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Directed emission of CdSe nanoplatelets originating from strongly anisotropic 2D electronic structure

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Intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy-efficient display technology. Here, we propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic, directed photoluminescence. Analysis of the two-dimensional k-space distribution reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters.
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Directed emission of CdSe nanoplatelets originating
from strongly anisotropic 2D electronic structure
Riccardo Scott1,JanHeckmann
1, Anatol V. Prudnikau2, Artsiom Antanovich2, Aleksandr Mikhailov2,
Nina Owschimikow1, Mikhail Artemyev2,JuanI.Climente
3, Ulrike Woggon1, Nicolai B. Grosse1
and Alexander W. Achtstein1*
Intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy-
efcient display technology. Here, we propose a new route to overcome intrinsic efciency limitations in light-emitting
devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic, directed photoluminescence.
Analysis of the two-dimensional k-space distribution reveals the underlying internal transition dipole distribution. The
observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition
dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further
enhanced by the optical local density of states and local elds. In contrast to the emission directionality, the off-resonant
absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the
oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of
semiconductor-based emitters.
Lightmatter interaction and emission characteristics in
semiconductors are mediated by the transition dipole moment
(TDM). This is determined by the Bloch functions (states)
associated with the bands involved in an optical transition and
the envelope function dening the selection rules1. In theory, the
properties of optical TDMs in two-dimensional (2D) materials
such as IIVI nanoplatelets2,3, nanobelts4and 2D transition-metal
dichalcogenides5can be highly anisotropic3,616, analogous to
those in molecules. Until now, however, very few reports support
this hypothesis apart from one study16 on 2D MoS
2
. We present a
combined experimental and theoretical approach to obtain quantitat-
ive information about the Bloch functions of 2D materials. Using
CdSe nanoplatelets we demonstrate that, in contrast to a resonantly
excited molecular system17, the absorption is due to an isotropic tran-
sition dipole distribution, whereas the heavy hole exciton in CdSe
platelets has a highly anisotropic transition dipole distribution
forming a bright plane that coincides with the platelet plane. Using
2D k-space spectroscopy, we show the correlation between the direc-
tional external radiation pattern of an oriented sample of 2D CdSe
nanoplatelets and the anisotropic intrinsic TDM distribution and
thus the underlying Bloch functions. Our theoretical analysis shows
that the semiconductor Bloch functions are strongly anisotropic and
hence directional for heavy hole excitons, and isotropic in the conti-
nuum where heavy hole (hh), light hole (lh) and the split-off (so)
bands contribute. This sets the platelets apart from systems in
which an intrinsically isotropic emission is reshaped only by the
dielectric contrast and the external photon density of states18.
Beyond intrinsic electronic effects, the anisotropic shape of the
nanoplatelets induces dielectric effects, further concentratingthe intrin-
sically enhanced emission perpendicular to the platelet. This is a funda-
mental advantage over (elongated) wurtzite CdSe quantum dots (QDs),
where the bright plane does not coincide with the dielectrically favoured
axis1922. Therefore, CdSe nanoplatelets can serve as efcient directional
emitters for photonic applications such as display technology.
2D k-space spectroscopy
Figure 1a shows the concept of our experiment, performed at room
temperature. An in-plane (xy) oriented monolayer of zincblende
(ZB) CdSe nanoplatelets with random azimuthal orientation was
deposited on fused-silica substrates by a Langmuir technique (see
Methods). Emitters placed in the sample plane (xyplane) may
have out-of-plane (OP) z-oriented dipoles and in-plane (IP)
dipoles. The sample was introduced into an inverted microscope-
like set-up, yielding a three-layer system consisting of substrate/
(ligands and nanocrystals)/air. This set-up allowed an angle- and
subsequently k-vector-dependent excitation and detection of the
photoluminescence (PL) signal. The k
x
and k
y
projections contain
information about the orientation of the dipoles (Fig. 1a). A mono-
layer of randomly oriented CdSe dots (ZB structure) with a similar
emission energy to the platelets was used as an isotropically
absorbing and emitting reference ensemble.
Figure 1b,c shows the results of the k-vector resolved emission of
oriented CdSe nanoplatelets and a CdSe QD reference (both excited
at normal incidence). The measured intensity proles of the k
x
and
k
y
projections are plotted next to the charge-coupled device images;
dashed lines indicate the selected regions (cuts). The k-scale is
normalized to the wavevector in air (k
0
=ω/c). Radiation with a wave-
vector |k
x,y
|>k
0
cannot couple into air, resulting in an increase in the
reected intensity collected by the objective for these k-vectors.
The k-dependent emission prole is considerably different for
platelets and dots, and can be related to a different fraction of IP
and OP transition dipoles. The s-polarization projections (k
y
cut)
originate only from IP dipoles; their intensity prole depends
only on the dielectric function and the thickness of the nanocrystal
monolayer (Supplementary Section D). We concentrate on cuts in
the k
x
direction, as only p-polarization I(kout
x) contains signal
from IP as well as OP dipoles. Also, to address IP and OP dipoles
in our 2D k-space spectroscopy, the excitation beam was p-polarized
and we scanned the excitation k-vectors along kin
x.
1Institute of Optics and Atomic Physics, Technical University of Berlin, Strasse des 17. Juni 135, Berlin 10623, Germany. 2Research Institute for Physical
Chemical Problems of Belarusian State University, Minsk 220006, Belarus. 3Departament de Química Física i Analítica, Universitat Jaume I, Castelló de la
Plana E-12080, Spain. These authors contributed equally to this work. *e-mail: achtstein@tu-berlin.de
ARTICLES
PUBLISHED ONLINE: 18 SEPTEMBER 2017 | DOI: 10.1038/NNANO.2017.177
NATURE NANOTECHNOLOGY | VOL 12 | DECEMBER 2017 | www.nature.com/naturenanotechnology 1155
© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
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