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Comparative Study Demonstrates Strong Size Tunability of Carrier-Phonon Coupling in CdSe-Based 2D and 0D Nanocrystals


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In a comparative study we investigate the carrier-phonon coupling in CdSe based core-only and hetero 2D as well as 0D nanoparticles. We demonstrate that the coupling can be strongly tuned by the lateral size of nanoplatelets, while, due to the weak lateral confinement, the transition energies are only altered by tens of meV. Our analysis shows that an increase of the lateral platelet area results in a strong decrease of the phonon coupling to acoustic modes due to deformation potential interaction, yielding an exciton deformation potential of 3.0 eV in line with theory. In contrast, the coupling to optical modes tends to increase with platelet area. This cannot be explained by Froehlich interaction, which is generally dominant in II-VI materials. We compare CdSe/CdS nanoplatelets to their equivalent, spherical CdSe/CdS nanoparticles. Universally, in both systems the introduction of a CdS shell is shown to result in an increase of the average phonon coupling, mainly related to an increase of the coupling to acoustic modes, while the coupling to optical modes is reduced with increasing CdS layer thickness. The demonstrated size and CdS overgrowth tunability has strong implications for applications like tuning carrier cooling and carrier multiplication -- relevant for solar energy harvesting applications. Other implications range from transport in nanosystems e.g. for fieldeffect transistors or dephasing control. Our results open up a new toolbox for the design of photonic materials.
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Cite this: Nanoscale, 2019, 11, 3958
Received 22nd November 2018,
Accepted 28th January 2019
DOI: 10.1039/c8nr09458f
A comparative study demonstrates strong size
tunability of carrierphonon coupling in
CdSe-based 2D and 0D nanocrystals
Riccardo Scott,
Anatol V. Prudnikau, §
Artsiom Antanovich,
Sotirios Christodoulou,
Thomas Riedl,
Guillaume H. V. Bertrand,
Nina Owschimikow,
Jörg K. N. Lindner,
Zeger Hens,
Iwan Moreels,
Mikhail Artemyev,
Ulrike Woggon
and Alexander W. Achtstein *
In a comparative study we investigate the carrierphonon coupling in CdSe based core-only and hetero
2D as well as 0D nanoparticles. We demonstrate that the coupling can be strongly tuned by the lateral
size of nanoplatelets, while, due to the weak lateral connement, the transition energies are only altered
by tens of meV. Our analysis shows that an increase in the lateral platelet area results in a strong decrease
in the phonon coupling to acoustic modes due to deformation potential interaction, yielding an exciton
deformation potential of 3.0 eV in line with theory. In contrast, coupling to optical modes tends to
increase with the platelet area. This cannot be explained by Fröhlich interaction, which is generally domi-
nant in IIVI materials. We compare CdSe/CdS nanoplatelets with their equivalent, spherical CdSe/CdS
nanoparticles. Universally, in both systems the introduction of a CdS shell is shown to result in an increase
of the average phonon coupling, mainly related to an increase of the coupling to acoustic modes, while
the coupling to optical modes is reduced with increasing CdS layer thickness. The demonstrated size and
CdS overgrowth tunability has strong implications for applications like tuning carrier cooling and carrier
multiplication relevant for solar energy harvesting applications. Other implications range from transport
in nanosystems e.g. for eld eect transistors or dephasing control. Our results open up a new toolbox
for the design of photonic materials.
Optoelectronic properties of semiconductor nanoparticles
attract increasing interest because of their promising appli-
cation potential. Particularly 2D semiconductors in the form
of nanoplatelets and sheets
receive growing attention due to
fast radiative lifetimes
related to strong exciton correlation
and the giant oscillator strength eect
allowing high
quantum yields,
promising lasing properties
and high
two-photon absorption.
Furthermore, their directed emis-
and polarization
and well-width dependent high
darkbright splitting
of 36 meV are of direct interest.
High exciton binding energies (of >100 meV) have been pre-
dicted and measured,
confirming the presence of
robust excitons even at room temperature. First predictions
have suggested strong tunability of the emission spectra and
decay times for platelets and their heterostructures.
Applications of these nanoparticles for ecient field eect
or strong electro-absorption response
been demonstrated.
In recent years, nanoplatelet heterostructures have attracted
growing attention due to various properties, which can be
engineered in these heterostructures.
On the other
hand the opto-electronic properties of core/shell quantum
dots are still an active research field.
First indications have
been found that nanoplatelets can exhibit unusually small
excitonphonon coupling.
The authors have investigated the
Electronic supplementary information (ESI) available. See DOI: 10.1039/
These authors contributed equally to this work.
§Current address: Physical Chemistry, TU Dresden, Bergstraße 66b, 01062
Dresden, Germany.
Institute of Optics and Atomic Physics, Technical University of Berlin,
Strasse des 17. Juni 135, 10623 Berlin, Germany. E-mail:
Research Institute for Physical Chemical Problems of Belarusian State University,
220006 Minsk, Belarus
ICFO-Institut de Ciencies Fotoniques, 08860 Castelldefels, Barcelona, Spain
Department of Physics, Paderborn University, Warburger Strasse 100,
33098 Paderborn, Germany
CEA Saclay, 91191 Gif-sur-Yvette, France
Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
Department of Chemistry, Ghent University, Krijgslaan 281 S3, 9000 Gent,
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temperature-dependent recombination dynamics, bandgap
and quantum yield in 2D CdSeCdS coreshell platelets,
which exhibit a strong thickness dependence. A detailed, com-
parative study of the (lateral) size and shape dependence of
excitonphonon coupling in dierent structures like 0D dots
and 2D platelets with and without a shell is still missing.
Therefore, we investigate in this manuscript at first the size
dependent excitonphonon coupling in core-only CdSe nano-
platelets. Furthermore, we compare the results with CdSe/CdS
core/shell nanoplatelets, as well as topologically similar CdSe/
CdS heterostructures in the form of CdSe/CdS core/shell
quantum dots based on an analysis of the temperature depen-
dent emission line width and bandgap renormalization. We
show that the excitonphonon coupling of these structures can
be tuned over about an order of magnitude by varying the
lateral size of core only platelets or introducing a CdS shell
atop of CdSe quantum dots or platelets, where we compare 0D
and 2D systems.
