Download Highly efficient blue photoluminescence from colloidal lead

INSTITUTE OF PHYSICS PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 39 (2006) 1477–1480
doi:10.1088/0022-3727/39/8/003
Highly efficient blue photoluminescence
from colloidal lead-iodide nanoparticles
C E Finlayson1 and P J A Sazio
Optoelectronics Research Centre (ORC), University of Southampton, Southampton SO17
1BJ, UK
E-mail: [email protected]
Received 16 December 2005, in final form 10 February 2006
Published 30 March 2006
Online at stacks.iop.org/JPhysD/39/1477
Abstract
We report the synthesis of solvent-stabilized lead-iodide nanoparticles,
using a convenient route involving coordinating solvents. The resultant
colloids show strong absorption features in the ultraviolet region of the
optical spectrum, which are consistent with the formation of semiconducting
nanocrystals of lead (II) iodide. An effective-mass approximation model of
quantum-confined states is in good agreement with the observed transition
energies, giving strong indications of the particle morphologies and
dimensions. Intense photoluminescence is also observed, with some spectral
tuning possible with ripening time, giving a range of emission photon
energies approximately spanning from 2.5 to 3.0 eV. We measure
photo-stable luminescence quantum efficiencies of around 20% in solution,
increasing to up to 30% if the coordinating ligand is exchanged for a
Lewis-base capping layer. This demonstrates the potential for the utilization
of lead-iodide nanocrystals in visible optoelectronics applications.
(Some figures in this article are in colour only in the electronic version)
The development and modification of synthesis routes to
semiconductor nanoparticles or ‘nanocrystals’ has represented
a strongly active area of research in the chemical sciences
for many years [1–4]. These materials are nanometrescaled, roughly spherical, chemically synthesized particles of
II–VI and III–V materials, which have attracted considerable
scientific interest due to the regime of extreme quantummechanical confinement possible when the particle dimensions
are smaller than that of the bulk-exciton Bohr radius [5–8].
Such nanocrystals exhibit discrete electronic states and
strongly size-tunable optical transitions in a fashion more akin
to atoms and molecules rather than the bulk semiconductor
material. In this domain, in between individual atoms or
molecules and bulk solids, the physical properties also depend
on the fragment size as well as the overall composition and
stoichiometry of the material. Hence, control of particle
size becomes potentially a very powerful tool in customizing
the physical properties of a wide range of semiconducting
materials. Common material systems, in which quantum
dots have been successfully synthesized and characterized,
1
Currently at Cavendish Laboratory, University of Cambridge, Cambridge
CB3 0HE, UK.
0022-3727/06/081477+04$30.00
© 2006 IOP Publishing Ltd
include CdSe [9] and CdS [10], operating in the visible
region of the spectrum and InAs [11] and PbSe [12] in the
near infrared. Surface atoms constitute a significant fraction
of the overall structure in such nanoscale structures, hence
surface contributions can no longer be neglected and this
can have a dramatic effect on the observed thermodynamic
properties. In extreme cases, it has been possible to produce
metastable bonding structures within nano-sized clusters that
are unachievable in the bulk material [13]. In addition, the
inherent control over the surface chemistry of these colloidal
materials makes them ideal for solution processing and liquid
phase deposition techniques. These desirable properties
have attracted a diverse range of demonstrated applications,
including, but not limited to, LEDs [14], photovoltaic and solar
cells [15], lasers and optical gain media [16], photonics [17],
biological detection and labelling [18] and as components in
molecular electronics [19].
