I. L. Martynov

Interaction of CdSe/ZnS core-shell semiconductor nanocrystals in solid thin films

The optical properties of CdSe/ZnS semiconductor nanocrystals with the core-shell structure are studied upon visible-laser excitation in a wide range of flux densities. It is demonstrated that the dimensional quantization effect is preserved in the films with a limiting high concentration of nanocrystals. A strong bathochromic shift of the absorption and luminescence peaks relative to the peak positions in the corresponding spectra of nanocrystals in films with a relatively low concentration of nanocrystals and solutions is caused by a high concentration of nanocrystals and the dipole moment related to the asymmetry of the nanoparticles. The shift is varied from 35 to 50 nm depending on the film thickness. The luminescence spectra of the films remain unchanged upon an increase in the laser intensity to 1 × 106 W/cm2. The laser action on the nanoparticle films is studied at intensities (5 × 106−1 × 109 W/cm2) higher than the damage threshold.

Hybrid heterostructures based on aromatic polyimide and semiconductor CdSe quantum dots for photovoltaic applications

A nanohybrid photoactive material based on aromatic polyimide (PI) doped with CdSe quantum dots (QDs) has been developed to be used in photovoltaic solar cells. The solar cell is based on a heterostructure of an ITO electrode covered with a layer of Cu–phthalocyanine and a layer of a PI–QD composite. The photovoltaic properties of the CuPc/PI:CdSe hybrid heterostructure at various QD concentrations in the PI matrix have been studied. Luminescent and transmission electron microscopy analyses have shown that the optimal QD mass concentration is 60%. The efficiency of the solar cell based on optimized PI:CdSe structures approaches those for the structures based on conventional MEH-PPV organic semiconductor. Moreover, the photovoltaic characteristics of the solar cell remain stable in the air for a long time (120 h). This is expected to considerably simplify the technology of manufacturing these hybrid solar cells. The mechanisms of the excitation and charge transfer from QDs to the organic semiconductors and...

Laser-induced photoprocesses in solutions and films of the CdSe/ZnS nanoparticles

The photophysical properties of solutions and films with relatively high concentrations of CdSe/ZnS nanoparticles are studied in the presence of the visible laser irradiation in a wide range of power densities. The short-wavelength wing detected in the photoluminescence spectra of the solutions of quantum dots is due to the selective laser excitation of small-size nanoparticles. A comprehensive analysis of the anti-Stokes photoluminescence of the nanoparticles in solutions and films indicates the thermal mechanism of this phenomenon. The dimensional quantization effect, narrow spectra, and a relatively high luminescence yield are retained in the films with a high nanoparticle concentration. The luminescence spectra of the films remain unchanged when the laser flux density increases to 1 × 106 W/cm2. The effect of the laser radiation on the nanoparticle films is studied at the flux densities exceeding the damage threshold (5 × 106–1 × 109 W/cm2).

Photoluminescence of CdSe/ZnS quantum dots in a porous silicon microcavity

It is known that manufacturing and applications of photonic crystals is currently an area of much interest. One of the focuses of special attention in this area is various microcavity (MC) devices. Porous silicon is one of the most promising materials for manufacturing such devices because it is simple to prepare, its optical parameters are precisely controllable, and it has an enormous surface area. This allows to inject different kinds of luminophores into porous silicon MC devices. Apparently, semiconductor quantum dots (QDs) are among the most interesting of them. QDs are characterized by a wide absorbance spectrum, large absorption cross-section, high quantum yield, and excellent photostability. To date, there have been few studies on QD injection into porous silicon photonic structures. In addition, many structures used lack the desired characteristics; the depth of QD penetration also remains a question. This is the first study to analyze the photoluminescence spectrum and kinetics of QDs in a high-quality porous silicon MC. A drastic narrowing of the luminescence spectrum has been observed after QD injection. We have found that the MC morphology considerably affects the penetration of QDs. The kinetics of photoluminescence has also been investigated. Measurements have shown a decrease in the QD characteristic photoluminescence decay time after QD injection into a porous silicon MC compared with the QD photoluminescence decay time in a toluene solution. However, we have not observed a significant difference between the photoluminescence decay times of QDs in an MC and in single-layer porous silicon.

Formation of anions of nitroaromatic compounds in gases during UV laser irradiation

The mechanism of formation of negative ions of trinitrotoluene, dinitrotoluene, and para-nitrotoluene in the gas phase under the action of laser irradiation was studied using ion mobility spectrometry. The gas mixture was ionized by the fourth harmonic radiation of a YAG:Nd3+ laser. The irradiance was varied within 2–5 MW/cm2. It was demonstrated that, although the test compounds are characterized by a high cross section of absorption of laser radiation the formation of their anions occurs largely through the multiphoton ionization of organic admixtures present in the gas mixture with the subsequent attachment of electrons formed. In a nitrogen medium, the attachment of electrons occurs directly, whereas in air, the O2− immediate plays an important role. Laser radiation causes the dissociation of the molecules under study, especially marked for para-nitrotoluene.