4.5 monolayer (ML) zinc blende (ZB) CdSe nanoplatelet samples
with dierent average lateral sizes of 17 × 6 nm
to 41 × 13 nm
and 4.5 monolayer (ML) thickness were synthesized according
to ref. 3 and 44 (see Methods) and characterized by trans-
mission electron microscopy (TEM). To produce CdSe/CdS core/
shell nanoplatelets CdSe core-only nanoplatelets with 17 ×
11 nm
size were coated with 1 to 3 ML CdS by a layer by layer
growth technique,
see Methods. The final lateral size of CdSe-
3 ML CdS core/shell platelets was 22 × 18 nm
. The samples
were embedded in a poly(laurylmethacrylate-co-methyl-
methacrylate) co-polymer on fused silica substrates. Zinc-blende
CdSe 4 nm (diam.) core and 25 ML CdS shell quantum dots
were synthesized according to ref. 40 and 46 and deposited in
the PBMA polymer on quartz substrates. The samples were
mounted in a CryoVac Micro cryostat. A 150 fs, 75.4 MHz rep-
etition rate Ti:Sa laser at 420 nm was used for confocal exci-
Fig. 1 PL emission of (a) 4.5 ML core-only platelets with varying lateral sizes, (b) CdSe 1 to 3 ML (CdS) CdSe/CdS core/shell nanoplatelets, and (c)
CdSe/CdS core/shell quantum dots at 4 K and 180 or 200 K as well as representative TEM images. Clearly a splitting can be observed for the core-
only nanoplatelets (a) at low temperatures in line with ref. 7. PL from core-only platelets is tted with two Voigt proles, and core/shell particles with
a modied Gaussian (black lines atop experimental curves).
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tation with a 0.4 NA objective (0.2 W cm
CW equivalent exci-
tation density) for the nanoplatelets and a 442 nm He:Cd laser
for the core/shell dot samples with comparable excitation
density. Time integrated detection was realized by using a
Horiba IHR550 spectrometer with an attached LN
cooled CCD.
Results and discussion
Fig. 1 shows temperature dependent PL spectra for two exemp-
lary temperatures of (a) 4.5 ML ZB CdSe core-only platelets with
varying lateral sizes, (b) CdSe-nML CdS (n=0,1,2,3)ZBnano-
platelets (17 × 11 nm
core size) and (c) of 4 nm (diameter) ZB
CdSe core and 2, 3, and 5 ML CdS shell CdSeCdS dots.
The increasing redshift with shell thickness (at fixed tem-
perature) in (b) and (c) can be attributed to a lowering of the
confinement related to lower conduction and valence band
osets resulting in increasing delocalization of the CdSe core
exciton into the CdS shell of the heterostructures. In CdSe
CdS coreshell nanoplatelets it has been shown that still type I
band alignment is maintained,
so that our observed redshift
here is not an eect of strongly dierent spatial electron and
hole localizations, but the band alignment.
CdSe core-only nanoplatelets
We first concentrate on the CdSe core-only nanoplatelets. In
line with recent results
we observe for the core-only platelets a
double emission with 2040 meV energy spacing. Fig. 2(a)
shows the temperature dependence of the centers of the
double emission, obtained from Voigt fits (shown in Fig. 1) in
the range of 4 to 300 K. We refer to the lower energy peak as
the ground state (GS) and the higher one as the excited state
(ES), without any presumptive attribution to the nature of the
states. We would like to point out here that their origin
excited and ground state excitons, an exciton plus trion or an
exciton plus an LO phonon replica is still under debate.
However, this does not aect our bandgap analysis, since both
states show the same temperature dependence of the bandgap
(Fig. 2a). Hence, we concentrate on the results for the energeti-
cally higher emission for the core-only sample. Furthermore,
studying the emission dynamics of CdSe nanoplatelets,
Biadala et al.
recently demonstrated a darkbright state fine
structure with energy splittings of about 5 meV. Even for the
presence of a higher bright and a lower dark state, Shornikova
et al.
have shown that the quasi dark exciton state does not
contribute relevantly to the PL emission above 10 K resulting
in no relevant impact on our measurements.
An initial observation for the nanoplatelets (Fig. 2a) is the
steeper redshift (high temperature slope) for increasing lateral
platelet size (area). At first we concentrate on the lateral size
dependence of the temperature-dependent bandgap shift and
exciton phonon coupling. Fits to a semi-empirical model for
the bandgap shift are given by
Emax ¼E0aep 1þ2
where a
an excitonphonon coupling strength and θ=ω/
an average phonon temperature are indicated as solid lines
in Fig. 2(a). The results for the zero temperature bandgap E
and θare displayed in Fig. 2(b)(d).
Using eqn (1) allows the analysis of the impact of lateral
confinement on the zero temperature excitonic bandgap E
CdSe nanoplatelets at low temperature as shown in Fig. 2(b).
Starting from the approximate expression for the confinement
energy related bandgap shift ΔEof an infinitely deep semi-
conductor quantum box in x,yand transversal zdirection the
observed bandgap is
Eg¼Eg;bulk EBþnz2π22
where E
is the bulk bandgap, E
is the exciton binding
energy, and μis the reduced exciton mass. M
is the exciton
mass and n
is the quantum number in the x,y,zdirection.
We note that NPLs are strongly confined in the z-direction
Fig. 2 (a) Temperature dependent excitonic bandgap of 4.5 ML core-
only nanoplatelets with dierent lateral sizes obtained from the Voigt
ts in Fig. 1. Solid lines are ts according to eqn (1) in order to obtain: (b)
lateral connement part of the zero temperature excitonic bandgap
versus (1/L
+ (1/L
with L
the lateral platelet dimensions. The inset
shows the same data plotted against (n/L
+ (1/L
, with n= 1 for the
GS and n= 2 for the ES. (c) The excitonphonon coupling a
and (d)
the average phonon temperature θ.
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(quantization of relative motion), while weakly confined in in-
plane directions
(quantization of center of mass motion).
Under the assumption of a lowest electron and hole state tran-
sition n
= 1 in all directions and taking the x-direction as
the long side of the rectangular platelet, the first excited state
would be (n
) = (2,1,1) and the ground state (1,1,1) (see
also ESIfor a detailed discussion). The exciton binding
energy depends only weakly on the lateral size
and can be
assumed to be constant. The z-quantization energy is also con-
stant for fixed thickness as well as the bulk bandgap E
A plot of dE=E
versus (1/L
+ (1/L
ing (1,1,1) for the upper and lower emission states from Fig. 1
is shown in Fig. 2. A linear dependence is fitted in this plot
= 0.98). The slope π
corresponds to the quasi-par-
ticle (e.g. exciton) mass. The dierent slopes for the upper and
lower emission indicate dierent masses of the emitting
species, inferring dierent species or ground and excited
states. A plot under the assumption of ground state (GS) (1,1,1)
and excited state (ES) (2,1,1) excitons leads to less good corre-
lation (R
= 0.94) for a fit to the shifts vs. (n/L
+ (1/L
and is
shown in the inset of Fig. 2(b). However, a definite answer to
which of the two models applies cannot be given based on the
lateral confinement. As shown in the ESI,comparing M
the literature values of the eective carrier masses in ZB CdSe
for excitons, positive and negative trions and biexcitons and
the ES and GS hypothesis also do not result in a final attribu-
tion. One possible scenario for the double emission discussed
based on the displayed results here is an exciton and biexciton
Fig. 2(b) also allows us to extrapolate the energy dierence
of the two states for laterally infinite platelets or sheets as the
dierence of the intersection points of the fit curves with the
ordinate. The extrapolation to laterally infinite platelets with
vanishing lateral confinement results in an energy spacing of
17 meV. From the two fit curves in Fig. 2(b) the area dependent
energy spacing can be given as
ΔE½eV¼1:600 102þ0:652 ð1=Lx2þ1=Ly2Þ;ð3Þ
if the corresponding lengths are given in nm.