Lead (II) iodide (PbI2 ) is a wide bandgap semiconductor,
with notable applications in radiation detectors and x-ray
imaging [20]. Bulk PbI2 has also previously been reported
to exhibit direct-bandgap luminescence at green wavelengths,
corresponding to a bandgap energy of 2.55 eV [21]. In this
Printed in the UK
1477
C E Finlayson and P J A Sazio
Figure 1. Absorbance spectra for a saturated solution of PbI2 in
tetrahydrofuran (thin solid line) and for an aliquot taken out of the
reaction vessel after 10 min (thick solid line), the latter showing a
series of sharp peaks in the ultra-violet region of the spectrum. The
dotted line shows the position of the nominal bulk bandgap energy
of 2.55 eV.
paper, we report the synthesis of highly photoluminescent,
size-tunable PbI2 nanoparticles, using a convenient colloidal
route, indicating the potential use of this material in
optoelectronics applications. In particular, the range of
emission wavelengths we observe (figure 4 inset) covers a gap
in terms of the II–VI colloidal materials, which are widely
used in the 1.2–2.5 eV photon energy range. The synthesis of
very small blue-emitting CdSe nanoparticles by metathesis has
been demonstrated [22]; however, the limited size-tunability
and poor surface stability of these materials has rather limited
their device exploitation.
Our convenient synthesis involves using coordinating
solvents, e.g. tetrahydrofuran (THF), to produce solventstabilized lead-iodide nanoparticles. Firstly, 100 mg of lead
iodide powder (99.99%, from Fluka) was dissolved in 15 ml
of THF. The resultant mixture was vigorously sonicated for
5 min, in order to produce a saturated solution, which was
then removed from any undissolved solid by centrifugation
and decantation. This deep-yellow coloured solution was then
placed into an enclosed flask at room temperature under an inert
nitrogen atmosphere and stirred continuously. Next, 10 ml of
anhydrous methanol, in which PbI2 is only slightly soluble,
was added to the vessel and a colour change from yellow to
colourless was noted. The vessel conditions were kept the
same for 24 h, with small aliquots being taken at intervals
in order to monitor the time evolution of the absorption and
photoluminescence (PL).
Figure 1 shows the absorption spectrum of a sample taken
10 min after initiation, as measured immediately after aliquot
removal, together with that of the saturated solution of PbI2
in THF. Measurements were carried out using a dual channel
spectrophotometer, with a matched reference cell containing 3
parts THF to 2 parts methanol and using a spectral wavelength
resolution of 2 nm. The spectrum after 10 min shows clear
signs of PbI2 nanoparticle formation, with the broad absorption
spectrum of the saturated solution being replaced by a series
of sharp peaks in the ultraviolet [23]. The blue-shifting of
these absorption features relative to the bulk bandgap energy is
indicative of the quantum-confinement effect in nanoparticles
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Figure 2. Results of an EMA model showing the quantum-confined
bandgap energy in PbI2 as a function of lateral dimension, a, for the
three lowest optical transitions. The experimentally observed
positions of the absorption peaks are marked on as horizontal
dashed lines for reference and the vertical dashed line corresponds
to a = 1.27 nm, giving a close fit to the data in all three cases where
l = 1.71 nm.
which are smaller than the bulk exciton radius of around 1.9 nm
in PbI2 , as has been previously confirmed by transmission
electron microscopy (TEM), [24] small-angle x-ray diffraction
studies [25] and Raman spectroscopy [26]. Mallik et al [27],
have attributed the features seen in the absorption spectra of
PbI2 nanocrystals to interband transitions in cylindrical or rodlike particles of a monomodal size-distribution, with variations
in size depending upon the exact nature of the solvent/matrix
medium used. This is in contrast to previous reports of the
various peaks representing first-order transitions in a number
of discrete particles sizes, possibly corresponding to ‘magic
number’ bonding configurations [23].
