[en] The interface formation between Mg and 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) was investigated by high resolution soft x-ray photoemission spectroscopy. The interface chemistry was obtained after fitting the core level spectra as a function of Mg thickness. At the initial stage of deposition, a strong chemical interaction between Mg and the single bonded oxygen atoms of PTCDA is observed leading to the formation of MgO and a modified organic molecule. Based on the experimental evidence, the molecular structure of the modified molecule is proposed. Moreover, the changes observed in the measured C_1s core level spectra are supported by density functional theory calculations

[en] The stationary and time-resolved polariton radiation in ultrahigh quality AIGaAs layers have been studied. It has been found that elastic exciton-exciton collisions lead to the appearance of a low-energy line of polariton radiation. We show that the rate of exciton-to-polariton transitions caused by elastic exciton-exciton collisions is determined not only by the density of the excitonic gas, but also by its temperature; this is in accordance with existing theoretical predictions

[en] We present a study of the optical phonon spectrum of relaxed In_xAl_1-xAs epitaxial layers grown by molecular-beam epitaxy on GaAs substrates in the whole range of In content (x = 0 divide 1) using macro- and micro-Raman and IR spectroscopies.

[en] Optical absorption spectra of x-ray irradiated CdS_1-xSe_x nanocrystals embedded in a borosilicate glass matrix are studied. The observed transformation of the confinement-related features in the spectra is related to the x-ray-induced negative ionization of the nanocrystals with charge transfer across the nanocrystal/matrix interface. The radiation-induced changes and their relaxation upon postirradiation storage as a function of nanocrystal size and composition are discussed.

[en] Index matching of guided modes in birefringent multilayered organic waveguides opens new prospects for the design of mode coupling and mode switching devices. We demonstrate index matching of guided modes in two multilayered structures, in (a) a PTCDA-Alq₃-PTCDA three-layer and (b) a PTCDA-Alq₃ effective medium multilayer waveguide. The optical waveguides were grown on a Pyrex substrate by organic molecular beam deposition. The occurrence of index matching was investigated both experimentally by measuring the effective refractive index dispersion of transverse electric and magnetic modes using the m-line technique and theoretically by modelling the index dispersion with a transfer matrix algorithm. (paper)

[en] Interference-enhanced Raman scattering (IERS) was observed for thin films of copper(II) hexadecafluorophthalocyanine (F_1_6CuPc) deposited on SiO_2 layers on a Si substrate. The enhancement of the Raman scattering originates from the interference of the light in the transparent SiO_2 layer. Stripes of SiO_2/Si with gradually varying oxide layer thickness were used as IERS substrates for a systematic study of the enhancement parameters. Raman measurements were carried out using three laser lines (325, 514.7, and 632.8 nm) in order to probe the F_1_6CuPc thin films at non-resonant and resonant Raman conditions. Spectroscopic ellipsometry was used to determine both the SiO_2 layer and thin film thicknesses. The intensity enhancement of the F_1_6CuPc Raman peaks occurs periodically, corresponding to the alternating interference conditions for the Raman scattered light with increasing SiO_2 layer thickness. The enhancement factors were calculated using the optical constants of the layers involved and the geometric parameters. This allows a straightforward application of IERS for optical studies of thin films and interfaces by calculating the dielectric thickness where maximum Raman enhancement is expected. (paper)

[en] A cubic-to-hexagonal phase transition in Si nanocrystals (NCs) is observed in situ by recording their Raman and photoluminescence (PL) spectra simultaneously. The formation of the hexagonal structure is concluded from the appearance of characteristic Raman peaks and intense PL with spectral positions reported previously for hexagonal Si. The phase transition occurs due to heating of the NCs by the laser beam, but it is supposed to be a photo-induced effect, as it does not occur as a consequence of solely thermal heating. Both the structural transition and concomitant switching of the bright PL emission are reversible and are supposed to be of interest from the viewpoint of application. (paper)

[en] A review of recent applications of Raman spectroscopy as a fast, sensitive, and non-destructive technique for exploring II–VI semiconductor nanocrystals fabricated by various methods (colloidal chemistry, Langmuir–Blodgett method, diffusion-limited growth) is presented. Specific size-related features revealed in the nanocrystal Raman spectra (phonon confinement, surface phonons) are analysed, as well as more complicated size effects for ultrasmall nanocrystals (NCs) related to the activation of the phonon density of states modified by surface reconstruction. Similarities and differences of the Raman scattering in II–VI and III–V or elemental (Si) semiconductor NCs are briefly analysed. Implementation of resonant conditions and application of infrared absorption analysis, complementary to the Raman spectroscopy—resulting in the observation of phonon modes ‘silent’ in conventional Raman scattering processes—are discussed. Furthermore, Raman spectroscopy is employed for fast and efficient assessment of the composition of matrix-embedded ternary II–VI nanocrystals, as well as more complicated multimode quaternary II–VI systems. Selective probing of electronic and vibrational spectra of different parts of heterogeneous NCs (such as core–shell systems) by tuning the excitation wavelength in resonant Raman scattering is considered. The analysis of phonon spectra is applied to the quantitative estimation of strain in the core and shell, and degree of interface intermixing, as well as to checking the surface oxidation. The above approaches and phenomena are further explored in more complex compound NCs beyond II–VI, such as CuInS₂/ZnS. Recent results in the field of surface- and tip-enhanced Raman spectroscopy and surface-enhanced infrared absorption are analysed showing the perspectives of Raman spectroscopy as a tool for investigation of single-nanocrystal phonon spectra. (topical review)

[en] The effect of shells of various thicknesses on the vibrational resonant Raman spectra of CdSe/ZnS core-shell nanoparticles is studied. The dependence of the core-shell structure on the method of shell deposition is derived from a comparison of the vibrational and photoluminescence spectra of nanoparticles. Along with the appearance of peaks attributed to the shell, the phonon spectrum of the core undergoes significant changes upon shell growth. The change of the CdSe LO peak lineshape in core-shell nanoparticles is discussed with respect to possible changes in the spectrum of both optical and acoustical phonons upon shell formation. Based on the observed decrease of the CdSe 2LO/LO peak intensity ratio, a weakening of exciton coupling to the CdSe LO phonon upon ZnS shell deposition is supposed. The change in the carrier localization volumes upon shell formation is discussed as a possible reason for the reduced coupling.

[en] Raman and photoluminescence (PL) spectra of amorphous As₂Se₃:Cd films show that laser irradiation can result in the local formation of CdSe nanocrystals (NCs). Due to a pronounced laser-induced drastic drop of the film viscosity, a strong local enhancement of diffusion of atoms in the laser spot area results in a material transfer from the laser spot (evidenced by SEM and AFM data) and simultaneously facilitates the formation of CdSe NCs. The difference of the PL maximum positions measured at a high laser power density P _exc and subsequently (from the same spot) at a lower P _exc value can be explained either by local heating of the sample by the high-P _exc illumination, or by the flexoelectric effect (additional strain in the CdSe NCs induced by the electric field gradient caused by the charging of the NC surface). Based on the spectral positions of the PL maximum, with the account of the Stokes shift, the average radii of the CdSe NCs formed are estimated. (paper)