Fig. 2(c) shows the electronphonon coupling parameter a
obtained from the temperature-dependence of the bandgap
according to eqn (1). For both, the ES and GS, a
tends to
increase linearly with the platelet area -with some scattering of
the data, perhaps due to not identical sample quality. A linear
fit to all data (with non-zero oset) yields a slope of 77 ±
15 μeV nm
A possible explanation for this linear increase is the coher-
ent delocalization of the 2D excitons over the whole nanoplate-
lets. The number of unit cell dipole moments contributing to
the exciton transition dipole moment increases with the plate-
let area via the giant oscillator strength eect (GOST).
elementary dipole moments coherently add up to the tran-
sition dipole moment of the exciton, which then couples to
the phonon modes, leading to a linear increase of the coupling
strength with the platelet area. Clear indications for this coher-
ent delocalization over the whole nanoplatelet and the result-
ing increase of the transition rates in our size range of nano-
platelets have been found in ref. 8 and 10. In particular the
scaling of the transition rates with the platelet area leads to a
quadratic area (volume) scaling of the two photon absorption
cross section with the lateral platelet size as well as a constant
intrinsic absorption in the continuum. We remark that these
eects of coherent delocalization are observed, as the nanopla-
telets are atomically flat systems
due to their anisotropic col-
loidal synthesis. In contrast epitaxial quantum wells suer e.g.
from interface roughness eects, which limit the spatial coher-
ence of excitons more strongly.
We remark that the trend of increased excitonphonon
coupling is also observed, if a
is converted in the dimension-
less HuangRhys parameter S, often used to quantify the
strength of phonon coupling, and that the resulting values are
comparable to laterally quasi infinite 2D transition dichalco-
genide materials
(see the ESI).
The linear increase of a
shown in Fig. 2(c) allows for a
control of electronphonon interaction and the accompanied
temperature-dependent bandgap red shift with the platelet
area. Additionally, it directly implies control over dephasing in
the system, as discussed later. Furthermore, this degree of
freedom is practically independent of the emission energy of
the system the bandgap shifts are less than 30 meV and
small compared to the 2.5 eV band gap that is controlled by
the plateletsthickness.
Fig. 2(d) shows the average phonon temperature θ.It
corresponds to a weighted average phonon energy of acoustic
and optical modes. The observed increase of θwith lateral
size may be related to alterations in the average coupling to
acoustic and optical phonons. It can be shown (later) that the
coupling to acoustic phonons in 2D systems scales with 1/A
(Abeing the platelet area), so that the contribution of acous-
tic modes is reduced with increasing platelet size. At the
same time the contribution of optical modes increases. Since
these exhibit energies higher than acoustic modes, the
average phonon energy (temperature) increases with lateral
CdSe/CdS core/shell nanoplatelets and dots
In the following section we concentrate on the CdSe/CdS
core/shell nanoplatelet and quantum dot samples. Fig. 3(a)
and (b) compare the temperature dependent excitonic band
gaps E
deduced from fits to the PL (Fig. 1) and fitted with
eqn (1). The CdSe/CdS platelet values presented here for
comparison are in agreement with those recently obtained
on CdSe/CdS NPLS.
Fig. 3(c) and (d) show the resulting
excitonphonon coupling a
, and (e) and ( f ) the average
phonon temperatures θ.
As seen in the case of core-only platelets in Fig. 2(c), the
excitonphonon interaction a
is linear in the platelet area A.
On the other hand, the density of states in a 2D system is pro-
portional to 1/d(inverse thickness) and alters the interaction
strength. For this reason, we plot in Fig. 3(c) the interaction
strength per CdSe nanoplatelet area versus 1/d
, the CdS
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layer thickness. The slope is equivalent to plotting a
surface area (2A)versus the inverse total CdS thickness of
. For quantum dots shown in Fig. 3(d) we use the inter-
face area given by 4π(d/2)
, with the core diameter d.We
observe a linear dependence with a slope of 4.1 ± 0.1 μeV nm
for the core/shell platelets and 40 ± 6 μeV nm
for the core/
shell dots.
Thus in addition to the lateral size a CdS shell also
allows for controlling the excitonphonon interaction in CdSe
nanoplatelets and dots, in line with the first indications in
ref. 12 and 42 in PL and Raman spectroscopy. Interestingly,
the same concept applies to topologically similar core/shell
quantum dots, which can be considered as spherical 2D CdSe/
CdS interface structures, where the interaction is proportional
to the interaction (interface) area. A decreasing coupling with
CdS shell thickness is consistent with the results of Lin
et al.,
who observed in Raman measurements that the
HuangRhys factor for LO-phonon coupling tended to decrease
upon introduction of a shell. There, a model assuming a
Fröhlich-only mechanism was not able to reproduce this
trend. They oer surface charging and related S- and P-type
wavefunction mixing as a possible mechanism for increasing
the eh separation and counteracting the Fröhlich coupling
trend. Finally, considering the slopes in Fig. 3(c) and (d), the
excitonphonon interaction per unit area in CdSe/CdS nano-
platelets is lower than that in CdSe/CdS dots. We remark that
as the platelets exhibit type I band alignment,
there are no
eects of spatially indirect exciton formation, which alters the
coupling to phonons. For CdSeCdS dots type I band align-
ment is also suggested,
but is still under discussion.
The performed renormalization by the surface area is also
substantiated by the results of Takagahara et al.
for spherical
CdSe core-only quantum dots. They found a 1/r
dence for coupling in dots, where ris the crystallite radius. In
Fig. 3(e) and (f ), we further observe a reduction of the average
phonon temperature with increasing CdS thickness. While the
starting point for core-only samples is comparable for platelets
and dots, the decrease is faster for platelets. This dierence
can be attributed to the dierent phonon densities of states
for the 2D system and the spherical CdSe/CdS hetero system,
so that a dierent scaling of the coupling to acoustic and
optical modes may be the reason for these findings. As for the
core-only platelets we recalculated from Fig. 3(c)(f) the
HuangRhys parameter S(see Fig. S1 in the ESI) and found
that Sdecreases with the CdS thickness in line with Fig. 3(c)
and (d).
Based on an analysis of the emission line width, we will
now investigate the acoustic and optical phonon coupling in
more detail.
Fig. 4(a) shows the temperature dependent linewidth (FWHM)
obtained from the log-normal function fits to the PL spectra in
Fig. 1. We fit the temperature dependence with the well known
where Δ
is the sum of the (temperature independent) inhomo-
geneous and zero-temperature linewidth. Δ
is the coupling
to acoustic and Δ
the coupling to LO phonons, with the LO
phonon energy E
= 25.4 meV.
(See also the ESIfor the dis-
cussion of line width related eects.)
