High Energy Photoluminescence Band Research Articles

Excitation dynamics of MAPb(I1-xBrx)3 during phase separation by photoirradiation: Evidence of sink, band filling, and Br-Rich phase coarsening

Here, we report the radiative recombination behavior of a binary perovskite, MAPb(I0.2Br0.8)3, during phase separation by investigating photoluminescence (PL) spectrum modulations under different laser irradiation conditions: power density, wavelength (379 and 470 nm), and irradiation time. During laser irradiation, the original PL spectrum was divided into three PL peaks assigned to the 1st I-rich, 2nd I-rich, and Br-rich phases. The observed modulations of the intensity and wavelength of the PL spectrum could be explained by the multiple effects of the charge carrier sink, band filling, and Br-rich phase coarsening. Under a weak laser power density, the observed PL predominantly showed the sink effects of the charge carriers through the excited state potential minimum. As the laser power density or irradiation time increased, new high-energy PL bands appeared owing to the band filling and Br-rich phase formation. With a further increase in the laser power or irradiation time, the PL from the Br-rich phase was intensified, but its PL peak position was red-shifted owing to the quantum size effect from the Br-rich phase coarsening. These variations in PL intensity and charge carrier behavior were carefully studied using time- and space-resolved fluorescence imaging microscopy.

Emission and structure variations at aging ZnO:Ag nanocrystals prepared by spray pyrolysis

The emission and structure of ZnO:Ag nanocrystal (NC) films obtained by ultrasonic spray pyrolysis have been studied in the as-grown state and after aging. The scanning electronic microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) methods have been applied. The films are characterized by the wurtzite crystal structure with the NC sizes about of 8.0–10.0 nm in the as-grown state and 6.0–8.0 nm after aging in ambient air. The Ag content was varied within the range of 1–4 at.% in the ZnO films. PL spectra of as-grown ZnO:Ag films are complex and can be presented as a set of PL bands at 10 K: the near band edge emission with the peak at 3.18 eV, two Ag doping—related PL bands centered at 2.95 and 2.68 eV, and native defect-related green (2.40 eV) and orange (1.90 eV) PL bands. The high PL intensities of Ag doping-related PL bands were detected for the Ag concentrations of 2–3 at.%. PL intensities of high energy PL bands (3.18, 2.95 and 2.68 eV) fall down at aging together with intensity stimulating the orange and green PL bands. The joint analysis of XRD and PL results permit to confirm that the 2.95 eV PL band is connected with the optical transition via the substitutional AgZn defects. It was shown as well that aging in ambient air is owing to the diffusion of oxygen interstitials in the ZnO:Ag NCs that leads to intensity increasing the orange PL band and modification of the AgZn radiative defects.

Luminescence properties ofCu2ZnSn(S,Se)4solar cell absorbers: State filling versus screening of electrostatic potential fluctuations

The power conversion efficiency of ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_{4}$ kesterite thin-film solar cells is mainly limited by a low open-circuit voltage ${V}_{\text{OC}}$. In the literature, this low ${V}_{\text{OC}}$ has been amongst others attributed to electrostatic potential fluctuations leading to fluctuating band edges. This assignment was mainly based on the observation of a shift of the photoluminescence (PL) maximum to higher photon energies as a function of excitation power, which was interpreted in terms of a screening of the electrostatic potential fluctuations. However, in this paper, we show evidence that the observed shift of the PL maximum is dominantly caused by state filling rather than screening. Our assignment is based on the investigation of the full PL line shape (not only of the PL maximum) and the PL yield as a function of excitation power and temperature. Further support of our interpretation is given by the observation of additional band-tail emission showing up as a second high-energy PL band at the highest excitation powers. The spectral position of this additional band coincides with the absorption-tail deduced from photoluminescence excitation spectroscopy (PLE).

