Supramolecular architectures and photoluminescent properties of triethanolammonium 4-nitrobenzoate salt and its Ni(II) complexes

Article abstract of DOI:10.1016/j.poly.2020.114893

Single crystals of new organic salt (HTEA)(4NB) (1) and its different types of Ni(II) complexes obtained from the same system NiCl₂-compound 1 under various experimental conditions: [Ni(4NB)(TEA)(H₂O)](4NB) (2), [Ni(4NB)(TEA)(H₂O)](4NB)?H₂O (3), [Ni(TEA)₂](4NB)₂ (4) and Ni(BIC)₂ (5), where TEA = triethanolammine, 4NB = 4-nitrobenzoate and BIC = N,N-bis(2-hydroxyethyl) glycinate, were successfully synthesized and structural characterized by FTIR spectroscopy and single crystal X-ray analysis. All new compounds represent supramolecular systems assembled by combining two or three organizing forces: metal-coordination (2–5), hydrogen bonds (1–5) and π-π stacking interactions (1–4). In compounds 2 and 3 the Ni(II) atom has an identical NO₅ environment, while in complex 4 and 5 the metal centres have a N₂O₄ coordination core. The ionic coordination complexes 2–4 are arranged into 2D hydrogen-bonding layers and further assembled via π-π stacking interactions in supramolecular 3D network structures, while complex molecules of 5 are organized into a hydrogen-bonded network. This study emphasizes the first structurally characterized binary glycinate Ni(II) complex obtained from NiCl₂?6H₂O-(HTEA)(4NB) system. Comparative solid-state photoluminescent study of four Ni(II) coordination compounds and initial organic salt reveals a correlation between crystal structure/packing and luminescent properties.

Full text of DOI:10.1016/j.poly.2020.114893

Chemical Physics 469-470 (2016) 25–30
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Ultrafast Exciton Dynamics in CdxHg(1ꢀx)Te alloy Quantum Dots
Marina A. Leontiadou a, , Ali Al-Otaify a,b, Stephen V. Kershaw c, Olga Zhovtiuk c, Sergii Kalytchuk c,
⇑
Derrick Mott d, Shinya Maenosono d, Andrey L. Rogach c, David J. Binks a
a School of Physics and Astronomy and Photon Science Institute, University of Manchester, Manchester M13 9PL, UK
b Department of Physics, College of Science, Qassim University, Saudi Arabia
c Department of Physics and Material Science, City University of Hong Kong, Hong Kong Special Administrative Region
d School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahida, Nomi, Ishikawa 923-1292, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Ultrafast transient absorption spectroscopy is used to investigate sub-nanosecond exciton dynamics in
CdxHg(1ꢀx)Te alloy colloidal quantum dots. A bleach was observed at the band gap due to state-filling,
the mono-exponential decay of which had a characteristic lifetime of 91 1 ps and was attributed to
biexciton recombination; no evidence of surface-related trapping was observed. The rise time of the
bleach, which is determined by the rate at which hot electrons cool to the band-edge, ranged between
1 and 5 ps depending on the pump photon energy. Measuring the magnitude of the bleach decay for dif-
ferent pump fluences and wavelengths allowed the quantum yield of multiple exciton generation to be
determined, and was 115 1% for pump photons with energy equivalent to 2.6 times the band gap.
Ó 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/4.0/).
Received 27 November 2015
In final form 2 February 2016
Available online 24 February 2016
Keywords:
Multiple exciton generation
Femtosecond transient absorption
spectroscopy
CdHgTe alloy quantum dot
1. Introduction
In particular, the rate of exciton recombination will determine
the fraction of photo-generated charges that can be extracted to
Colloidal quantum dots (QDs) have potential use in a number of
applications, including in photovoltaic cells [1–3], displays and
lighting [4], and photorefractive devices [5]. For solar cells, size-
control of these materials allows the band gap to be tuned so that
it is optimum for exploitation of the solar spectrum [6,7]. Mercury
telluride (HgTe) is particularly well-suited for use as the photo-
absorbing species in solar cells because its almost zero bulk band
gap allows HgTe QDs to be size-tuned throughout the near-infra-
red region [2]. This type of QD benefits from additional flexibility
because the band gap of QDs can also be composition-tuned via
alloying with cadmium to produce CdxHg(1ꢀx)Te [8]. Furthermore,
the band gap and the valence band split-off energies come into res-
onance at a certain composition value [9] which can lower the
energy required for carrier multiplication, also known as multiple
exciton generation (MEG) [10–13]. Hence, with the combined
effect of size- and composition-tuning, this resonance could be
moved towards the optimum energy for exploitation of the solar
spectrum [9]. Thus CdxHg(1ꢀx)Te QDs with split-off energy differ-
ence ꢁ1 eV are of particular interest as the photo-absorbing spe-
cies for the next generation solar cells.
