Elena V. Ushakova

van der Waals metal-organic framework as an excitonic material for advanced photonics

Synergistic combination of organic and inorganic nature in van der Waals metal-organic frameworks supports different types of robust excitons that can be effectively and independently manipulated by light at room temperature, and opens new concepts for all-optical data processing and storage.

Self-Organization of Colloidal PbS Quantum Dots into Highly Ordered Superlattices

X-ray structural analysis, together with steady-state and transient optical spectroscopy, is used for studying the morphology and optical properties of quantum dot superlattices (QDSLs) formed on glass substrates by the self-organization of PbS quantum dots with a variety of surface ligands. The diameter of the PbS QDs varies from 2.8 to 8.9 nm. The QDSLs period is proportional to the dot diameter, increasing slightly with dot size due to the increase in ligand layer thickness. Removal of the ligands has a number of effects on the morphology of QDSLs formed from the dots of different sizes: for small QDs the reduction in the amount of ligands obstructs the self-organization process, impairing the ordering of the QDSLs, while for large QDs the ordering of the superlattice structure is improved, with an interdot distance as low as 0.4 nm allowing rapid charge carrier transport through the QDSLs. QDSL formation does not induce significant changes to the absorption and photoluminescence spectra of the QDs. However, the luminescence decay time is reduced dramatically, due to the appearance of nonradiative relaxation channels.

Measurement of the luminescence decay times of PbS quantum dots in the near-IR spectral range

Methods for recording luminescence decay times of semiconductor PbS quantum dots (QDs) with optical transitions in the near-IR spectral range have been analyzed. A measuring complex for spectral and kinetic analysis in the near-IR range (0.8–2.0 μm) in the time interval from several tens of nanoseconds to several tens of microseconds is described. In this complex, a semiconductor picosecond laser is used as an excitation source; luminescence decay times are recorded by a fast InGaAs photodiode, a high-speed amplifier, and a high-frequency oscilloscope; and the measurement results are multiply averaged (up to a million times) by a program. The technical features of this method are discussed and compared with the characteristics of techniques based on photon counting or application of more powerful radiation sources, and the limitations on sensitivity are analyzed. The results of measuring the luminescence decay kinetics of PbS QDs 2.7–7.6 nm in size prepared in the form of solutions and incorporated into matrices are reported.

Comparative analysis of Raman spectra of PbS macro- and nanocrystals

A comparative analysis of the Raman spectra of bulk PbS and PbS nanocrystals with average diameters of 6.6 and 3.8 nm has been performed. It is shown that the bands due to the LO phonons at the Γ point of the Brillouin zone dominate in the first- and second-order Raman spectra of the nanocrystals. Based on the analysis of the relative band intensities for the first- and second-order spectra a qualitative conclusion is drawn: the electron-LO-phonon interaction in PbS nanocrystals increases with a decrease in their size.

Dissociative CdSe/ZnS Quantum Dot-Molecule Complex for Luminescent Sensing of Metal Ions in Aqueous Solutions

The optical properties of dissociative luminescent sensors based on a complex consisting of highly luminescent hydrophobic core/shell CdSe/ZnS quantum dots (QDs) and 1-(2-pyridilazo)-2-naphtol (PAN) molecules in organic solutions and a polymer film are reported. It is demonstrated, using Ni2+ and Co2+ ions as an illustrative example, that the QD/PAN sensor may have applications in the quantitative luminescent sensing of metal ions in aqueous solutions.

Note: Near infrared spectral and transient measurements of PbS quantum dots luminescence

We describe an experimental setup for the characterization of luminescence from nanostructures. The setup is intended for steady-state and time-resolved luminescence measurements in the near-infrared region. The setup allows us to study spectral luminescence properties in the spectral range of 0.8-2.0 μm with high spectral resolution and kinetic luminescence properties between 0.8 and 1.7 μm with a time resolution of 3 ns. The capabilities of the system are illustrated by taking luminescence measurements from PbS quantum dots. We established the size dependencies of the optical properties of the PbS quantum dots over a wide spectral range. Finally, the energy transfer process was studied with a high temporal and spectral resolution.

Optical properties of ordered superstructures formed from cadmium and lead chalcogenide colloidal nanocrystals

The optical properties of three-dimensional ordered superstructures formed on glass substrates by self-assembly of cadmium selenide or lead sulfide nanocrystals (NCs) are investigated and compared to the optical properties of the initial NC colloidal solutions. The formation of the superstructures is strongly correlated to the presence of oleic acid molecules on the surface of the NCs. It is found that the absorption band of the NCs in the superstructures is broadened and shifted to shorter wavelengths in comparison with the absorption band of the NCs in solution. The luminescence spectra of the NCs in the superstructures also differ from the spectra of the NCs in solution. The observed modification of optical properties of superstructures is a manifestation of interactions between the NCs and the chemical environment within the superstructures.

Size-dependent room-temperature luminescence decay from PbS quantum dots

We study size dependence of kinetic and spectral properties of near-infrared luminescence from PbS quantum dots in colloidal solution. Luminescence lifetimes are found to lie between 250 ns for the largest quantum dots and 2:5 μs for the smallest ones, while the Stokes shift is found to increase from 4-5 to 300 meV. These results are explained by the presence of the long-living in-gap state, with the size-dependent energy. Analytical modeling shows that the relaxation from this state is dominant in small quantum dots and negligible in large ones. Biexponential luminescence decay with the size-dependent recombination rates is predicted for quantum dots of all sizes.

Self-organization of lead sulfide quantum dots of different sizes

X-ray structural analysis is used for investigation of structures, obtained by self-organization from nanoparticles - lead sulfide (PbS) quantum dots (QDs) of different sizes - deposited on a glass substrate or embedded in a porous matrix. These nanostructures obtained by both methods represent ordered close-packed structures of the nanoparticles. The configuration of obtained structures does not depend on the nanoparticle size and type of substrate and matrix.

Nanoscale Generation of White Light for Ultrabroadband Nanospectroscopy

Achieving efficient localization of white light at the nanoscale is a major challenge due to the diffraction limit, and nanoscale emitters generating light with a broadband spectrum require complicated engineering. Here we suggest a simple, yet highly efficient, nanoscale white-light source based on a hybrid Si/Au nanoparticle with ultrabroadband (1.3-3.4 eV) spectral characteristics. We incorporate this novel source into a scanning-probe microscope and observe broadband spectrum of photoluminescence that allows fast mapping of local optical response of advanced nanophotonic structures with submicron resolution, thus realizing ultrabroadband near-field nanospectroscopy.