19 research outputs found
p-State Luminescence in CdSe Nanoplatelets: The Role of Lateral Confinement and an LO Phonon Bottleneck
We report excited state emission from p-states at excitation fluences well
below ground state saturation in CdSe nanoplatelets. Size dependent exciton
ground state-excited state energies and dynamics are determined by three
independent methods, time-resolved photoluminescence (PL), time-integrated PL
and Hartree renormalized kp calculations -- all in very good agreement.
The ground state-excited state energy spacing strongly increases with the
lateral platelet quantization. Our results suggest that the PL decay of CdSe
platelets is governed by an LO-phonon bottleneck, related to the reported low
exciton phonon coupling in CdSe platelets and only observable due to the very
large oscillator strength and energy spacing of both states
A comparative study demonstrates strong size tunability of carrier–phonon coupling in CdSe-based 2D and 0D nanocrystals
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 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 dominant in II–VI 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 field effect transistors or dephasing control. Our results open up a new toolbox for the design of photonic materials.TU Berlin, Open-Access-Mittel - 201
Directed emission of CdSe nanoplatelets originating from strongly anisotropic 2D electronic structure
ntrinsically 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
Linear and Two-Photon Absorption in Zero- and One-Dimensional CdS Nanocrystals: Influence of Size and Shape
Colloidal
CdS nanocrystals show strongly enhanced two-photon absorption
(TPA) cross sections δ<sup>(2)</sup> of 10<sup>4</sup> to 10<sup>5</sup> GM over a wide NIR spectral range, making them ideal markers
for confocal two-photon microscopy. We present a systematic study
of the size and shape dependence of the linear and TPA cross sections
for colloidal CdS dots and rods of diameters <i>d</i> between
2 and 5 nm. On the basis of <i>z</i>-scan measurements,
we observe that the TPA cross section of colloidal CdS dots at 800
nm in and near the absorption continuum is manly shape and volume
dependent, whereas it is less influenced by the electronic confinement.
In the case of resonance to the lowest excitonic transition, a significant
confinement-induced TPA enhancement with respect to the off-resonant
case is observed. Colloidal CdS rods can exhibit a factor on the order
of 10 larger δ<sup>(2)</sup> compared to CdS dots of the same
diameter. In very small CdS rods a non-negligible three-photon absorption
is found and assigned to a change in valence band symmetry
Directed Two-Photon Absorption in CdSe Nanoplatelets Revealed by k‑Space Spectroscopy
We show that two-photon absorption (TPA) is
highly anisotropic in CdSe nanoplatelets, thus promoting them
as a new class of directional two-photon absorbers with large
cross sections. Comparing two-dimensional k-space spectroscopic
measurements of the one-photon and two-photon
excitation of an oriented monolayer of platelets, it is revealed
that TPA into the continuum is a directional phenomenon. This
is in contrast to one-photon absorption. The observed
directional TPA is shown to be related to fundamental band
anisotropies of zincblende CdSe and the ultrastrong anisotropic
confinement. We recover the internal transition dipole
distribution and find that this directionality arises from the intrinsic directionality of the underlying Bloch and envelope
functions of the states involved. We note that the photoemission from the CdSe platelets is highly anisotropic following either
one- or two-photon excitation. Given the directionality and high TPA cross-section of these platelets, they may, for example, find
employment as efficient logic AND elements in integrated photonic devices, or directional photon converters
A strain-induced exciton transition energy shift in CdSe nanoplatelets: The impact of an organic ligand shell
We study the influence of surface passivating ligands on the 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 a large shift of exciton transition energy 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 the experimental data with theoretical calculations we demonstrate that the exciton energy shift is mainly caused by the 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 this agrees well with the 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 the optical properties of 2D semiconductor nanocrystals over a broad region thus widening the range of possible applications of AIIBVI nanoplatelets in optics and optoelectronics.ChemE/Opto-electronic Material
A strain-induced exciton transition energy shift in CdSe nanoplatelets: The impact of an organic ligand shell