produce the output of a photovoltaic cell. Surface states produced
by dangling bonds are known to enable a non-radiative recombina-
tion pathway that traps band-edge charges on a picosecond time-
scale [14–16]. These surface states can also trap hot charges,
introducing an additional cooling pathway and thus increasing
the overall cooling rate [17]. This influences the quantum yield
(QY) of photo-generated charges because it competes with MEG,
which uses the energy of an absorbed photon in excess of the band
gap to form additional electron–hole pairs [18]. MEG has been
predicted to be efficient in CdxHg(1ꢀx)Te QDs because impact ion-
ization, the corresponding effect in bulk materials, is significant
in CdHgTe avalanche photodiodes [19]. The additional excitons
produced by MEG can only contribute to photocurrent, and thus
improve the efficiency of solar cells based on QDs [20], if charge
extraction can be achieved within the lifetime of the resulting mul-
tiexcitons, which also occurs on an ultrafast time-scale.
The sub-nanosecond exciton dynamics of HgTe QDs have been
recently studied in detail using ultrafast transient absorption spec-
troscopy [21]. In contrast, only initial results have been reported
for CdxHg(1ꢀx)Te QDs to date [9], using an ultrafast transient grating
(TG) technique. An increased TG component was reported for a
pump wavelength of 290 nm and was attributed to the additional
biexcitons formed by MEG. However, whilst the decay time con-
The utility of QDs in solar cells often depends on the dynamics
of the photo-generated excitons on a sub-nanosecond time-scale.
stant,
sdecay, for pumping above the MEG threshold was found,
⇑
Corresponding author.
E-mail address: marina.leontiadou@manchester.ac.uk (M.A. Leontiadou).
the TG method did not allow the recovery of sdecay for pumping
http://dx.doi.org/10.1016/j.chemphys.2016.02.003
0301-0104/Ó 2016 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

26
M.A. Leontiadou et al. / Chemical Physics 469-470 (2016) 25–30
below the MEG threshold. This prevented the confirmation that the
TG observed above and below the MEG threshold was decaying at
the same characteristic rate, and hence attributable due to biexci-
ton recombination in both cases. Moreover, the potential effects of
surface-trapping on the TG were not investigated and so their
influence on the reported QY values cannot yet be excluded. In this
work, ultrafast transient absorption (UTA) spectroscopy is used to
further study the exciton dynamics in CdxHg(1ꢀx)Te QDs, allowing a
comparison of decay lifetimes above and below MEG threshold and
thus confirmation that it is indeed MEG producing the observed
differences in response. The rise-time of the UTA signal is also
investigated enabling the electron cooling time to be determined
for different pump wavelengths. The possibility of sub-nanosecond
surface trapping is also investigated allowing the enhanced
response at the shortest pump wavelengths to be more confidently
attributed to MEG. Finally, the MEG QY is determined with a signif-
icantly improved accuracy and compared to the previous result.
Scientific, Variomag Mini) at ꢁ1000 rpm throughout the experi-
ment, unless otherwise stated. More details on the experimental
set-up can be found elsewhere [23,24].
3. Results
3.1. Spectroscopic Characterization
Figure 1 shows the absorption (solid line) and PL (dashed line)
spectra of the CdxHg(1ꢀx)Te alloy QDs sample. The PL spectrum
has a peak centred at 1043 nm with a FWHM of about 150 nm.
In comparison, the absorption spectrum shows a broad shoulder,
blue-shifted from the PL peak by approximately 130 nm, at about
920 nm, with a weak tail extending as far as the PL peak. These
spectra are very similar to those observed for HgTe QDs, the forms
of which were explained using detailed electronic structure mod-
elling [21]. These calculations showed the PL peak corresponded
to the conduction band minimum (CBM) to valance band
maximum (VBM) transition. However, the same calculations also
showed that the VBM-to-CBM absorbing transition is very weak
compared to the transition from the VBM to the CBM-1 state.