Optical emission fromSiO2-embedded silicon nanocrystals: A high-pressure Raman and photoluminescence study

We investigate the optical properties of high-quality Si nanocrystals $(\mathrm{NCs})/\mathrm{Si}{\mathrm{O}}_{2}$ multilayers under high hydrostatic pressure with Raman scattering and photoluminescence (PL) measurements. The aim of our study is to shed light on the origin of the optical emission of the Si $\mathrm{NCs}/\mathrm{Si}{\mathrm{O}}_{2}$. The Si NCs were produced by chemical-vapor deposition of Si-rich oxynitride $(\mathrm{SRON})/\mathrm{Si}{\mathrm{O}}_{2}$ multilayers with 5- and 4-nm SRON layer thicknesses on fused silica substrates and subsequent annealing at 1150 \ifmmode^\circ\else\textdegree\fi{}C, which resulted in the precipitation of Si NCs with an average size of 4.1 and 3.3 nm, respectively. From the pressure dependence of the Raman spectra we extract a phonon pressure coefficient of $8.5\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/\mathrm{GPa}$ in both samples, notably higher than that of bulk $\mathrm{Si}\phantom{\rule{0.28em}{0ex}}(5.1\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/\mathrm{GPa})$. This result is ascribed to a strong pressure amplification effect due to the larger compressibility of the $\mathrm{Si}{\mathrm{O}}_{2}$ matrix. In turn, the PL spectra exhibit two markedly different contributions: a higher-energy band that redshifts with pressure, and a lower-energy band which barely depends on pressure and which can be attributed to defect-related emission. The pressure coefficients of the higher-energy contribution are $(\ensuremath{-}27\ifmmode\pm\else\textpm\fi{}6)$ and $(\ensuremath{-}35\ifmmode\pm\else\textpm\fi{}8)\phantom{\rule{0.16em}{0ex}}\mathrm{meV}/\mathrm{GPa}$ for the Si NCs with a size of 4.1 and 3.3 nm, respectively. These values are sizably higher than those of bulk $\mathrm{Si}\phantom{\rule{0.28em}{0ex}}(\ensuremath{-}14\phantom{\rule{0.16em}{0ex}}\mathrm{meV}/\mathrm{GPa})$. When the pressure amplification effect observed by Raman scattering is incorporated into the analysis of the PL spectra, it can be concluded that the pressure behavior of the high-energy PL band is consistent with that of the indirect transition of Si and, therefore, with the quantum-confined model for the emission of the Si NCs.

Physical reasons of emission transformation in infrared CdSeTe/ZnS quantum dots at bioconjugation

The core/shell CdSeTe/ZnS quantum dots (QDs) with emission at 780–800nm (1.55–1.60eV) have been studied by means of photoluminescence (PL) and Raman scattering methods in the nonconjugated state and after conjugation to different antibodies (Ab): (i) mouse monoclonal [8C9] human papilloma virus Ab, anti-HPV 16-E7 Ab, (ii) mouse monoclonal [C1P5] human papilloma virus HPV16 E6+HPV18 E6 Ab, and (iii) pseudo rabies virus (PRV) Ab. The transformations of PL and Raman scattering spectra of QDs, stimulated by conjugated antibodies, have been revealed and discussed.The energy band diagram of core/shell CdSeTe/ZnS QDs has been designed that helps to analyze the PL spectra and their transformations at the bioconjugation. It is shown that the core in CdSeTe/ZnS QDs is complex and including the type II quantum well. The last fact permits to explain the nature of infrared (IR) optical transitions (1.55–1.60eV) and the high energy PL band (1.88–1.94eV) in the nonconjugated and bioconjugated QDs. A set of physical reasons has been analyzed with the aim to explain the transformation of PL spectra in bioconjugated QDs. Finally it is shown that two factors are responsible for the PL spectrum transformation at bioconjugation to charged antibodies: (i) the change of energy band profile in QDs and (ii) the shift of QD energy levels in the strong quantum confinement case. The effect of PL spectrum transformation is useful for the study of QD bioconjugation to specific antibodies and can be a powerful technique for early medical diagnostics.

