Dielectrically enhanced excitons in semiconductor-insulator quantum wires: Theory and experiment

We present both theoretical and experimental investigations of optical properties of excitons in semiconductor–insulator quantum wires. Spectra of linear and nonlinear absorption, photoluminescence and its polarization, photoluminescence excitation, time-resolved photoluminescence of GaAs, CdSe, and InP quantum wires 4–6 nm in diameter, crystallized in dielectric matrix, demonstrate the prominent excitonic behavior. In these structures an essential difference of dielectric constants of constituent materials leads to a considerable enhancement of excitons, the binding energies ranging from 120 meV to 260 meV and exciton transitions being well distinguished in nanowires with sufficient dispersion of diameter even at room temperature. A theoretical approach to calculations of the exciton parameters in a semiconductor–insulator cylindrical quantum wire of finite diameter is developed. This approach accounts for a band-gap renormalization due to the spatial confinement and self-image effect, as well as for a dielectric enhancement of the electron-hole interaction. The calculated exciton transition energies and absorption spectra are consistent with the experimental results

Optical properties of excitons in semiconductor quantum wires

Spectra of luminescence and linear and nonlinear absorption of semiconductor quantum wires crystallized in a transparent dielectric matrix have been investigated and interpreted in terms of excitonic transitions and filling of the exciton phase space. The calculated energies of excitonic transitions are in qualitative agreement with experimental data. The estimated values of exciton binding energies (>100 meV) in semiconductor quantum wires embedded in dielectric are a factor of several tens higher than in bulk semiconductors. The cause of this increase in the exciton's binding energy is not only dimensional quantization, but also the enhancement of Coulomb interaction, i.e. stronger attraction between electrons and holes owing to the large difference between dielectric constants of semiconductor filament and dielectric matrix

Pump-probe studies of photoluminescence of InP quantum wires embedded in dielectric matrix

Persistent changes in the photoluminescence (PL) spectra of InP quantum wires (QWRs) embedded in chrysotile asbestos in the presence of the strong resonant narrow band excitation were observed. Induced suppression and enhancement of PL in different regions of spectra and the blue shift of the PL have been demonstrated. Three onsets related to the LA momentum conserving phonons of InP have been observed in the changed spectrum. These spectral changes are long preserved at 2 K and depend on the intensity and the photon energy of strong resonant narrow-band excitation. They can be erased by the exposure of the below-absorption-edge light. The observed spectral changes are caused by trapping of charges at the surface of the nanostructures and their tunneling into traps in the matrix. The enhancement and suppression of PL of InP nanostructures can be explained by the competition of the following processes: saturation of the capture centers by strong narrow-band resonance light, charge-induced blue shift of the PL spectrum and Auger autoionization and subsequent Auger quenching of radiative recombination of carriers. The large blue shift of the maximum of the PL under high excitation regime is caused by the strong Coulomb interaction anisotropy in semiconductor–insulator QWRs owing to the image charge effect. For comparison and better understanding of the origin of these effects, the experiment with InP quantum dots embedded in chrysotile asbestos has also been carried out

Macrocycle as a “Container” for Dinitramide Salts

Dinitramic acid salts are promising components as oxidizers and burning-rate modifiers of high-energy compositions. However, most of these salts are not free of drawbacks such as hygroscopicity. Therefore, their application under special conditions of use and storage is limited. The synthesis and storage of stable dinitramic acid salts is a topical issue. Here, we synthesized an adduct starting from the nickel salt of dinitramic acid with carbohydrazide and glyoxal to settle the problem of stability and storage of that salt. The chemical composition of the adduct was confirmed by infrared spectroscopy and elemental analysis. The Ni content was determined by atomic emission spectroscopy. Thermogravimetric DSC and TGA analyses showed the adduct to have three decomposition stages. The adduct exhibits a good thermal stability and a low sensitivity to mechanical stimuli. Here, the adduct is demonstrated to be a promising burning-rate inhibitor of pyrotechnic compositions

Linear and nonlinear excitonic absorption in semiconducting quantum wires crystallized in a dielectric matrix

Spectra of linear and nonlinear absorption of GaAs and CdSe semiconducting quantum wires crystallized in a transparent dielectric matrix (inside chrysotile-asbestos nanotubes) have been measured. Their features are interpreted in terms of excitonic transitions and filling of the exciton phase space in the quantum wires. The theoretical model presented here has allowed us to calculate the energies of excitonic transitions that are in qualitative agreement with experimental data. The calculated exciton binding energies in quantum wires are a factor of several tens higher than in bulk semiconductors. The cause of this increase in the exciton binding energy is not only the size quantization, but also the “dielectric enhancement,” i.e., stronger attraction between electrons and holes owing to the large difference between permittivities of the semiconductor and dielectric matrix