We study the influence of surface passivating ligands on the 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 a large shift of exciton transition energy 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 the experimental data with theoretical calculations we demonstrate that the exciton energy shift is mainly caused by the 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 this agrees well with the 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 the optical properties of 2D semiconductor nanocrystals over a broad region thus widening the range of possible applications of AIIBVI nanoplatelets in optics and optoelectronics.ChemE/Opto-electronic Material
Design of cross-linked polyisobutylene matrix for efficient encapsulation of quantum dots
Funding: This work was supported by the EU Horizon 2020 Project MiLEDi (779373) and by the Belarusian Republican Foundation for Fundamental Research (grant X21MC-007). D. I. Shiman, E. Ksendzov, E. A. Bolotina, and S. V. Kostjuk acknowledge the support by the Erasmus+ Traineeship Programme for Higher Education.Photoluminescent quantum dots (QDs) are a prominent example of nanomaterials used in practical applications, especially in light-emitting and light-converting devices. Most of the current applications of QDs require formation of thin films or their incorporation in solid matrices. The choice of an appropriate host material capable of preventing QDs from degradation and developing a process of uniform incorporation of QDs in the matrix have become essential scientific and technological challenges. In this work, we developed a method of uniform incorporation of Cu–Zn–In–S (CZIS) QDs into a highly protective cross-linked polyisobutylene (PIB) matrix with high chemical resistance and low gas permeability. Our approach involves the synthesis of a methacrylate-terminated three-arm star-shaped PIB oligomeric precursor capable of quick formation of a robust 3D polymer network upon exposure to UV-light, as well as the design of a special ligand introducing short PIB chains onto the surface of the QDs, thus providing compatibility with the matrix. The obtained cross-linked QDs-in-polymer composites underwent a complex photostability test in air and under vacuum as well as a chemical stability test. These tests found that CZIS QDs in a cross-linked PIB matrix demonstrated excellent photo- and chemical stability when compared to identical QDs in widely used polyacrylate-based matrices. These results make the composites developed excellent materials for the fabrication of robust, stable and durable transparent light conversion layers.Publisher PDFPeer reviewe
Time-Resolved Stark Spectroscopy in CdSe Nanoplatelets: Exciton Binding Energy, Polarizability, and Field-Dependent Radiative Rates
We present a study of the application
potential of CdSe nanoplatelets
(NPLs), a model system for colloidal 2D materials, as field-controlled
emitters. We demonstrate that their emission can be changed by 28%
upon application of electrical fields up to 175 kV/cm, a very high
modulation depth for field-controlled nanoemitters. From our experimental
results we estimate the exciton binding energy in 5.5 monolayer CdSe
nanoplatelets to be <i>E</i><sub>B</sub> = 170 meV; hence
CdSe NPLs exhibit highly robust excitons which are stable even at
room temperature. This opens up the possibility to tune the emission
and recombination dynamics efficiently by external fields. Our analysis
further allows a quantitative discrimination of spectral changes of
the emission energy and changes in PL intensity related to broadening
of the emission line width as well as changes in the intrinsic radiative
rates which are directly connected to the measured changes in the
PL decay dynamics. With the developed field-dependent population model
treating all occurring field-dependent effects in a global analysis,
we are able to quantify, e.g., the ground state exciton transition
dipole moment (3.0 × 10<sup>–29</sup> Cm) and its polarizability,
which determine the radiative rate, as well as the (static) exciton
polarizability (8.6 × 10<sup>–8</sup> eV cm<sup>2</sup>/kV<sup>2</sup>), all in good agreement with theory. Our results
show that an efficient field control over the exciton recombination
dynamics, emission line width, and emission energy in these nanoparticles
is feasible and opens up application potential as field-controlled
emitters
Electronic Structure and Exciton–Phonon Interaction in Two-Dimensional Colloidal CdSe Nanosheets
We study the electronic structure of ultrathin zinc-blende
two-dimensional
(2D)-CdSe nanosheets both theoretically, by Hartree-renormalized k·p
calculations including Coulomb interaction, and experimentally, by
temperature-dependent and time-resolved photoluminescence measurements.
The observed 2D-heavy hole exciton states show a strong influence
of vertical confinement and dielectric screening. A very weak coupling
to phonons results in a low phonon-contribution to the homogeneous
line-broadening. The 2D-nanosheets exhibit much narrower ensemble
absorption and emission linewidths as compared to the best colloidal
CdSe nanocrystallites ensembles. Since those nanoplatelets can be
easily stacked and tend to roll up as they are large, we see a way
to form new types of multiple quantum wells and II–VI nanotubes,
for example, for fluorescence markers