to overcome this limitation.
Thus we have presented a brief review of the motivation, techniques, salient features and
some novel applications of single quantum dot optical spectroscopy.
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