The results for core-only nanoplatelets are shown in
Fig. 4(b) and (c). While there is a clear trend of a reduction of
with the platelet area, only a by trend increase is observed
for Δ
. Both trends are in line with the results in Fig. 2(d),
where with increasing lateral size LO phonons provide increas-
ingly major contribution to the average phonon temperature. θ
approaches the LO phonon temperature of 300 K (equal
to 25.4 meV) for large platelets. The demonstrated strong area
scaling of the acoustic phonon interaction can be understood
from the underlying theory of deformation potential inter-
of acoustic phonons. According to Fermis golden
rule the transition rate Γis proportional to the absolute value
squared of transition matrix element. The eective defor-
mation potential coupling u
Fig. 3 Temperature dependent excitonic bandgaps deduced from ts
to PL in Fig. 1 for 4.5 ML core/shell nanoplatelets (a) and 4 nm diameter
CdSe core/shell dots (b) with 03 ML and 05 ML CdS shell, respect-
ively. Excitonphonon coupling a
per interface area for the platelets
(c) and dots (d) versus the inverse CdS thickness. For the platelets the
system has two interfaces as well as their average phonon temperature θ
(e) + (f ) determined from ts in (a) and (b).
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the phonon wave number q(absolute value of the wave vector),
the volume Ωof the structure under consideration, the density
ρ, the sound velocity v
and the deformation potential dier-
ence of D
of electrons and holes. The corresponding
matrix element is M
f(q) is the form factor that can be shown
to be (1 + (qa
for a hydrogen (S-)like lowest envelope function and small
acoustic phonon wave vectors, as estimated from ref. 7 and 60.
Due to Fermis golden rule the transition rate Γscales with
the square of u
. In our platelets with dimensions L
volume Ω=L
, the quantized phonon wave vector is q=
. Following ref. 61 and 59, and assum-
ing small acoustic phonon wave vectors (as e.g. also shown in
ref. 62), the third contribution is the dominant one and is
given by the z-breathing mode frequency (taken from ref. 60)
since L
. The contribution of acoustic phonon scatter-
ing to the homogeneous line width is Δ
according to eqn (4) Δ
T, this leads, apart from a
scaling factor C
, to the following dependency
ΔAC ¼CDP2Lx2þLy2þLz2
where the last line is an approximation for L
and fixed
well width L
. Hence, we can understand the decrease of the
acoustic phonon coupling with increasing platelet area as
shown in Fig. 4(b). A fit to the function above is indicated.
is a material dependent propor-
tionality constant, resulting from the considerations discussed
above. Using an eective exciton mass from ref. 7, a ZB CdSe
sound velocity of v
= 2.26 km s
an acoustic phonon
energy of E
= 3.42 meV calculated
from the sound velocity,
a platelet thickness of 4.5 ML and a mass density of ρ= 5.66
we obtain from the fit to eqn (5) |D
| = 3.03 ±
0.3 eV. This value is in excellent agreement with a value of
2.87 eV calculated from elastic constants based on DFT
and results on ZB CdSe.
It further shows that the
deformation potential is unaltered by the strong anisotropic
size quantization in CdSe nanoplatelets. The agreement of the
deformation potentials directly justifies our approach, the val-
idity of our fits to the emission lines and the assumptions of
our model. It proves that we demonstrate for the first time an
independent control of the acoustic phonon coupling or
dephasing from the transition energy in a nanosystem. As the
homogeneous line-width is shown to be acoustic phonon scat-
tering limited, we obtain a direct control over dephasing in
our system via the lateral platelet area. A tuning range of more
than a factor of four with increasing values for small platelets
Fig. 4 Temperature dependent line width of CdSe platelets (a), CdSe/CdS nanoplatelets (17 × 11 nm
CdSe core) (d) and nano-dots with 4 nm core
size (g), with ts to eqn (4). Resulting acoustic Δ
(b) and LO phonon Δ
(c) contribution to the linewidth as a function of the nanoplatelet area. (b)
shows a 1/At according to eqn (5). (e, h) and (f, i) acoustic Δ
and LO phonon Δ
contribution for core/shell platelets and dots versus the CdS
layer thickness.
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gives great prospects that for smaller platelets even higher
values can be obtained.
Slight deviations of the actual platelets from the fit curve in
Fig. 4(b) may be related to slight variation of sample quality or
size determination by TEM.
We remark that the observed trend of a decrease of acoustic
phonon coupling with nanostructure size is in-line with calcu-
lations by Kelley
and Takagahara et al.
for spherical CdSe
quantum dots. With increasing size the exciton coupling to
acoustic phonons was observed to decrease due to decreasing
deformation potential coupling. Our results for large platelets
are comparable to large dots, for which Takagahara et al.
found the coupling to be about 7 μeV K
. For small quantum
dots, of diameter comparable to the thickness of our 4.5 ML
platelets (1.4 nm), they obtained 100 μeV K
in good agree-
ment with the values of smaller platelets as shown in Fig. 4(b).
We also note that they found a 1/r
dependence for dots,
where ris the crystallite radius. This corresponds to an inverse
surface area dependence of the nanocrystal analogous to our
findings for the nanoplatelets (1/A).
This inverse relationship between coupling to low energy
acoustic modes and platelet area Afurther substantiates our
interpretation of the increase of the average phonon temperature
θas shown in Fig. 2(d) with A. For larger lateral sizes the contri-
bution of low energy acoustic modes decreases due to the
reduced coupling while optical modes with higher phonon ener-
gies increasingly contribute to the average phonon temperature.
In Fig. 4(c) we further observe a by trend increase of the
coupling to LO-phonons with increasing nanoplatelet area.
The coupling to optical modes tends to increase with Aand
then saturate for large platelet areas towards values
reported by Chia et al. for laterally infinite MBE grown CdSe
epilayers (20 meV).
As the Fröhlich interaction is con-
sidered the dominating mechanism
for the LO phonon
coupling in polar semiconductor nano-structures we briefly
investigate whether this can explain the observed complicated
behavior directly. The corresponding interaction potential
results in a predicted
scaling Δ
where the first proportionality is an approximation due to
and the last proportionality due to L
for fixed
well width L
. A nonlinear increase for larger platelet areas is
expected for more quadratic nanoplatelets. The observed
scatter of data points may be related to the lateral aspect ratio
sensitivity (via the (L
dependence), as the used platelets
have varying aspect ratios. However, the actual trend in Fig. 4(c)
cannot be fully explained by this simple theory. Another expla-
nation for deviations from the simple scaling theory presented
could be a varying defect density which alters the LO phonon
coupling, as e.g. the number of defect sites scales with the
platelet area. A detailed analysis, however, is beyond the scope
of this article.
In the following we analyze the cases of CdSe/CdS platelets
and dots for varying CdS layer thicknesses. Fig. 4(e) and (f )
show the coupling to acoustic and optical modes. Δ
can be
(roughly) approximated with a linear increase with the CdS
layer thickness. The 1 ML CdS sample is excluded from the
linear fit shown in Fig. 3(e). The trend which is in line with
the reduction of the average phonon temperature in Fig. 3(c + d)
via increasing coupling to low energy acoustic phonons may
be interpreted in terms of an increase of the interaction
volume with increasing shell thickness. In bulk the acoustic
branches of the phonon dispersion in CdSe and CdS are quite
similar and have a continuous spectrum. Here we have quan-
tized acoustic modes, but the energy spectrum is still quite
similar in both materials, which can be seen from the fact that
the acoustic sound velocities 2.3 km s
and 2.6 km s
CdSe and CdS (calculated based on ref. 60) are quite similar.