Excitons in InP quantum wires with dielectric barriers

Spectra of linear absorption and photoluminescence (PL) at different polarization of the laser excitation; photoluminescence excitation (PLE) spectra and time - resolved PL of the InP quantum wires (QWRs) crystallized in transparent nanotubes of chrysotile asbestos have been measured. We evaluate the exciton ground state binding energy as large as 200 meV. In the presence of the strong resonant laser pumping of InP QWRs (the energy of laser photon coincides with the absorption band of QWRs) the induced suppression and enhancement of PL in different parts of the spectra and the blue shift (about 13meV) of the PL maximum relatively to the energy of laser photon have been observed. The PL features of InP QWRs has been explained by saturation of the capture centers and Auger process

Excitons in InP quantum wires with dielectric barriers

Spectra of linear absorption and photoluminescence (PL) at different polarization of the laser excitation; photoluminescence excitation (PLE) spectra and time - resolved PL of the InP quantum wires (QWRs) crystallized in transparent nanotubes of chrysotile asbestos have been measured. We evaluate the exciton ground state binding energy as large as 200 meV. In the presence of the strong resonant laser pumping of InP QWRs (the energy of laser photon coincides with the absorption band of QWRs) the induced suppression and enhancement of PL in different parts of the spectra and the blue shift (about 13meV) of the PL maximum relatively to the energy of laser photon have been observed. The PL features of InP QWRs has been explained by saturation of the capture centers and Auger process

Optical properties of excitons in semiconductor (InP)-insulator quantum wires

Features in the spectra of absorption, luminescence, and luminescence efficiency obtained under sample excitation with differently polarized laser radiation, and the nonlinear dependence of the luminescence intensity on the excitation level are explained as due to excitonic transitions in semiconductor (InP)-insulator (chrysotile asbestos) quantum wires. The measured excitonic-transition energies in the quantum wires are in quantitative agreement with calculations. The calculations took into account both the size quantization in a quasi-one-dimensional structure and the “dielectric enhancement” of excitons (the noticeable increase of the exciton binding energy and of the excitonic-transition oscillator strength associated with the increased attraction between the electron and the hole due to the large difference between the dielectric permittivities of the semiconductor and the dielectric matrix)

A New Approach for the Synthesis of 2,3,4а,6,7,8а,9,10-Octaaza-4,8-dioxo-3,4,4a,7,8,8а,9,9a,10,10а-decahydroanthracene and High-Energy Performance Characterization of Its Dinitramide Salt

A simple, one-pot regioselective method for the synthesis of a high-nitrogen tricycle, 2,3,4а,6,7,8а,9,10-octaaza-4,8-dioxo-3,4,4a,7,8,8а,9,9a,10,10а-decahydroanthracene, with a yield of 27% was developed on a starting urea basis as a result of studies focused on finding new, more efficient approaches to the synthesis of high-energy derivatives of dinitramic acid (DNA). This tricycle was further treated to furnish 2,3,4а,6,7,8а,9,10-octaaza-4,8-dioxo-3,4,4a,7,8,8а,9a,10а-octohydroanthracene-9,10-ion-bis(dinitramide). The resultant salt of dinitramic acid exhibited inhibitory properties towards the burning rate of pyrotechnic compositions, reducing it by 30%, and possessed good thermal stability due to a high decomposition temperature above 260 °C and a low sensitivity to mechanical stimuli. The structural features of the new tricycle-based dinitramide salt were characterized via 2D NMR spectroscopy and double-focusing sector mass spectrometry (DFS)

Linear and nonlinear optical properties of excitons in semiconductor-dielectric quantum wires

The characteristic features of the absorption, luminescence and photoluminescence excitation spectra of InP nanocrystals crystallized in chrysotile asbestos nanotubes and CdS nanocrystals crystallized in hollow channels of a dielectric template have been explained in terms of exciton transitions in semiconductor–dielectric quantum wires. In these structures, the dielectric confinement effect leads to a considerable increase of the exciton binding energy—the Coulomb attraction between electron and hole is considerably enhanced as a result of the difference between the permittivities of the semiconductor and insulator. The kinetics of porous InP photoluminescence at high excitation by picosecond laser pulses has been explained by the slowing down of the intraband energy relaxation, collective exciton–exciton (electron) interaction and Auger recombination in nanostructures