The absorption shoulder thus corresponds to this strong transition
at higher energy, with a contribution from deeper in the valance
band to the CBM also possible, and it is the weak tail extending
to the PL peak that is produced by the lower energy VBM-to-
CBM transition. We attribute the almost identical spectra observed
here for CdHgTe QDs to the same cause. The position of the absorp-
tion edge is well-suited to the exploitation of the solar spectrum by
a photovoltaic cell utilizing MEG [12]. As described in previous
work [9], the composition of QDs of known diameter can be esti-
mated from the position of the absorption edge. In this case this
process yielded a value of x = 0.5.
2. Experimental Method
2.1. Sample Preparation
CdxHg(1ꢀx)Te alloy QDs were produced from CdTe QDs by an ion
exchange method, which was described in detail in a previous
report [9] and so is only outlined here. The CdTe QDs, in aqueous
solution and stabilized with thioglycerol, were synthesized using
the one-pot method reported by Wu et al. [22]. These were contin-
ually converted into CdHgTe QDs by mixing with aliquots of mer-
cury perchlorate solution, Hg(ClO4)2ꢂ2.5H2O stabilized in alkaline
solution with a small amount of additional ligand. The resulting
CdHgTe alloy QDs were transferred into an organic solution by
exchanging the thioglycerol for dodecanthiol, with the final pro-
duct being CdxHg(1ꢀx)Te QDs in tetrachloroethylene solvent (TCE).
Transmission electron microscope image (see Fig. S1 in SI) shows
that the QDs produced are 4.5 0.5 nm in diameter. For optical
characterization the samples were placed in 10 mm thick quartz
cuvettes and diluted using TCE to obtain an absorbance at the band
edge of between 0.2 and 0.5.
Figure 2 shows the pump-induced intensity change spectrum
for the QD sample for pumping at a wavelength of 500 nm and flu-
ence of 16 l
J/cm2 obtained at the time corresponding to the peak
signal (occurring at ꢁ4 ps after the arrival of the pump pulse for
this pump wavelength); for comparison, the corresponding spectra
obtained for the TCE and under the same pumping conditions is
also shown. The spectrum for the QDs comprises two overlapping
peaks centred at approximately 920 nm and 1050 nm. In contrast,
the spectrum for the TCE is featureless except for a sharp peak at
the exactly half the pump wavelength, which is attributed to
pump light scattered by the sample into the spectrometer and
2.2. Spectroscopic Techniques
The photoluminescence (PL) and steady-state absorption spec-
tra were measured using a Horiba Jobin Yvon Fluorolog model
iHR(FL3-22) spectrofluorometer and a Perkin Elmer Lambda 1050
UV/Vis/NIR spectrometer, respectively.
The ultrafast transient absorption experiments were performed
with the use of a mode-locked Ti:sapphire laser oscillator (Spectra-
Physics Tsunami) and amplifier (Spectra Physics SpitfirePro) sys-
tem, producing 100 fs, 1 mJ pulses at a wavelength of 800 nm
and at a repetition rate of 1 kHz. A tunable pump beam was gener-
ated using 95% of the amplifier output, which was passed through
an optical parametric amplifier (Light Conversion, TOPAS) with
subsequent harmonic generator, before reaching the sample. At
the sample, the pump beam had a diameter of 3 mm and a fluence
up to 20 l
J/cm2. An optical chopper was used to modulate the
pump at 500 Hz. The remainder of the amplifier output was used
to produce the probe beam; it was directed, via a 200 mm long
delay stage (Newport ILS), to a 2 mm thick sapphire crystal in order
to generate the white light continuum. The probe was split into
sample and reference beams; the sample beam was focussed to
1 mm diameter, whilst the reference beam bypassed the sample.
Both beams were directed into a monochromator (Acton Research,
SpectraPro 2500i) and detected by two silicon photodiodes. The
signal amplitude was recorded with the use of a lock-in amplifier
(Stanford Research Systems, SR830) synchronized to the optical
chopper. All samples were stirred by a magnetic stirrer (Thermo
Figure 1. Optical Characterization. Absorbance (solid line) and PL (dash line)
spectra for the sample of CdxHg(1ꢀx)Te alloy QDs.