Micro-Raman and micro-photoluminescence study of bio-conjugated core–shell CdSe/ZnS nanocrystals

The micro-Raman and micro-photoluminescence spectra of non-conjugated and conjugated with antibody against S6K2 commercial CdSe/ZnS quantum dots (QDs) were investigated under different excitation wavelengths and at different temperatures. In the photoluminescence (PL) spectra, the additional PL band shifted on 0.6–0.65eV to higher energies from the CdSe/ZnS QD exciton PL band is revealed. The relative intensity of this band is found to be several times larger in bio-conjugated QDs, than in the non-conjugated ones. The characteristics of both PL bands (the PL intensity, spectral position and half-width of the PL band) vary similarly under continuous laser light irradiation, storage of the QD samples in the atmospheric ambience as well as during the temperature change. In the Raman spectra recorded under excitation resonant with the high-energy PL band, the additional Raman peaks at about 300cm−1 and 600cm−1, which are close to the frequency of LO and 2LO phonons of bulk CdS, are found. It is proposed that alloyed QDs with chemical composition close to CdS are responsible for the additional high-energy PL band. The possible reasons for the formation of the alloyed QDs are discussed.

The evolution of photoluminescence from Si cluster to nanocrystal in Si-rich oxide/SiO2 multilayer films

The evolution of photoluminescence (PL) from Si cluster to nanocrystal in Si-rich oxide (SRO)/SiO2 multilayer films is reported. Two PL bands can be obtained when the SRO layer thickness is no larger than 2 nm, and the high energy PL band (blue band) is related to the interband transition of Si cluster caused by quantum confinement effect (QCE), while the low energy band (red band) is related to the interface recombination. Both Si clusters and nanocrystals exist in the film when the SRO layer is increased to 3 nm, and the intense PL in the red band suggests that Si nanocrystals becomes the main light-emitting centers. The blue band disappears when the SRO layer is increased to 4 nm, and the PL mechanisms of the red band are QCE and interface defects in Si nanocrystals.

Exciton-light coupling and polaritons in SiC nanocrystals

The paper presents the results of SiC:N nanocrystal characterization using the photoluminescence (PL) and X-ray diffraction (XRD) techniques. Photoluminescence study of porous SiC:N (PSiC) layers with different PSiC thicknesses and SiC nanocrystal (NC) sizes reveals the intensity stimulation mainly for the high energy PL bands. The early investigation of temperature dependences of the high energy PL bands has shown that these PL bands related to the free exciton emission in the different SiC polytypes. The SiC polytypes in the original n-type SiC:N wafers and in porous SiC layers were confirmed by XRD study. The intensity enhancement of exciton-related PL bands in big size (50–250nm) SiC NCs is attributed to the realization of the exciton week confinement and exciton-light coupling with the formation of polaritons in the SiC NCs. The numerical simulation of exciton radiative recombination rates and radiative lifetimes for the different exciton emissions has been done using a model of exciton–light coupling in SiC NCs. The experimental and numerically calculated results have been compared and discussed.

Re-charging the luminescence states in CdSe/ZnS quantum dots at the conjugation to Osteopontin antibodies

The paper presents the original study of photoluminescence (PL) and Raman scattering spectra of core–shell CdSe/ZnS quantum dots (QDs) covered by the amine-derivatized polyethylene glycol (PEG) with luminescence interface states. First commercially available CdSe/ZnS QDs with emission at 640nm (1.94eV) covered by PEG polymer have been studied in nonconjugated states. PL spectra of nonconjugated QDs are characterized by a superposition of PL bands related to exciton emission in a CdSe core and to the hot electron–hole recombination via high energy luminescence states. The study of high energy PL bands in QDs at different temperatures has shown that these PL bands are related to luminescence interface states at the CdSe/ZnS or ZnS/polymer interface. Then CdSe/ZnS QDs have been conjugated with biomolecules—the Osteopontin antibodies. It is revealed that the PL spectrum of bioconjugated QDs changed essentially with decreasing hot electron–hole recombination flow via luminescence interface states. It is shown that the QD bioconjugation process to Osteopontin antibodies is complex and includes the covalent and electrostatic interactions between them. The variation of PL spectra due to the bioconjugation is explained on the basis of electrostatic interaction between the QDs and biomolecule dipoles that stimulates re-charging QD interface states. The study of Raman scattering of bioconjugated CdSe/ZnS QDs has confirmed that the antibody molecules have the electric dipoles. It is shown that CdSe/ZnS QDs with luminescence interface states are promising for the study of bioconjugation effects with specific antibodies and can be a powerful technique in biology and medicine.