Hence there is no strong mode confinement due to the CdSe
CdS interface and to a first approximation the acoustic modes
can be considered as to extend over the whole particle, includ-
ing the CdS shell. In contrast these arguments do not apply to
the optical modes, as the energies of LO phonons in CdSe
(210 cm
) and CdS (302 cm
) are well distinct. Hence, an
increased shell thickness and wavefunction delocalization may
tend to decrease the LO coupling as the core (LO) phonon
modes cannot couple to the shell modes and the wavefunction
overlap of the exciton with the core mode decreases relatively
for stronger delocalization into thicker CdS shells.
A reduction of the coupling to LO phonons is further
observed in (f ). Lin et al.
measured a non Fröhlich-like
reduction of the coupling (HuangRhys factor S) with increas-
ing shell thickness in CdSe/CdS quantum dots in-line with
our result. This seems to be a universal trend in hetero-nano-
platelets and quantum dots. Furthermore, the decreasing
coupling to high energy LO-phonons with increasing layer
thickness is also reflected in the reduction of the average
phonon temperature θas seen in Fig. 3(e) and (f). This also
confirms the consistency of our whole analysis via the inde-
pendent experimental quantities, bandgap and line width.
The same trends of acoustic and optical phonon
coupling with CdS shell thickness discussed now for platelets
(Fig. 4(e + f )) are also observed for the topologically similar
CdSe/CdS dots. On an absolute scale, they show smaller coup-
ling than nanoplatelets. This may stem from smaller transition
dipole moments in dots as compared to platelets coupling to
phonons, or a not perfect spherical shape of the nanoparticles.
The dots are smaller than the nanoplatelets. This may result in
a smaller number of acoustic or LO-phonon modes in the
system to which the exciton can couple, leading to an overall
reduction in CdSe/CdS dots compared to platelets.
Furthermore, due to dierent crystallographic orientations on
the dot surface, CdS monolayers are not as ideally smooth as
on the platelets, which may result e.g. in the larger data point
The demonstration of the tuneability of excitonphonon
interaction in colloidal quantum-wells by lateral size and shell
growth has strong application potential, for example, for
tuning carrier cooling, which is relevant e.g. in solar energy
harvesting applications. Furthermore, carrier multiplication is
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a very active field, where carrier cooling and carrier multipli-
cation are competing processes. The former tunes the multi-
carrier pair generation eciencies and poses a promising way
to boost solar cell eciencies. Hence 2D materials of finite
lateral size, like the nanoplatelets in this manuscript, with a
control of the carrier cooling rates are desirable model
systems, for both basic understanding of the involved pro-
cesses as well as optimization of the performance of solar
energy converters. Also for transport in nanosystems our
results show the feasibility of tuning excitonphonon inter-
action, for example in field-eect transistors, varying the
lateral size of the structure and e.g. tuning from ballistic to
diusive transport. A further interesting implication of our
results is that the quantum yield of nanoplatelets with or
without a shell, which has been found to be considerably
will depend on the phonon interaction, which
governs exciton cooling and relaxation in non-radiative chan-
nels. For large platelets or core/shell platelets with thin shells
the average phonon coupling is the highest, so that e.g.
phonon assisted nonradiative recombination is stronger.
These findings can be used for emitters of optimized photo-
luminescence eciency. Our results open up a new toolbox for
the design of photonic materials.
In summary we have shown that CdSe nanoplatelets possess a
lateral confinement of 30 meV, which can be adjusted via the
lateral size of the nanoplatelets. We demonstrated that
excitonphonon coupling in CdSe nanoplatelets can be varied
strongly via the lateral size or the thickness of a CdS shell. The
coupling to acoustic phonons decreases with the platelet area
by a factor of 4, in line with deformation potential theory.
The resulting 3.03 eV excitonacoustic phonon deformation
potential is in line with theory predictions. The coupling to
LO-phonons is observed to decrease with decreasing platelet
area. We further demonstrated that a CdS shell results in an
increased coupling to acoustic modes and reduced coupling to
optical modes for both CdSeCdS coreshell platelets and
quantum dots. The absolute coupling strengths are lower for
quantum dots than for nanoplatelets, which may be attributed
to the higher transition dipole moments to which the phonon
modes couple in nanoplatelets and more phonon modes to
couple to in the larger nanoplatelets. We further demonstrated
that the average excitonphonon coupling strength increases
linearly with the platelet area for core-only platelets, while it is
linear in the interface area versus the inverse CdS layer thick-
ness in core/shell platelets as well as in nanodots. As the
trends for both core/shell structures are similar, there is a uni-
versal mechanism for 2D platelets and similar coreshell dots
also exhibiting a 2D interface. Our results regarding the tun-
ability of phonon interaction are of immediate interest for
carrier cooling and multiexciton generation, relevant for solar
light conversion, for transport and high quantum yield emit-
ters and open up a new toolbox for the design of photonic
materials. We remark that further in-depth studies beyond the
scope of this article are necessary to give a definite answer to
the nature of the two emissive states in CdSe nanoplatelets,
however our experimental results presented in this paper are
not influenced by this.
Synthesis and characterization
Nanoplatelets. 4.5 ML-thick CdSe NPls were prepared using a
modified procedure based on the protocols described in ref. 3
and 44. 170 mg of cadmium myristate, 12 mg of elemental sel-
enium and 15 ml of octadecene-1 were charged into a three-
necked flask. The flask was degassed for 15 minutes, purged with
of cadmium acetate dihydrate and 21 mg of anhydrous cadmium
acetate was swiftly added into the reaction mixture. The ratio of
anhydrous/hydrate allows tuning the aspect ratio. When the tem-
perature reached 240 °C, the flask was kept at this temperature
for 45 s to 2 min. After that, the reaction mixture was cooled to
80 °C and nanocrystals were precipitated with isopropanol and
hexane and isolated by centrifugation. The cadmium sulfide shell
was deposited onto CdSe NPL cores by a colloidal atomic layer
deposition technique.
At first, 2 ml of hexane colloidal solution
of CdSe NPls were mixed with 2 ml of N-methylformamide
(NMF). Then 10 μL of 40 wt% ammonium sulfide aqueous solu-
tion was added and NPls were transferred into the NMF phase
due to the growth of a sulfide surface layer. After 10 minutes of
vigorous mixing the hexane layer was discarded and then, in
order to remove the excess of sulfur precursor, the NPLs were pre-
cipitated with isopropanol, isolated by centrifugation and redis-
persed in fresh NMF. To ensure complete removal of the
unreacted sulfide the precipitation step was repeated twice. In
order to complete the deposition of 1 ML of CdS by growing a
cadmium layer, 20 μLof0.1Msolutionofcadmiumacetatein
water were added to the solution of sulfide-covered NPLs in NMF.