M.A. Leontiadou et al. / Chemical Physics 469-470 (2016) 25–30
27
h
mp;th ¼ Egð2 þ meeff =mehff
Þ
ð1Þ
For CdHgTe, meeff =mehff ꢃ 0:13 [28], and thus the MEG threshold
occurs at hmp,th ꢃ 2.1Eg. Taking the PL peak as a measure of Eg, this
relation yields hmp,th = 2.5 eV for the samples studied, which corre-
sponds to a pump wavelength of ꢁ500 nm. The MEG process can
result in more than one additional exciton but this would only
occur for an additional increase hmp above Eg of at least the same
size [20], i.e. a value of at least 3.2Eg for CdHgTe QDs. However,
the largest hmp value used in this study is equivalent to 2.8Eg, and
so under these conditions MEG can create only one extra exciton,
forming a biexciton.
Biexcitons are also formed when a QD absorbs two photons per
pump pulse, the probability of which is determined by the pump
fluence and the absorption cross-section at the pump wavelength
[29]. Recent work has shown that surface states can trap hot carri-
ers significantly, so that absorbed photons produce band edge exci-
tons with a sample-specific efficiency [30]. However, it is not
necessary to know this efficiency, since the band edge occupation,
F, can be determined directly from the peak fractional transmit-
Figure 2. Pump-induced intensity change,
D
I/I spectra taken for CdxHg(1ꢀx)Te alloy
QDs (solid line) and TCE (dash line). Pumping was at a wavelength of 500 nm with a
spot size of 4 mm and a fluence of 16
intensity (dotted line).2
l
J/cm2. Also shown for comparison is the PL
tance change observed,
[20]:
DT/T|pk,, using the following expression
undergoing second-order diffraction from the grating therein. The
first peak in the QD spectrum is at the same spectral position as the
absorption edge, and is attributed solely to the bleach caused by
state-filling by photo-generated charges, whilst the second is at
the position that corresponds to the PL peak shown in Fig. 1 (and
reproduced in Fig. 2 to allow direct comparison) and will have con-
tributions from both bleaching and emissive processes. Direct sur-
face-trapping of photogenerated charges has been shown to result
in characteristic photo-induced absorption (PIA) features in the
vicinity of the band-edge [14,25,26]. However, the pump-induced
intensity change spectrum shown in Fig. 2 shows no signs of any
such PIA consistent with no significant surface-trapping occurring
on this time-scale. The first peak lies outside the PL band and so
does not correspond to a transition involving both the valence
band maximum (VBM) and the conductance band minimum
(CBM) simultaneously. Rather, it must correspond to transitions
involving either the VBM or the CBM and another state, with the
bleach produced by state-filling of whichever of the band edge
states is involved. Similar behaviour has been reported previously
for HgTe QDs, and was supported by detailed electronic structure
calculations [21]. In contrast the PL band corresponds to a CBM
to VBM transition, and here both PL and emission stimulated by
the probe beam, as well as bleaching, can result in an increased
intensity at the detector. However, the PL intensity is independent
of the time-delay between the pump and probe pulses and so
appears as a constant background in intensity change transients.
Figure S2a in the SI compares transients obtained for probe wave-
lengths of 920 nm and 1064 nm i.e. outside and within the PL band.
A significant background is observed for the latter but none for the
former, confirming this interpretation. Thus, in order to simplify
the analysis, a probe wavelength of 920 nm was used for the stud-
ies described from hereon since that yields an absorption bleach
only, and allows the pump-induced response of the CQDs to be
expressed in terms of the fractional change in transmittance pro-
duced by the bleach.
ꢀ
ꢀ
ꢀ
ꢀ
D
T
T
Apr
A
¼
Fð1 ꢀ eꢀA
Þ
ð2Þ
pk
where Apr and A are the sample absorbance at the probe and the
pump wavelengths, respectively. Thus F is used as a measure of
band edge excitation in this study.
Regardless of how they are produced, biexcitons decay into
single excitons with a lifetime that is typically several tens of
picoseconds; for instance, the lifetime reported for HgTe QDs was
49 2 ps [16]. This is observed in transient absorption data as a
sub-nanosecond decay of the bleach feature from its peak value,
which is due to state-filling by both biexcitons and single excitons,
to a smaller value corresponding to state-filling just by the single
excitons that remain. Single excitons have a much longer lifetime,
typically several nanoseconds or more, and so the bleach compo-
nent due to single excitons is observed on the time-scale of the
experiment as a constant or a nearly constant plateau [20]. The
ratio of the peak to the plateau bleach value, R, is given by [27]:
R ¼ dQYhNi½1 ꢀ expðꢀhNiÞꢄꢀ1
ð3Þ
where QY corresponds to the average number of excitons generated
per absorbed photon, hNi is the number of photons absorbed per QD
per pump pulse and is proportional to F, and d is a factor that takes
into account the decay of the single exciton over the duration of the
transient. At sufficiently low pump fluences, where hNi is negligible,
R/d ꢃ QY, which will be greater than unity if MEG is significant.
