Anatol Prudnikau

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.

CdSe-CdS nanoheteroplatelets with efficient photoexcitation of central CdSe region through epitaxially grown CdS wings.

We synthesized a new type of optically active semiconductor nanoheterostructure based on CdSe nanoplatelets with epitaxially grown CdS flat branches or wings. CdS branches work as efficient photonic antenna in the blue spectral region, enhancing the excitation of CdSe band edge emission. The formation of CdSe-CdS nanoheteroplatelets instead of CdSe/CdS core-shell nanoplatelets was achieved using short-chain Cd ethylhexanoate and sulfur in octadecene as precursors for CdS overgrowth in the presence of acetate salt.

Optically and Electrically Controlled Circularly Polarized Emission from Cholesteric Liquid Crystal Materials Doped with Semiconductor Quantum Dots

Novel types of electro- and photoactive quantum dot-doped cholesteric materials have been engineered. UV-irradiation or electric field application allows one to control the degree of circular polarization and intensity of fluorescence emission by prepared quantum dot-doped liquid crystal films.

p-State luminescence in CdSe nanoplatelets : role of lateral confinement and a longitudinal optical phonon bottleneck

We evidence excited state emission from p states well below ground state saturation in CdSe nanoplatelets. Size-dependent exciton ground and excited state energies and population dynamics are determined by four independent methods: time-resolved PL, time-integrated PL, rate equation modeling, and Hartree renormalized k·p calculations-all in very good agreement. The ground state-excited state energy spacing strongly increases with the lateral platelet quantization. Depending on its detuning to the LO phonon energy, the PL decay of CdSe platelets is governed by a size tunable LO phonon bottleneck, related to the low exciton-phonon coupling, very large oscillator strength, and energy spacing of both states. This is, for instance, ideal to tune lasing properties. CdSe platelets are perfectly suited to control the exciton-phonon interaction by changing their lateral size while the optical transition energy is determined by their thickness.

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

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.

Determination of Concentration of Amphiphilic Polymer Molecules on the Surface of Encapsulated Semiconductor Nanocrystals

We present a method for the determination of the average number of polymer molecules on the surface of A(II)B(VI) luminescent core-shell nanocrystals (CdSe/ZnS, ZnSe/ZnS quantum dots, and CdS/ZnS nanorods) encapsulated with amphiphilic polymer. Poly(maleic anhydride-alt-1-tetradecene) (PMAT) was quantitatively labeled with amino-derivative of fluorescein and the average amount of PMAT molecules per single nanocrystal was determined using optical absorption of the dye in the visible spectral range. The average amount of PMAT molecules grows linearly with the surface area of all studied nanocrystals. However, the surface density of the monomer units increases nonlinearly with the surface area, because of the increased competition between PMAT molecules for Zn-hexanethiol surface binding sites. The average value of zeta potential (ζ = -35 mV) was found to be independent of the size, shape, and chemical composition of nanocrystals at fixed buffer parameters (carbonate-bicarbonate buffer, pH 9.5 and 5 mM ionic strength). This finding is expected to be useful for the determination of the surface density of remaining carboxyl groups in PMAT-encapsulated nanocrystals.

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.

Multilayers of CdSe/CdS/ZnCdS Core/Wings/Shell Nanoplatelets Integrated in a Polymer Waveguide

In this work, fabrication of multilayers of colloidal CdSe/CdS/ZnCdS core/wings/shell nanoplatelets (NPls) on solid substrates by layer-by-layer deposition technology is successfully demonstrated for the first time. Integration of multilayers of nanoplatelets into polymeric waveguide resulted in strong dichroism in absorption and polarization effects in photoluminescence with preferable absorption and emission of TE (horizontal) mode that evidences about in-plane orientation of NPls relative to the planar waveguide. The propagation of the excitation and emission light along the waveguide was thoroughly investigated both theoretically and experimentally for different number of NPl layers.

Anisotropy of Structure and Optical Properties of Self-Assembled and Oriented Colloidal CdSe Nanoplatelets

Abstract Atomically flat colloidal nanoplatelets with strong one-dimensional confinement represent the most recently discovered type of quantum-confined cadmium chalcogenide nanocrystals. In almost a decade a great progress has been achieved in the colloidal synthesis of nanoplatelets and understanding of their basic optical and electronic properties. However, up until recently methods of their controlled spatial orientation were quite scarce, what in turn hindered obtaining reliable information on their anisotropic structural and optical properties arising from their shape. In this paper we provide a mini-review of recent advances in this field of study.

Time-resolved stark spectroscopy in CdSe nanoplatelets: A route to field controlled emitters

We study [1] the application potential of CdSe nanoplatelets (NPLs) [2], a model system for colloidal 2D materials, as field-controlled emitters and their properties. We show that their luminescence emission can be modulated by 28% upon application of electrical fields up to 175 kV/cm. This is a very high modulation depth for field-controlled nanoemitters. Based on our experimental results we estimate the exciton binding energy in 5.5 monolayer CdSe nanoplatelets to be Eb = 170 meV. Therefore CdSe NPLs exhibit highly robust excitons being stable even at room temperature. This allows to tune the emission and recombination dynamics efficiently by external electric fields.