Polymer-II-VI Nanocrystals Blends: Basic Physics and Device Applications to Lasers and LEDs

Raman study of colloidal Cu2ZnSnS4 nanocrystals obtained by "green" synthesis modified by seed nanocrystals or extra cations in the solution

The method of affordable colloidal synthesis of nanocrystalline Cu₂ZnSnS₄ (CZTS) is developed, which is suitable for obtaining bare CZTS nanocrystals (NCs), cation substituted CZTS NCs, and CZTS-based hetero-NCs. For the hetero-NCs, the synthesized in advance NCs of another material are introduced into the reaction solution so that the formation of CZTS takes place preferably on these "seed" NCs. Raman spectroscopy is used as the primary method of structural characterization of the NCs in this work because it is very sensitive to the CZTS structure and allows to probe NCs both in solutions and films. Raman data are corroborated by optical absorption measurements and transmission electron microscopy on selected samples. The CdTe and Ag NCs are found to be good seed NCs, resulting in a comparable or even better quality of the CZTS compound compared to bare CZTS NCs. For Au NCs, on the contrary, no hetero-NCs could be obtained under the given condition. Partial substitution of Zn for Ba during the synthesis of bare CZTS NCs results in a superior structural quality of NCs, while the introduction of Ag for partial substitution of Cu deteriorates the structural quality of the NCs.

Special Issue "II-VI Semiconductor Nanocrystals and Hybrid Polymer-Nanocrystal Systems"

The continuous need to improve the performance of photonic, electronic and optoelectronic devices has stimulated research toward the development of innovative semiconducting materials which display better properties with respect to standard bulk semiconductors [...].

Investigation of the exciton relaxation processes in poly(9,9-dioctylfluorene-co-benzothiadiazole):CsPbI1.5Br1.5 nanocrystal hybrid polymer–perovskite nanocrystal blend

The combination of lead halide perovskite nanocrystals and conjugated polymer in a blend film opens the way to the realization of hybrid active layers with widely tunable optical and electrical properties. However, the interaction between the polymeric and the perovskite component of the blends is mainly unexplored to date. In this work we perform temperature-dependent photoluminescence and time resolved photoluminescence measurements in order to deeply investigate the photophysics of a poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT):CsPbI_1.5Br_1.5 nanocrystal hybrid film. Our results suggest that the primary interaction channel is charge transfer, both from F8BT to the NCs and from the NCs to F8BT, while Förster resonant energy transfer has no visible effects. Moreover, we show that the charge transfer is assisted by energy migration within the F8BT excited state distribution and that it is dependent on the local micromorphology of the film. Our work improves the current understanding of the polymer:perovskite NC interactions in hybrid films, and it is expected to be relevant for the development of hybrid organic–perovskite optoelectronic devices.

The emission properties of a hybrid polymer:perovskite nanocrystals (NCs) blend film are investigated, evidencing that the main interaction process is not Förster transfer, but instead bidirectional polymer → NC and NC → polymer charge transfer.

Photoinduced Enhancement of Photoluminescence of Colloidal II-VI Nanocrystals in Polymer Matrices

The environment strongly affects both the fundamental physical properties of semiconductor nanocrystals (NCs) and their functionality. Embedding NCs in polymer matrices is an efficient way to create a desirable NC environment needed for tailoring the NC properties and protecting NCs from adverse environmental factors. Luminescent NCs in optically transparent polymers have been investigated due to their perspective applications in photonics and bio-imaging. Here, we report on the manifestations of photo-induced enhancement of photoluminescence (PL) of aqueous colloidal NCs embedded in water-soluble polymers. Based on the comparison of results obtained on bare and core/shell NCs, NCs of different compounds (CdSe, CdTe, ZnO) as well as different embedding polymers, we conclude on the most probable mechanism of the photoenhancement for these sorts of systems. Contrary to photoenhancement observed earlier as a result of surface photocorrosion, we do not observe any change in peak position and width of the excitonic PL. Therefore, we suggest that the saturation of trap states by accumulated photo-excited charges plays a key role in the observed enhancement of the radiative recombination. This suggestion is supported by the unique temperature dependence of the trap PL band as well as by power-dependent PL measurement.

Narrowband Spontaneous Emission Amplification from a Conjugated Oligomer Thin Film

In this paper, we studied the laser and optical properties of conjugated oligomer (CO) 1,4-bis(9-ethyl-3-carbazo-vinylene)-9,9-dihexyl-fluorene (BECV-DHF) thin films, which were cast onto a quartz substrate using a spin coating technique. BECV-DHF was dissolved in chloroform at different concentrations to produce thin films with various thicknesses. The obtained results from the absorption spectrum revealed one sharp peak at 403 nm and two broads at 375 and 428 nm. The photoluminescence (PL) spectra were recorded for different thin films made from different concentrations of the oligomer solution. The threshold, laser-induced fluorescence (LIF), and amplified spontaneous emission (ASE) properties of the CO BECV-DHF thin films were studied in detail. The ASE spectrum was achieved at approximately 482.5 nm at a suitable concentration and sufficient pump energy. The time-resolved spectroscopy of the BECV-DHF films was demonstrated at different pump energies.

Deep Blue Light Amplification from a Novel Triphenylamine Functionalized Fluorene Thin Film

The development of high performance optically pumped organic lasers operating in the deep blue still remains a big challenge. In this paper, we have investigated the photophysics and the optical gain characteristics of a novel fluorene oligomer functionalized by four triphenylamine (TPA) groups. By ultrafast spectroscopy we found a large gain spectral region from 420 to 500 nm with a maximum gain cross-section of 1.5 × 10⁻¹⁶ cm² which makes this molecule a good candidate for photonic applications. Amplified Spontaneous Emission measurements (ASE) under 150 fs and 3 ns pump pulses have revealed a narrow emission at 450 nm with a threshold of 5.5 μJcm⁻² and 21 μJcm⁻² respectively. Our results evidence that this new fluorene molecule is an interesting material for photonic applications, indeed the inclusion of TPA as a lateral substituent leads to a high gain and consequently to a low threshold blue organic ASE.