M. S. Sokolikova

Organic light-emitting diode with an emitter based on a planar layer of CdSe semiconductor nanoplatelets

Colloidal CdSe semiconductor nanoplatelets with characteristic longitudinal sizes of 20–70 nm and thicknesses of several atomic layers are synthesized. The spectra and kinetics of the photoluminescence of these quasi-two-dimensional nanostructures (quantum wells) at room and cryogenic temperatures are investigated. A hybrid light-emitting diode with the electron and hole transport layers based on TAZ and TPD organic compounds, respectively, and the active “emissive” element based on a layer of such single-component nanoplatelets is designed. The spectral and electrical characteristics of the fabricated device, emitting at a wavelength of λ = 515 nm, are determined. The use of quasi-two-dimensional nanostructures of this kind (nanoplatelets) is promising for the fabrication of hybrid light-emitting diodes with pure colors.

Synthesis of quasi-two-dimensional colloidal cadmium selenide nanoparticles and formation of sulfide monolayer on their surfaces

We have carried out a solution synthesis of quasi-two-dimensional CdSe nanoparticles (nanoplates) using oleic acid as a stabilizer. Three populations of CdSe nanoplates with different thicknesses quantified by one CdSe monolayer were obtained. The absorption spectra of CdSe nanoplates exhibit narrow excitonic bands (with bandwidths of ∼60 meV), whose positions are determined by the nanoplate thickness and are 461, 503, and 551 nm for the three populations. We have systematically studied how formation of sulfide monolayer on the surface of CdSe nanoplates affects their optical properties. The red shift and significant broadening of absorption bands was shown.

Electroluminescence of colloidal quasi-two-dimensional semiconducting CdSe nanostructures in a hybrid light-emitting diode

We report on the results of studying quasi-two-dimensional nanostructures synthesized here in the form of semiconducting CdSe nanoplatelets with a characteristic longitudinal size of 20–70 nm and a thick-ness of a few atomic layers. Their morphology is studied using TEM and AFM and X-ray diffraction analysis; the crystal structure and sizes are determined. At room and cryogenic temperatures, the spectra and kinetics of the photoluminescence of such structures (quantum wells) are investigated. A hybrid light-emitting diode operating on the basis of CdSe nanoplatelets as a plane active element (emitter) is developed using the organic materials TAZ and TPD to form electron and hole transport layers, respectively. The spectral and current-voltage characteristics of the constructed device with a radiation wavelength λ = 515 nm are obtained. The device triggering voltage is 5.5 V (visible glow). The use of quasi-two-dimensional structures of this type is promising for hybrid light-emitting diodes with pure color and low operating voltages.

Synthesis and optical properties of quasi-2D CdSxSe1 − x nanoparticles

Colloidal 2D CdSxSe1 − x nanoparticles have been synthesized by a solution method in octadecene using oleic acid as a stabilizer. Growth of quasi-2D nanoparticles has been promoted by the presence of cadmium acetate in the reaction mixture. The resulting nanoparticles are platelets with lateral sizes 20–30 nm. The absorption and luminescence spectra of these nanoparticles show narrow bands of lh-e and hh-e exciton transitions corresponding to 2D systems. The spectral position of the lowest energy hh-e transition monotonically changes within 382–461 nm with a change in the composition of nanoparticles. The observed absorption bands are broader than those for the individual CdSe and CdS nanoparticles. The suggested method makes it possible to vary the exciton band position for quasi-2D nanoparticles by changing their composition.

Synthesis, morphology, and optical properties of colloidal CdTe/CdSe and CdTe/CdS nanoheterostructures based on CdTe tetrapods

We report the synthesis of colloidal CdTe/CdSe and CdTe/CdS nanoheterostructures based on CdTe tetrapods with a CdSe or CdS shell. The shell growth takes place on the lateral faces of the tetrapod legs, whereas the leg length remains essentially unchanged. Both the CdSe and CdS shell growth shifts the luminescence of the nanoheterostructures to the near-IR range, up to 850 nm, with a quantum yield of up to 20%. Analysis of the kinetics of the shift of the excitonic absorption band during nanoheterostructure growth suggests that the growth rate of CdS exceeds that of CdSe. The variation of the luminescence wavelength with shell thickness is compared to numerical modeling results.

Density of surface states in colloidal CdSe nanoplatelets

With the use of a novel thermoluminescence technique, a new type of surface traps is observed in colloidal CdSe nanocrystals of planar geometry (nanoplatelets). The emptying of these traps, which cannot be detected by means of the conventional thermally stimulated luminescence scheme, proceeds via a cascade process involving their own excited states. An energy-level diagram of nanoplatelets taking into account the specific characteristics of these traps is proposed and the density of surface states, featuring two peaks at energies of 100 and 280 meV, is determined.

Extinction and luminescence coefficients of CdSe/CdTe, CdTe/CdSe, and CdTe/CdS heterostructures based on colloidal CdSe and CdTe nanocrystals

This paper discusses the optical properties of colloidal CdSe/CdTe, CdTe/CdSe, and CdTe/CdS heterostructures synthesized by growing a shell on CdSe and CdTe nanocrystals. The extinction coefficients of the first exciton absorption band and the extinction coefficients at 400 nm are determined as a function of the amount of shell material added during the growth of the heterostructures. It is shown that the resulting heterostructures have extinction coefficients 1–2 orders of magnitude greater than the initial cores: up to 0.3×107cm−1M−1 in the case of CdTe/CdS and CdTe/CdSe and up to 1.6×107cm−1M−1 in the case of CdSe/CdTe. It is shown that the luminescence wavelength in the 550–950-nm range can be controlled by varying the shell size for CdSe/CdTe heterostructures.

Strain effects on optical properties of tetrapod-shaped CdTe/CdS core–shell nanocrystals

Abstract The exciton states of strained CdTe/CdS core–shell tetrapod-shaped nanocrystals were theoretically investigated by the numerical diagonalization of a configuration interaction Hamiltonian based on the single-band effective mass approximation. We found that the inclusion of strain promotes the type-II nature by confining the electrons and holes in nonadjacent regions. This carrier separation is more efficient than that with type-II spherical nanocrystals. The strain leads to a small blue shift of the lowest exciton energy. Its magnitude is smaller than the red shift induced by increasing the shell thickness. Moreover, the larger arm diameter reduces the influence of both shell thickness and strain on the exciton energy. Considering the broken symmetry, a randomness induced carrier separation was revealed which is unique to a branched core–shell structure. The calculated shell thickness dependence of the emission energy agrees well with available experimental data.