In particular, absorption peaks at photon energies of 3.39,
4.23 and 5.14 eV are observed; in order to investigate the
origins of these transitions in terms of particle morphology
and size, we have developed an effective-mass approximation
(EMA) model of the system. PbI2 has an unusual bonding
structure of layered hexagonal planes (lattice parameter
4.55 Å), which are bound by Van der Waals forces. Assuming
this bonding structure is replicated in our nanocrystals [27],
we can use an EMA model whereby we have symmetrical 2D
quantum confinement in the plane of the hexagonal layers and
1D confinement in the plane normal to the layers (i.e. the ‘caxis’); this effectively corresponds to a cylindrical quantumwell, which we approximate to be infinitely deep relative to
the surrounding medium. The EMA model used is based
around the following expression for how the quantum-confined
bandgap varies as a function of the length, l, along the c-axis
and the lateral dimension, a, of the crystal:
π 2 h̄2
βms 2 a 2 m1
Em,s,n = Eg +
n2 , (1)
+
2m1 a 2
π
l
m2
where Eg is the bulk bandgap energy (2.55 eV), m1 and m2
are the effective masses of carriers in the planes perpendicular
and parallel to the c-axis respectively, βms is the sth zero of
the circular Bessel function Jm (s) and n is a quantum number
taking on integral values. The values of effective mass used
were m1 (e) = 0.25 me , m2 (e) = 1.25 me , m1 (h) = m2 (h) =
1.1 me [28]. Figure 2 shows the results of a process to match
Blue photoluminescence from lead-iodide nanoparticles
Figure 3. Absorbance spectra of a series of aliquots taken from the
reaction vessel at various times, showing the ripening of particles
over a 24 h time course.
the energies of the 3 lowest transitions to E011 , E012 and E013 ,
by iteration of the independent variables a and l. We find
that a suitable match occurs using the values l = 1.71 nm
and a = 1.27(±0.05) nm, giving strong evidence that the
PbI2 nanocrystals are rod-like shaped, with the long dimension
orientated along the c-axis, with aspect ratio ∼1.4 and a lateral
dimension of ∼1.27 nm.
Figure 3 shows how the absorption spectrum changes
during the 24 h after the initiation of our synthesis. We
consistently see three discrete ultraviolet absorption peaks,
with only small changes in the spectral position and broadening
of the features occurring with ripening, implying reasonable
particle shape and size integrity. In contrast to these previous
reports, which do not describe the effects of ripening, the
relative intensities of the peaks are not constant, but are
observed to change with time; the peak at around 5.2 eV in
photon energy becoming notably the dominant feature and
expanding into a broader feature after 24 h. We speculate
that these effects may be due to variations in the relative
oscillator strength of transitions during ripening; this might
possibly be caused by changes in the solvent interaction and
collective effects on the electron wavefunctions associated with
transitions; we note similar effects, attributed to photolysis,
in the work of Micic et al [25]. These issues represent a
significant future challenge in the modelling and understanding
of such colloidal PbI2 nanocrystals. We propose that our
method of using a THF/methanol mixture offers advantages
over previous reports because the interaction of these solvents
produces a colloidal PbI2 product which is effectively ‘solvent
stabilized’, in a similar fashion to the use of pyridine
in previously reported metathesis-type reactions [22]. In
addition, our method avoids the generation of un-reacted
precursor species, such as I−
3 , which may substantially affect
the optical properties of the colloidal product. We find that
a solvent volume ratio of the order of 3 parts THF to 2
parts methanol produces the best results in terms of nascent
nanocrystal formation, having experimented with a range of
serial dilution ratios of methanol in THF.
Figure 4 shows the PL spectra of aliquots taken at
time intervals of 10 min, 1 h and 24 h after initiation. The
aliquots were transferred into glass vials and illuminated with
Figure 4. Normalized PL spectra of selected aliquots from the
reaction vessel, with the low energy tail of the absorbance spectrum
from an aliquot taken 24 h after initiation also shown for reference.
The inset shows PL exhibited by samples of solvent stabilized PbI2
nanocrystals, showing emission peaking at around λ = 450 nm,
photon energy ∼3.0 eV (centre) and 500 nm, photon energy
∼2.5 eV (right). The samples were transferred directly from the
reaction vessel into vials and illuminated by a hand-held UV lamp.