Figure 3 shows the transmittance transients,
DT(t)/T for the
CdHgTe QDs induced by pumping at a wavelength of 400 nm and
at range of excitation levels. The maximum value of the pump-
induced bleach varies linearly with pump fluence, as shown in
the SI (see Fig. S2b). The decay from this bleach peak is well-
described by a mono-exponential decay for F = 1.2, 3.1 and 6.9%,
and a global fit to these transients yields a time constant of
sB = 91 1 ps. Similar decay components have been reported previ-
ously for other QDs and attributed to the accumulation of charges
trapped on the surface [31–33]. Such surface trapped charges leads
to the formation of trions an electron–hole pair is photo-generated
whilst the geminate partner of the trapped charge remains in the
QD [34]. However, this effect is ameliorated by rapidly stirring or
flowing the sample and using only moderate pump fluences. A
comparison of the transients obtained for stirred and static sam-
ples of the CdHgTe QDs for moderate pump fluence is shown in
the SI (see Fig. S3). No significant difference is evident between
3.2. Ultrafast exciton dynamics
The quantum yield, QY, of MEG can be determined by studying
how the dynamics of the bleach peak depend on pump photon
energy, hmp. The onset of MEG occurs at a threshold value of hmp,th
that is determined by both the size-dependent band gap, Eg, and
the ratio of the effective masses of the electron and hole,
meeff =mehff [27]:

28
M.A. Leontiadou et al. / Chemical Physics 469-470 (2016) 25–30
pump wavelengths used – further transients acquired for a range
excitation levels are given in the SI. As shown in Fig. 4, the decays
from the bleach peak resulting from pumping at each of these
wavelengths can all be well-described by a mono-exponential
function with a 91 ps lifetime, showing that the sub-nanosecond
dynamics observed both above and below the threshold for MEG
are consistent with biexciton formation and recombination.
Confirming that the observed dynamics are free of trapping
effects in this way enables Eq. (2) to be used to determine the QY
of MEG. Figure 5 shows the values of R determined for pump
wavelengths below and above the MEG threshold as a function of
excitation level, F. Also shown in Fig. 5 are fits of Eq. (3) to the data.
The value of d used for the fits was calculated from
ꢁ
ꢂ
b
tplateau ꢀ tpeak
d ¼ exp
ð4Þ
sPL
where (tplateau ꢀ tpeak) is the period between the peak bleach and a
time by which biexciton recombination is complete and the bleach
transient has reached a constant plateau, which in this study was
chosen to be 5sB ꢃ 0.5 ns; sPL and b are the lifetime and stretching
exponent, respectively, that characterize the PL decay. The PL
transient obtained for this sample has been previously reported
Figure 3. Transmittance transients at a probe wavelength of 920 nm for the Cdx-
Hg(1ꢀx)Te alloy QDs induced by pumping at a wavelength of 400 nm and various
band edge excitation levels, F, as shown in the legend. Each of the transients shown
is the average of 10 scans and the delay line step size was 0.2 mm. Also shown are
mono-exponential fits to the data for F = 1.2%, 3.1% and 6.9% and a bi-exponential fit
for F = 11.5%.
[9] to be
d = 1.021 0.001. Using this value resulted in the fits shown and
yielded QY = 1.01 0.01, 1.15 0.01 and 1.05 0.02 for h p = 1.4Eg,
sPL = 64.9 0.6 ns and b = 0.80 0.01, which gives
m
the transients, and hence we conclude that the observed sub-
nanosecond decay from the peak bleach is not due to the trion
recombination that results from the accumulation of trapped
charges. As noted above, the absence of any PIA features in the
transmittance change spectrum is consistent with no significant
charge trapping on a sub-nanosecond time-scale, and we thus con-
clude the decay with a characteristic lifetime of 91 ps is due to
biexciton recombination.