After 20 minutes of mixing the NPLs were purified from the
excess of the cadmium precursor by the precipitation procedure
described above. NPLs were then transferred into hexane by
adding 10 μL of oleic acid and thorough mixing. Second and
third layers were grown by repeating the steps described above.
Zinc-blende CdSe core-only and 25 ML CdS shell quantum
dots. These were synthesized according to ref. 40 and 46 based
on 4 nm CdSe cores and deposited in PBMA polymers on
quartz substrates. Sizes and diameters were determined by
TEM in all samples.
TEM analysis of all samples was performed through
dierent instruments (Jeol ARM200F operated at 200 kV and
Zeiss Leo 906E at 120 kV). At least 50 platelets per sample were
used for size determination.
Conicts of interest
There are no conflicts to declare.
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R. S., U. W. and A. W. A acknowledge the DFG grants WO477-1/
32 and AC290-2/1., M. A. the CHEMREAGENTS program, and
A. A. the BRFFI grant no. X17KIG-004.
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... Studies on thermal line broadening of CdSe NPL emission are typically limited to measurements up to room temperature. 15,16 However, temperatures relevant for application in lamps and displays are much higher, even exceeding 150°C in high power w-LEDs. At elevated temperatures, homogeneous broadening due to exciton− phonon coupling dominates over inhomogeneous broadening. ...
... 4 Most studies related to application of semiconductor NPLs and QDs as spectral converters in w-LEDs rely on measurements at room temperature. 15,38 However, for on-chip application luminescent materials in w-LEDs, temperatures as high as 150°C are reached. It is therefore important to consider spectral line widths and temperature-induced shifts at 150°C for QDs and NPLs and compare these with traditional phosphors based on microcrystalline insulator materials doped with Eu 2+ , Ce 3+ , or Mn 4+ . ...
Full-text available
Nanoplatelets (NPLs) of CdSe are an emerging class of luminescent materials, combining tunable and narrow emission bands with high quantum yields. This is promising for application in white light LEDs (w-LEDs) and displays. The origin of the narrow spectral width of exciton emission in core NPL compared to core-shell NPL and quantum dot (QD) emission is not fully understood. Here we investigate and compare temperature dependent emission spectra of core and core-shell CdSe NPLs and QDs. A wide temperature range, 4 to 423 K, is chosen to gain insight in contributions from ho-mogeneous and inhomogeneous broadening and also to extend measurements into a temperature regime that is relevant for operating conditions in w-LEDs (T∼423 K). The results show that temperature induced homogeneous broadening does not strongly vary between the various CdSe nanostructures (ΔEhom ≈ 60-80 meV at 423 K) indicating that electron-phonon coupling strengths are similar. Only for the smallest QDs stronger coupling is observed. The origin of the narrow bandwidth reported at 300 K for core CdSe NPLs is attributed to a very narrow inhomogeneous linewidth. At 423 K the spectral width of NPL exciton emission is still superior to that of QDs. A comparison with traditional w-LED phosphors is made to outline advantages (tunability, narrow band-width, high efficiency) and disadvantages (color shift, stability issues) of NPLs for application in w-LEDs.
... The finite lateral size of NPLs then also affects the COM energy of excitons, as discussed by Richter [15]. The presence of these states has also been invoked to explain the exciton dynamics probed by transient resonant four-wave mixing [3] and transient PL spectra at temperatures below 200 K [25,26]. The latter exhibits two PL peaks with a lateral size-dependent energy difference of tens of millielectronvolts. ...
... The exponential factor is due to the classical Maxwell-Boltzmann (MB) distribution over thermalized COM motional states. The broadening of each of these states is modelled by the Voigt distribution [23,25], ...
We show that the finite lateral sizes of ultrathin CdSe nanoplatelets strongly affect both their photoluminescence and optical absorption spectra. This is in contrast to the situation in quantum wells, in which the large lateral sizes may be assumed to be infinite. The lateral sizes of the nanoplatelets are varied over a range of a few to tens of nanometers. For these sizes excitons experience in-plane quantum confinement, and their center-of-mass motion becomes quantized. Our direct experimental observation of the discretization of the exciton center-of-mass states can be well understood on the basis of the simple particle-in-a-box model.
... meV/°C reported for the emission of 4.5 ML NPLs. 30,31 The intensity of the NPL absorption in Figure 2D increases over time. In addition, the absorption maximum shifts noticeably between the first few displayed spectra (light blue to green), more than expected from temperature effects. ...
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The growth of two-dimensional platelets of the CdX family (X = S, Se, or Te) in an organic solvent requires the presence of both long- and short-chain ligands. This results in nanoplatelets of atomically precise thickness and long-chain ligand-stabilized Cd top and bottom surfaces. The platelets show a bright and spectrally pure luminescence. Despite the enormous interest in CdX platelets for optoelectronics, the growth mechanism is not fully understood. Riedinger et al. studied the reaction without a solvent and showed the favorable role for short-chain carboxylates for growth in two dimensions. Their model, based on the total energy of island nucleation, shows favored side facet growth versus growth on the top and bottom surfaces. However, several aspects of the synthesis under realistic conditions are not yet understood: Why are both short- and long-chain ligands required to obtain platelets? Why does the synthesis result in both isotropic nanocrystals and platelets? At which stage of the reaction is there bifurcation between isotropic and 2D growth? Here, we report an in situ study of the CdSe nanoplatelet reaction under practical synthesis conditions. We show that without short-chain ligands, both isotropic and mini-nanoplatelets form in the early stage of the process. However, most remaining precursors are consumed in isotropic growth. Addition of acetate induces a dramatic shift toward nearly exclusive 2D growth of already existing mini-nanoplatelets. Hence, although myristate stabilizes mini-nanoplatelets, mature nanoplatelets only grow by a subtle interplay between myristate and acetate, the latter catalyzes fast lateral growth of the side facets of the mini-nanoplatelets.
We investigate the lateral size tunability of the exciton diffusion coefficient and mobility in colloidal quantum wells by means of line width analysis and theoretical modeling. We show that the exciton diffusion coefficient and mobility in laterally finite 2D systems like CdSe nanoplatelets can be tuned via the lateral size and aspect ratio. The coupling to acoustic and optical phonons can be altered via the lateral size and aspect ratio of the platelets. Subsequently the exciton diffusion and mobility become tunable since these phonon scattering processes determine and limit the mobility. At 4 K the exciton mobility increases from ∼ 4 × 10³ cm² V⁻¹ s⁻¹ to more than 1.4 × 10⁴ cm² V⁻¹ s⁻¹ for large platelets, while there are weaker changes with size and the mobility is around 8 × 10¹ cm² V⁻¹ s⁻¹ for large platelets at room temperature. In turn at 4 K the exciton diffusion coefficient increases with the lateral size from ∼ 1.3 cm² s⁻¹ to ∼ 5 cm² s⁻¹, while it is around half the value for large platelets at room temperature. Our experimental results are in good agreement with theoretical modeling, showing a lateral size and aspect ratio dependence. The findings open up the possibility for materials with tunable exciton mobility, diffusion or emission line width, but quasi constant transition energy. High exciton mobility is desirable e.g. for solar cells and allows efficient excitation harvesting and extraction.