Radiative interface state study in CdSe/ZnS quantum dots covered by polymer

Abstract The paper presents the transformation of photoluminescence (PL) spectra of nonconjugated and bioconjugated core/shell CdSe/ZnS QDs covered by PEG polymer at the aging in ambient air. Studied QDs are characterized by the sizes: (i) 3.6–4.0 nm and color emission with the maxima at 560–565 nm (2.19–2.25 eV) and (ii) 5.2–5.3 nm and with emission at 605–610 nm (2.02–2.08 eV). The part of 565 nm CdSe/ZnS QDs has been bioconjugated to the mouse anti PSA (Prostate-Specific Antigen) antibodies and the part of 605 nm QDs has been bioconjugated to the antihuman IL10 (Interleukin 10) antibodies using the commercially available 565 nm and 605 nm QD conjugation kits. It is revealed that the aging process in ambient air has the very strong impact on PL spectra of nonconjugated core/shell CdSe/ZnS QDs covered with PEG polymer. The aging process relates to the polymer modification in ambient air that is accompanied by the three effects: (i) polymer transparency increasing for the emission of CdSe cores (2.03 or 2.20 eV), (ii) the intensity stimulation of high energy PL bands (2.37, 2.73 and 3.06 eV) related to the interface states at the ZnS/PEG polymer interface and (iii) the elastic strain modification in QD systems. The concentration of interface states at the ZnS/polymer interface increases at the aging of PEG polymer in ambient air.

Emission related to exciton‐polariton coupling in porous SiC

AbstractThe paper presents the results of SiC nanocrystal (NC) characterization using the photoluminescence (PL), its temperature dependence and X‐ray diffraction (XRD) techniques. It is revealed that original n‐type 6H‐SiC wafers and porous 6H‐SiC layers consisted inclusions of 2H‐SiC, 4H‐SiC, and 8H‐SiC polytypes. The photoluminescence study has shown that in porous SiC layers the emission intensity of high energy PL bands enlarges. Temperature dependences of high energy PL bands testify that these PL bands related to excitons bounded at nitrogen donors in different SiC polytypes. The PL intensity enhancement of donor‐bound exciton PL bands in the big size (50‐250nm) SiC nanocrystals has been attributed to the realization of exciton weak confinement and exciton‐polariton coupling in SiC nanocrystals. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Relation between conductance, photoluminescence bands and structure of ITO nanoparticles prepared by various chemical methods

To study the relation between conductance, photoluminescence bands and structure of indium tin oxide (ITO), nano- and microparticles of ITO were synthesised using various methods, including hydrothermal, oxalate precursor decomposition and Pechini type sol–gel combustion methods. The resultant powders were analysed by X-ray diffraction (XRD), resistance measurement, photoluminescence (PL) spectroscopy and scanning electron microscopy techniques. The crystallographic and microstructural parameters of samples were refined from the XRD patterns utilising Rietveld method. Two PL bands were observed in 475 and 560 nm and it was found that the low-energy PL band intensity presents direct relation with conductivity of samples while the other one shows inverse relation. Pechini samples demonstrated the highest conductivity and low-energy PL band intensity, while hydrothermal sample has low conductivity. It was suggested that the low-energy PL band is the characteristics of excitons trapped in oxygen vacancy while the high-energy PL band is attributed to the excitons trapped in other defects centre. The colour of samples and data resulted from XRD pattern refinement were also validated. According to the refinement results and Popa's model, all samples show anisotropic micro-strain and crystallite size. It was also deduced that hydrothermal sample was textured in the 〈111〉 direction.