2.6Eg, and 2.8Eg, respectively. The uncertainty in these QY values
comes predominantly from the fits to the data shown in Figs. 5
and S5, the contribution from the uncertainty in d being negligible.
The first of these QY values is not significantly different from unity,
as expected for pumping below the MEG threshold. In contrast, the
other two pump photon energies are greater than hmp,th and conse-
quently yield a QY increased by MEG.
The highest QY value obtained here was that of 1.15 0.01
For the maximum excitation level, F = 11.5%, an additional fast
component appears at the beginning of decay. This transient is
well-described a bi-exponential decay function with a long time
constant equal to sB and a short time constant of 11 1 ps, which
is attributed to the decay of triexcitons. A pump fluence that is suf-
ficiently high to create a substantial population of biexcitons will
necessarily produce a significant number of triexcitons as well
[29]. Triexciton decay is not normally observed in transient
absorption studies because of the twofold degeneracy of the s-like
CBM commonly found in QDs [35]. State-filling of the CBM by
photo-generated electrons is responsible for the pump-induced
bleach observed for these QD for a probe wavelength tuned to
the absorption edge [35]. The decay of a triexciton into a biexciton
does not change the population of such a s-like CBM, since it is sat-
urated by the presence of either, and hence the transmission of a
probe beam tuned to the absorption edge is not affected. (QDs
based on lead chalcogenides are a notable exception to this rule
since they have an 8-fold degenerate CBM [36]. However, in this
case, the beam used to probe the CdHgTe QDs is tuned to absorp-
tion shoulder and not the absorption edge. As discussed in Sec-
tion 3.1, if CdHgTe QDs have a similar electronic structure to that
calculated for HgTe QDs [21], which the similarity of the absor-
bance and PL spectra between the two types of QDs indicates that
they do, then the absorption shoulder will have a significant contri-
bution from the VBM to CBM-1 transition. For HgTe QDs, the VBM
is p-like [37] and so will have a degeneracy greater than two, and
hence will not be completely filled by a biexciton. Unlike in many
other types of QD, triexciton decays will thus be observable in our
samples of CdHgTe QDs when pumped at sufficiently high fluence.
Transmittance transients were also obtained for pumping at
wavelengths of 370 nm, and 750 nm, corresponding to band gap
multiples of 2.8Eg, and 1.4Eg and can be found in the SI (see
Fig. S4). Figure 4 compares example transients for each of the
when photo-exciting with h
of 1.39 0.13 was recently reported for CdHgTe QDs [9] when
photo-exciting with p = 4.28 eV, which was described as
corresponding to 3Eg. However, the authors used the position of
the absorption shoulder as a measure of the Eg rather than that
of the PL peak, as in this study. As shown in Fig. 1, there is a large
mp = 2.6Eg. In contrast, a quantum yield
hm
Figure 4. Comparison of transmittance change transients produced by excitation
wavelengths of 370 nm (dash line), 400 nm (solid line), and 750 nm (crosses); the
band edge excitation levels were that of F = 6.7%, 6.9% and 5.1% respectively. In each
case, the probe beam wavelength was set to 920 nm, the position of the bleach
peak. Each trace is the average of 10 scans with a delay line step size of 0.2 mm. Also
shown are mono-exponential fits to the 400 nm and 750 nm decays yielding a time
constant of 91 ps, and a bi-exponential fit to the 370 nm decay, with time constants
of 11 ps and 91 ps.

M.A. Leontiadou et al. / Chemical Physics 469-470 (2016) 25–30
29
Figure 5. Peak to plateau ratio, R, of the transmittance transients induced by a pump beam of wavelengths of (a) 1.4 times and (b) 2.6 times the band edge for CdxHg(1ꢀx)Te
alloy QDs as a function of F; the fit shown is to Eq. (3)). See also Figure S4 in SI the 2.8Eg case.
difference in spectral position between the PL peak and the absorp-
tion shoulder, and so the two different approaches result in signif-
icantly different values of Eg. Moreover, as discussed in Section 3.1,
electronic structures for HgTe QDs indicate that absorption shoul-
der does not correspond to the VBM to CBM transition and thus is
not equivalent to Eg. Using instead the position of the PL peak
described in Ref. [9], for a better comparison with this work, yields
Eg = 1.24 eV, so that hmp = 4.28 eV now corresponds to 3.5Eg. An
increase from QY = 1.15 0.01 to 1.39 0.13 as hmp rises from
2.6Eg to 3.5Egis similar to the increase predicted for HgTe QDs over
the same range using a tight-binding model of MEG [21], suggest-
ing that the QY values reported in Ref. [9] and here are consistent.
