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 k$\cdot$p 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⁽²⁾ of 10⁴ to 10⁵ 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⁽²⁾ 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>_B = 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^–29 Cm) and its polarizability, which determine the radiative rate, as well as the (static) exciton polarizability (8.6 × 10^–8 eV cm²/kV²), 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