We show that the finite lateral sizes of ultrathin CdSe nanoplatelets strongly affect both their photolumi-nescence and optical absorption spectra. This is in contrast to the situation in quantum wells, in which thelarge lateral sizes may be assumed to be infinite. The lateral sizes of the nanoplatelets are varied over a rangeof a few to tens of nanometers. For these sizes excitons experience in-plane quantum confinement, and theircenter-of-mass motion becomes quantized. Our direct experimental observation of the discretization of theexciton center-of-mass states can be well understood on the basis of the simple particle-in-a-box model
Type-II heterostructures are key elementary components in optoelectronic, photovoltaics and quantum devices. The staggered band alignment of materials leads to the stabilization of indirects excitons (IXs) i.e. correlated electron-hole pairs experiencing spatial separation with novel properties, boosting optical gain and promoting strategies for the design of information storage, charge separation or qubit manipulation devices. Planar colloidal CdSe/CdTe core-crown type-II nested structures, grown as nanoplatelets (NPLs) are the focus of the present work. By combining low temperature single NPL measurements and electronic structure calculations we gain insights into the mechanisms impacting the emission properties. We are able to probe the sensitivity of the elementary excitations (IXs, trions) with respect to the appropriate structural parameter (core size). Neutral IXs, with binding energies reaching 50 meV, are shown to dominate the highly structured single NPL emission. The large broadening linewidth that persists at the single NPL level clearly results from strong exciton-LO phonon coupling (Eph = 21 meV) whose strength is poorly influenced by trapped charges. The spectral jumps (≈ 10 meV) in the photoluminescence recorded as a function of time are explained by the fluctuations in the IX electrostatic environment considering fractional variations (≈ 0.2 e) of the non compensated charge defects.
We present a theoretical study combined with experimental validations demonstrating that CdSe nanoplatelets are a model system to investigate the tuneability of trions and excitons in laterally finite 2D semiconductors. Our results show that the trion binding energy can be tuned from 36 meV to 18 meV with lateral size and decreasing aspect ratio, while the oscillator strength ratio of trion to exciton decreases. In contrast to conventional quantum dots the trion oscillator strength in a nanoplatelet at low temperature is smaller than that of the exciton. The trion and exciton Bohr radii become lateral size tunable, e.g. from ~3.5 to 4.8 nm for the trion. We show that dielectric screening has strong impact on these properties. By theoretical modeling of transition energies, binding energies and oscillator strength of trion and and exciton and comparison to experimental findings we demonstrate that these properties are lateral size and aspect ratio tunable and can be engineered by the dielectric confinement, allowing to suppress e.g. detrimental trion emission in devices. Our results strongly impact further in-depth studies, as the demonstrated lateral size tunable trion and exciton manifold is expected to influence properties like gain mechanisms, lasing, quantum efficiency and transport even at room temperature due to the high and tunable trion binding energies.
We show theoretically that carriers confined in semiconductor colloidal nanoplatelets (NPLs) sense the presence of neighbor, cofacially stacked NPLs in their energy spectrum. When approaching identical NPLs, the otherwise degenerate energy levels redshift and split, forming (for large stacks) minibands of several meV width. Unlike in epitaxial structures, the molecular behavior does not result from quantum tunneling but from changes in the dielectric confinement. The associated excitonic absorption spectrum shows a rich structure of bright and dark states, whose optical activity and multiplicity can be understood from reflection symmetry and Coulomb tunneling. We predict spectroscopic signatures which should confirm the formation of molecular states, whose practical realization would pave the way to the development of nanocrystal chemistry based on NPLs.
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Current colloidal synthesis methods for CdSe nanoplatelets (NPLs) routinely yield samples that emit –in discrete steps– from 460 nm to 550 nm. A significant challenge lies with obtaining thicker NPLs, to further widen the emission range. This is at present typically achieved via colloidal atomic layer deposition onto CdSe cores, or by synthesizing NPL core/shells structures. Here we demonstrate a novel reaction scheme, where we start from 4.5 monolayer (ML) NPLs and increase the thickness in a two-step reaction that switches from 2D to 3D growth. The key feature is the enhancement of the growth rate of basal facets by the addition of CdCl2, resulting a series of nearly monodisperse CdSe NPLs with thicknesses between 5.5 ML and 8.5 ML. Optical characterization yielded emission peaks from 554 nm up to 625 nm, with a line width (fwhm) of 9-13 nm, making them one of the narrowest colloidal nanocrystal emitters currently available in this spectral range. The NPLs maintained a fast emission lifetime of 5-11 ns. Finally, due to the increased red shift of the NPL band edge, photoluminescence excitation spectra revealed several high-energy peaks. Calculation of the NPL band structure allowed us to identify these excited-state transitions, and spectral shifts are consistent with a significant mixing of light and split-off hole states. Clearly, chloride ions can add a new degree of freedom to the growth of 2D colloidal nanocrystals, yielding new insights into the both the NPL synthesis as well as their opto-electronic properties.
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We study experimentally and theoretically the exciton-phonon interaction in MoSe2 monolayers encapsulated in hexagonal BN, which has an important impact on both optical absorption and emission processes. The exciton transition linewidth down to 1 meV at low temperatures makes it possible to observe high-energy tails in absorption and emission extending over several meV, not masked by inhomogeneous broadening. We develop an analytical theory of the exciton-phonon interaction accounting for the deformation potential induced by the longitudinal acoustic phonons, which plays an important role in exciton formation. The theory allows fitting absorption and emission spectra and permits estimating the deformation potential in MoSe2 monolayers. We underline the reasons why exciton-phonon coupling is much stronger in two-dimensional transition metal dichalcogenides as compared to conventional quantum well structures. The importance of exciton-phonon interactions is further highlighted by the observation of a multitude of Raman features in the photoluminescence excitation experiments.
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Among the colloidal quantum dots, 2D nanoplatelets present exceptionally narrow optical features. Rationalizing the design of heterostructures of these objects is of utmost interest; however, very little work has been focused on the investigation of their electronic properties. This work is organized into two main parts. In the first part, we use 1D solving of the Schrödinger equation to extract the effective masses for nanoplatelets (NPLs) of CdSe, CdS, and CdTe and the valence band offset for NPL core/shell of CdSe/CdS. In the second part, using the determined parameters, we quantize how the spectra of the CdSe/CdS heterostructure get affected by (i) the application of an electric field and (ii) by the presence of a dull interface. We also propose design strategies to make the heterostructure even more robust.
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The optical properties of atomically thin transition metal dichalcogenide (TMDC) semiconductors are shaped by the emergence of correlated many-body complexes due to strong Coulomb interaction. Exceptional electron-hole exchange predestines TMDCs to study fundamental and applied properties of Coulomb complexes such as valley depolarization of excitons and fine-structure splitting of trions. Biexcitons in these materials are less understood and it has been established only recently that they are spectrally located between exciton and trion. Here we show that biexcitons in monolayer TMDCs exhibit a distinct fine structure on the order of meV due to electron-hole exchange. Ultrafast pump-probe experiments on monolayer WSe$_2$ reveal decisive biexciton signatures and a fine structure in excellent agreement with a microscopic theory. We provide a pathway to access biexciton spectra with unprecedented accuracy, which is valuable beyond the class of TMDCs, and to understand even higher Coulomb complexes under the influence of electron-hole exchange.