The anisotropy of the optical properties of ternary semiconductors formed by elements of III and VI groups

AbstractThe GaSxSe1‐x and GaSexTe1‐x single crystals with the composition parameter x in the range of 0 ≤ x ≤ 0.3 and 0.7 ≤ x ≤ 1 respectively have been obtained. The absorption and photoluminescence (PL) measurements have been carried out for two polarizations: E ∥ C and E ⊥ C (E is the electrical field vector of light wave and C is the crystallographic axis normal to the cleaved plane). The anisotropy of optical absorption reveals itself on the low energy wing of the n =1 free exciton band. The anisotropy coefficient increases with the temperature decrease. The edge of the absorption band is determined by direct n =1 excitons for E ⊥ C polarization and by indirect transitions at low absorption coefficient for E ∥ C polarization. The photoluminescence spectra of GaS0.2Se0.8 crystals at 78 K in E ∥ C and E ⊥ C polarizations are identical to those of GaSe. The high energy photoluminescence band is determined by the radiative emission of direct bounded excitons. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin Dependent Photoinduced Absorption in a-Si:H

ABSTRACTWe have studied photoexcitation dynamics in undoped a-Si:H from 80 K to 300 K by the techniques of photoluminescence (PL), photoinduced absorption (PA) and their respective versions of optically detected magnetic resonance, namely PLDMR and PADMR. Both PL and PA spectra are composed of low and high energy bands, respectively. Using their respective temperature dependences we correlate the low energy PA band (∼ 0.4 eV) to the high energy PL band (∼1.3 eV) and the high energy PA band (∼ 1 eV) to the low energy PL band (∼ 0.8 eV). We also found that the PADMR spectrum is composed of three main contributions: a narrow resonance at g ∼ 2 (FWHM = 15 G), a broad resonance at g ∼ 2 (160 G) and a narrow asymmetric resonance at g ∼ 4 (20 G). By measuring their photon energy spectrum we found that the g ∼ 4 and the broad g ∼ 2 resonances are correlated with each other and therefore are due to trapped electron-hole pairs in the triplet spin configuration.

Investigation of Porous Silicon Films Structure by Optical Methods

ABSTRACTThe structure of aged porous silicon (PS) has been investigated using reflectivity, photoluminescence (PL), optical absorption and photoconductivity decay. An optical anisotropy in a perpendicular to surface direction is observed for PS samples below a critical porosity of 70%. The photostability of the PL spectra imply three different structures of PS, these should be attributed to quantum size structures, polymeric type structures and fluor-related structures. The absorption edge is deconvoluted into a band-to-band absorption with energy ranging between Eg = 1.6–2 eV and an absorption shoulder at hu ≤ Eg. The high energy PL band around 2.3 eV resembles that of fluoroindate glass. The observed rapid lifetime at high injection level is attributed to excitonic Auger-type recombination.

Recombination processes ina-Si: H. A study by optically detected magnetic resonance

We report cw and time-resolved optically detected magnetic resonance data on samples of $a$-Si: H prepared by reactive sputtering, glow-discharge, and chemical vapor deposition. Two distinct luminescence processes are inferred near 1.3-1.4 and 0.9 eV. An enhancing signal at $g=2.0065$ is observed on the 0.9-eV photoluminescence (PL) band and is linked to distant-pair radiative recombination between a dangling bond and a hole localized in a valence-band tail state. The high-energy PL band shows a quenching resonance at $g=2.005$, which can be associated with shunt processes involving dangling bonds. Enhancing signals are also observed on this high-energy band: In lowdefect-density sputtered samples, a 200-G-wide line at $g\ensuremath{\simeq}2.01$ is associated with short radiative lifetimes (\ensuremath{\sim}ms) and narrows for longer lifetimes (\ensuremath{\sim}s). In all other samples studied, a narrower line (19 G) at $g=2.0078$ is linked to short lifetimes and broadens for longer lifetimes. Both signals are consistent with a model in which the high-energy PL band arises from radiative recombination of carriers localized in band tail states, and are successfully explained by exchange interaction.