The same model also predicts that QY does not monotonically rise
with increasing hmp, but rather that there are several peaks in QY,
particularly as hmp grows from ꢁ2Eg to 3.5Eg. This is similar to
the drop in QY observed here for CdHgTe QDs as hmp rises from
2.6Eg and 2.8Eg. Similar behaviour has been also reported previ-
ously in a detailed theoretical study of MEG in
on the same tight-binding model [38]. That work attributed the
peaks found in the calculated MEG QY as h p varied to optical tran-
a-Sn QDs based
Figure 6. Initial bleach rise times for CdxHg(1ꢀx)Te alloy QDs when pumped with
mp above (circles and squares) and below (solid line) the MEG threshold. Each of
the transients shown is the average of 10 scans and the delay line step size was
h
m
sitions within the near-band edge electronic structure with partic-
0.02 mm.
ularly high rates of MEG.
The MEG QY for other types of QDs with an absorption edge sim-
ilarly well-suited to the solar spectrum have also been reported. The
MEG thresholds for isolated PbSe [32,39] and PbS [23] QDs have
been reported to be significantly greater, at ꢁ3Eg, than the minimum
of 2Eg required by energy conservation [12], so no significant MEG
increase in rate is similar in size to the accuracy of the measurement.
Nevertheless, this observation is consistent with an increased rate of
MEG increasing both the QY and the overall cooling rate.
4. Summary and conclusions
would occur at the values of h
threshold for PbSe QDs reduces to hmp,th ꢃ 2Eg when the QDs are
deposited as a film and at h p = 3Eg the QY rises to 1.4 [40]. In con-
trast, for InAs QDs hmp,th ꢃ 2Eg and at h p = 3Eg QY = 1.35 [41]. Ge
p = 3Eg.
mp used here or Ref.[9]. However, the
Ultrafast transient absorption spectroscopy has been used to
investigate the exciton dynamics in CdxHg(1ꢀx)Te alloy QDs. At
modest pump fluences, the sample was found to be free of the
effects of surface trapping. Hence the transient decays obtained
below the MEG threshold where attributed to biexciton recombi-
nation created by two photons, while in the above MEG threshold
case the decay of the transients is attributed to bi-exciton recom-
bination due to both the absorption of two photons and MEG. A
contribution due to triexciton formation at higher pump fluences
was also observed. The QY of MEG was found to be 1.15 0.01
when the sample was photo-excited with a photon energy 2.6
times the band gap; this result found to be consistent with a
previous study based on an alternative technique.
m
m
[42] and Si [43] QDs both have hmp,th ꢃ 2Eg and QY = 2.0 at h
m
However, these QDs were formed in a silica matrix and so are not
solution processable like colloidal QDs. Thus, the CdHgTe QDs stud-
ied here combine a solar-suitable band gap and competitive MEG
efficiency with solution-phase processing.
The rise of the bleach signal corresponds to the cooling of hot
excitons to the band edge, and is shown in Fig. 6 for hmp = 1.4Eg,
2.6Eg and 2.8Eg. The time taken for the signal to rise from zero to
its maximum value was measured to be 1.1 0.1 ps, 4.1 0.1 ps
and 5.1 0.2 ps for these pump photon energies, respectively. An
energy of hmp ꢀ Eg is lost by each initially photogenerated hot exci-
ton during this process and so these cooling times correspond to a
rate of energy of loss of 0.43 0.04 eV psꢀ1, 0.46 0.01 eV psꢀ1
,
Author contributions
and 0.42 2 eV psꢀ1, respectively. MEG is an additional process by
which the hot exciton loses energy and so would be expected to
increase the rate of energy loss. The highest rate is observed for
the value of hmp corresponding to the highest QY, although the
MAL and AlO performed the experiments and analyzed the
data; SVK, ALR, OZ and SK provided the materials; SM and DM
provided the TEM image; MAL and DJB wrote the paper.

30
M.A. Leontiadou et al. / Chemical Physics 469-470 (2016) 25–30
[13] S. ten Cate, C.S.S. Sandeep, Y. Liu, M. Law, S. Kinge, A.J. Houtepen, J.M. Schins, L.
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