CdSe/CdTe core-crown type-II nanoplatelet heterostructures are two-dimensional semiconductors that have attracted interest for use in light-emitting technologies due to their ease of fabrication, outstanding emission yields and tuneable properties. Despite this, the exciton dynamics of these complex materials, and in particular how they are influenced by phonons, is not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL) and temperature-dependent structural measurements to investigate CdSe/CdTe nanoplatelets with a thickness of four monolayers. We show that charge-transfer (CT) excitons across the CdSe/CdTe interface are formed on two distinct timescales: initially from an ultrafast (~70 fs) electron transfer and then on longer timescales (~5 ps) from the diffusion of domain excitons to the interface. We find that the CT excitons are influenced by an interfacial phonon mode at ~120 cm-1 which localizes them to the interface. Using low-temperature photoluminescence (PL) spectroscopy we reveal that this same phonon mode is the dominant mechanism in broadening the CT PL. On cooling to 4 K the total PL quantum yield reaches close to unity, with an ~85 % contribution from CT emission and the remainder from an emissive sub-bandgap state. At room temperature, incomplete diffusion of domain excitons to the interface and scattering between CT excitons and phonons limit the PL quantum yield to ~50%. Our results provide a detailed picture of the nature of exciton-phonon interactions at the interfaces of 2D heterostructures and explain both the broad shape of the CT PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that to maximise the PL quantum yield both improved engineering of the interfacial crystal structure and diffusion of domain excitons to the interface, e.g. by altering the relative core/crown size, are required.
We investigate the impact of shell growth on the carrier dynamics and exciton–phonon coupling in CdSe–CdS core–shell nanoplatelets with varying shell thickness. We observe that the recombination dynamics can be prolonged by more than one order of magnitude, and analyze the results in a global rate model as well as with simulations including strain and excitonic effects. We reveal that type I band alignment in the hetero platelets is maintained at least up to three monolayers of CdS, resulting in approximately constant radiative rates. Hence, observed changes of decay dynamics are not the result of an increasingly different electron and hole exciton wave function delocalization as often assumed, but an increasingly better passivation of nonradiative surface defects by the shell. Based on a global analysis of time-resolved and time-integrated data, we recover and model the temperature dependent quantum yield of these nanostructures and show that CdS shell growth leads to a strong enhancement of the photoluminescence quantum yield. Our results explain, for example, the very high lasing gain observed in CdSe–CdS nanoplatelets due to the type I band alignment that also makes them interesting as solar energy concentrators. Further, we reveal that the exciton-LO-phonon coupling is strongly tunable by the CdS shell thickness, enabling emission line width and coherence length control.
We show that electron-hole correlation can be used to tune interband and intraband optical transition rates in semiconductor nanostructures with at least one weakly confined direction. The valence-to-conduction band transition rate can be enhanced by a factor (L/aB)N -- with L the length of the weakly confined direction, aB the exciton Bohr radius and N the dimensionality of the nanostructure -- while the rate of intraband and inter-valence-band transitions can be slowed down by the inverse factor, (aB/L)N. Adding a hitherto underexplored degree of freedom to engineer excitonic transition rates, this size dependence is of interest for various opto-electronic applications. It also offers an interpretation of the superlinear volume scaling of two-photon absorption (TPA) cross-section recently reported for CdSe nanoplatelets, thus laying foundations to obtain unprecedented TPA cross sections, well above those of conventional two-photon absorbers. Further, our concept explains the background of the validity of the universal continuum absorption approach for the determination of particle concentrations via the intrinsic absorption. Potential applications of our approach include low excitation intensity confocal two-photon imaging, two-photon autocorrelation and cross correlation with much higher sensitivity and unprecedented temporal resolution as well as TPA based optical stabilization and optimizing of inter-subband transition rates in quantum cascade lasers.
We study the temperature-dependent photoluminescence of monolayer and bilayer molybdenum ditelluride in the temperature range between 5 K and room temperature. We disentangle the effects of interactions of excitons with acoustic and optical phonons and show that molybdenum ditelluride excitons have an unusually small coupling with acoustical phonons. This observation, together with the large luminescence yield which can be obtained from the bilayer, puts forward molybdenum ditelluride as a robust and bright light source in the near infrared range. The scaling of luminescence wavelength and linewidth of the molybdenum ditelluride bilayer differs from the observations in the monolayer by effects that can be traced to symmetry and wellwidth. This suggests a similar band alignment of mono- and bilayer, in contrast to other transition metal dichalcogenides.
Band alignments are essential for understanding the optical properties and carrier transfer of core/shell QDs. As CdSe/CdS core/shell QDs with increasing shell thickness represent red-shifted absorption and luminescence spectra, weakened oscillator strength of the lowest electronic transition, elongated luminescence lifetime, they are assigned to quasi-type II band alignment. However, femtosecond transient absorption spectroscopy with state selective excitation revealed a type I band alignment of the CdSe/CdS QDs with thin CdS shell, in which the excited electron is localized in CdSe core with core excitation while delocalized in the whole QDs with shell excitation, even though a quasi-type II carrier distribution was observed with steady-state spectroscopy. In the type I core/shell QDs, CdS shell acts as an energy barrier in surface electron and hole trapping processes. Especially, the time constant of hole trapping process of CdSe core (~10 ps) was elongated ten times owing to tunnel effect through the high energy barrier of CdS shell, which was estimated from the decay related with the biexcitonic induced spectral shift. The biexcitonic spectral shift induced by ~100 ps hole trapping process was also observed at 1S(e)-2S3/2(h) transition. Our results by transient absorption spectroscopy with state selective excitation are useful to clarify band alignment and carrier distribution of heteronanostructures, which could help to objectively extract charge carriers in photovoltaic applications.
We study the influence of surface passivating ligands on optical and structural properties of zinc blende CdSe nanoplatelets. Ligand exchange of native oleic acid with aliphatic thiol or phosphonic acid on the surface of nanoplatelets results in the large shift of the exciton transition for up to 240 meV. Ligand exchange also leads to structural changes (strain) in the nanoplatelet’s core analysed by wide-angle X-ray diffraction. By correlating experimental data with theoretical calculations we demonstrate that the exciton energy shift is mainly caused by a ligand-induced anisotropic transformation of the crystalline structure altering the well width of the CdSe core. Further the exciton reduced mass in these CdSe quantum wells is determined by a new method and agrees well with expected values substantiating that ligand-strain induced changes in the colloidal quantum well thickness are responsible for the observed spectral shifts. Our findings are important for theoretical modeling of other anisotropically strained systems and demonstrate an approach to tune optical properties of 2D semiconductor nanocrystals over a broad region thus widening the range of possible applications of AIIBVI nanoplatelets in optics and optoelectronics.