Fabrication of composite materials from semiconductor quantum dots and organic polymers for optoelectronics and biomedicine: role of surface ligands

Recent advances in the fields of application of the composites based on quantum dots (QDs) as optical converters for the light emitting devices, solar cells and biofunctional nanoprobes for detection of markers and medical diagnostics are considered. The possibilities of application of various QD—ligand—polymer combinations depending on desired photophysical properties in the final composite are analyzed. An attempt is made to predict the key future trends in the fabrication and application of hybrid nanocomposites for biomedicine and optoelectronics.

Effect of surface ligands on the performance of organic light-emitting diodes containing quantum dots

Quantum dots (QDs) have numerous applications in optoelectronics due to their unique optical properties. Novel hybrid organic light-emitting diodes (OLEDs) containing QDs as an active emissive layer are being extensively developed. The performance of QD–OLED depends on the charge transport properties of the active layer and the degree of localization of electrons and holes in QDs. Therefore, the type and the density of the ligands on the QD surface are very important. We have fabricated OLEDs with a CdSe/ZnS QD active layer. These OLEDs contain hole and electron injection layers consisting of poly(9-vinyl carbazole) and ZnO nanoparticles, respectively. The energy levels of these materials ensure efficient injection of charge carriers into the QD emissive layer. In order to enhance the charge transfer to the active QD layer and thereby increase the OLED efficiency, the QD surface ligands (tri-n-octyl phosphine oxide, TOPO) were replaced with a series of aromatic amines and thiols. The substituents were expected to enhance the charge carrier mobility in the QD layer. Surprisingly, the devices based on the original TOPO-coated QDs were found to have the best performance, with a maximum brightness of 2400 Cd/m2 at 10 V. We assume that this was due to a decrease in the charge localization within QDs when aromatic ligands are used. We conclude that the surface ligands considerably affect the performance of QD–OLEDs, efficient charge localization in QD cores being more important for good performance than a high charge transfer rate.

The influence of the quantum dot/polymethylmethacrylate composite preparation method on the stability of its optical properties under laser radiation

Photoluminescent semiconductor nanocrystals, quantum dots (QDs), are nowadays one of the most promising materials for developing a new generation of fluorescent labels, new types of light-emitting devices and displays, flexible electronic components, and solar panels. In many areas the use of QDs is associated with an intense optical excitation, which, in the case of a prolonged exposure, often leads to changes in their optical characteristics. In the present work we examined how the method of preparation of quantum dot/polymethylmethacrylate (QD/PMMA) composite influenced the stability of the optical properties of QD inside the polymer matrix under irradiation by different laser harmonics in the UV (355 nm) and visible (532 nm) spectral regions. The composites were synthesized by spin-coating and radical polymerization methods. Experiments with the samples obtained by spin-coating showed that the properties of the QD/PMMA films remain almost constant at values of the radiation dose below ~10 fJ per particle. Irradiating the composites prepared by the radical polymerization method, we observed a monotonic increase in the luminescence quantum yield (QY) accompanied by an increase in the luminescence decay time regardless of the wavelength of the incident radiation. We assume that the observed difference in the optical properties of the samples under exposure to laser radiation is associated with the processes occurring during radical polymerization, in particular, with charge transfer from the radical particles inside QDs. The results of this study are important for understanding photophysical properties of composites on the basis of QDs, as well as for selection of the type of polymer and the composite synthesis method with quantum dots that would allow one to avoid the degradation of their luminescence.

Silicon photonic structures with embedded polymers for novel sensing methods

At present time research and development of a new generation of optical sensors using conjugated polymers, in particular sensors of explosives are actively underway. Nevertheless, the problems of the sensitivity, selectivity, and stability of such sensors are still of great interest. One of the ways to solve the problem is the creation of luminescence sensors based on photonic crystals with a high specific surface area, which have significant sorption ability and allow to effective modulate emission properties of luminophores. In this paper, porous silicon microcavities with embeded organic polyphenylenevinylene- (PPV) and polyfluorene- (PF) type polymers were created. It was shown that polymer infiltration in porous silicon microcavities leads to modification of their luminescence properties, which is expressed in narrowing of the emission spectrum and changing of its directional pattern. It was demonstrated that such structures exhibit sensitivity to saturated vapors of trinitrotoluene. The structures proposed can be treated as a basis for development of new type of sensors used for detection of vapors of nitroaromatic compounds.

Optoelectronic Properties of Semiconductor Quantum Dot Solids for Photovoltaic Applications

Quantum dot (QD) solids represent a new type of condensed matter drawing high fundamental and applied interest. Quantum confinement in individual QDs, combined with macroscopic scale whole materials, leads to novel exciton and charge transfer features that are particularly relevant to optoelectronic applications. This Perspective discusses the structure of semiconductor QD solids, optical and spectral properties, charge carrier transport, and photovoltaic applications. The distance between adjacent nanoparticles and surface ligands influences greatly electrostatic interactions between QDs and, hence, charge and energy transfer. It is almost inevitable that QD solids exhibit energetic disorder that bears many similarities to disordered organic semiconductors, with charge and exciton transport described by the multiple trapping model. QD solids are synthesized at low cost from colloidal solutions by casting, spraying, and printing. A judicious selection of a layer sequence involving QDs with different size, composition, and ligands can be used to harvest sunlight over a wide spectral range, leading to inexpensive and efficient photovoltaic devices.