The vial on the left contains a standard rhodamine 6G dye for the
purposes of quantitative PL efficiency determination.
a broadband (approximately λ = 340–380 nm, peaking at
360 nm) ultraviolet lamp of a few watts total intensity. The PL
was detected using a fibre-coupled CCD spectrometer, with a
spectral resolution of better than 1 nm, and all measurements
were at room temperature and in air. Figure 4 shows how
the peak photon energy of the PL shifts approximately from
2.5 to 3.0 eV during the ripening phase of the synthesis,
indicating the spectral tunability of luminescence may be
controllable by further optimization of such parameters as
the ripening time and, also, the solvent ratios used. Using a
reference dye, rhodamine 6G in ethanol (see figure 4 inset),
of known optical density at the excitation wavelength of
the UV source, it was also possible to make a quantitative
calculation of the PL efficiency (or quantum-yield) of the
PbI2 nanocrystals in solution. The conditions and geometry
of excitation and detection were kept identical, according to
previously reported protocols [12], in the spectral intensity
measurement of both sample and then reference. Using the
usual ‘photons-out/photons-in’ definition of PL efficiency and
taking into account the nominal efficiency of the dye, it was
then possible to calculate the sample quantum-yield. In the
case of solvent stabilized nanocrystals, which had been ripened
for 24 h, we report a measured PL efficiency of 19%. An
organic Lewis-base, dodecylamine (∼1 mg ml−1 ), was then
added into the sample vial. It is expected that the lone electronpair of dodecylamine will ligate strongly to the surfaces of
the nanocrystals, replacing the coordinating solvent, hence
passivating any surface states or traps. Such surface traps
may provide non-radiative decay channels, which compete
with PL and, indeed, we observe the PL efficiency to increase
to around 30% after the addition of dodecylamine. It is also
noted that the absorption and PL spectra of the sample are
essentially unchanged by the addition of the amine, indicating
strongly that any photophysical changes are unlikely to be
due to effects of particle de-agglomeration or energy transfer
between adjacent nanocrystals [29]. The addition of an excess
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C E Finlayson and P J A Sazio
of such a Lewis base also provides a way of isolating the PbI2
nanocrystals from the coordinating solution during growth and,
also, a method of precipitation for the purposes of storage and
further solution processing. In order to test the photostability
of our amine-capped PbI2 nanocrystals, the vial used for PL
efficiency measurements was continuously irradiated under a
UV source of a few watts intensity, in air, for one hour. No
significant changes to either the PL spectrum or efficiency
were measured. We note that this high PL efficiency and
photostability is achieved without the need for any widebandgap epitaxial shell around the nanocrystalline core, as
is the case with chalcogenide quantum-dot emitters, such as
CdSe [3, 17].
The exact nature of the PL tunability is unclear, even
though the very high observed PL efficiencies suggest a
predominantly radiative bandgap transition is involved. The
invariance of the spectral position of the lowest transition in
absorption, relative to the PL band, even after 24 h, suggests
that any particle growth during the ‘ripening’ phase is limited;
however, the ripening is clearly having an effect on the relative
intensities and broadening of the absorption peaks and the
wavelength of the PL emission. We speculate that the tuning
may not be directly caused by quantum-confinement effects,
but rather deep traps or surface states (i.e. non-confined states)
additionally play some role in the Stokes shifting between
absorption and PL and also the radiative mechanism, as has
been previously observed in electroluminescence studies [21].
It is noted that such small nanoparticles will have a very
high surface-to-volume ratio and such non-confined states
may, indeed, have some sensitivity to the subtle effects of
particle ripening and detailed luminescence lifetime studies
may further elucidate this issue in the future.
In conclusion, the synthesis of solvent-stabilized leadiodide nanoparticles, using a convenient route involving THF
as a coordinating solvent, is reported. The resultant colloids
show strong absorption features in the ultraviolet region of the
optical spectrum, which are consistent with the formation of
semiconducting nanocrystals of PbI2 and an EMA model of
the system strongly suggests rod-like particles with a lateral
dimension of 1.27 nm. Intense blue PL is also observed, with
some spectral tuning possible with ripening time, suggesting
the suitability of these materials for use in visible wavelength
optoelectronics applications. The unusual nature of this
tuning, relative to the invariant absorption features, suggests
that the origin of PL may be more complicated than simple
bandgap luminescence between quantum-confined states.
Using a reliably quantitative method, we measure photo-stable
luminescence quantum efficiencies of around 20% in solution,
1480
increasing to up to 30% if the coordinating ligand is exchanged
for a Lewis-base donor, such as dodecylamine.
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