X-ray Excited Optical Luminescence Measurements Research Articles

Scanning x-ray excited optical luminescence of heterogeneity in halide perovskite alloys

Understanding the optoelectronic properties of optically active materials at the nanoscale often proves challenging due to the diffraction-limited resolution of visible light probes and the dose sensitivity of many optically active materials to high-energy electron probes. In this study, we demonstrate correlative synchrotron-based scanning x-ray excited optical luminescence (XEOL) and x-ray fluorescence (XRF) to simultaneously probe local composition and optoelectronic properties of halide perovskite thin films of interest for photovoltaic and optoelectronic devices. We find that perovskite XEOL stability, emission redshifting, and peak broadening under hard x-ray irradiation correlates with trends seen in photoluminescence measurements under continuous visible light laser irradiation. The XEOL stability is sufficient under the intense x-ray probe irradiation to permit proof-of-concept correlative mapping. Typical synchrotron XRF and nano-diffraction measurements use acquisition times 10–100x shorter than the 5-second acquisition employed for XEOL scans in this study, suggesting that improving luminescence detection should allow correlative XEOL measurements to be performed successfully with minimal material degradation. Analysis of the XEOL emission from the quartz substrate beneath the perovskite reveals its promise for use as a real-time in-situ x-ray dosimeter, which could provide quantitative metrics for future optimization of XEOL data collection for perovskites and other beam-sensitive materials. Overall, the data suggest that XEOL represents a promising route towards improved resolution in the characterization of nanoscale heterogeneities and defects in optically active materials that may be implemented into x-ray nanoprobes to complement existing x-ray modalities.

Spatially Resolved Dynamics of Cobalt Color Centers in ZnO Nanowires.

The dynamics of color centers, being a promising quantum technology, is strongly dependent on the local environment. A synergistic approach of X-ray fluorescence analysis and X-ray excited optical luminescence (XEOL) using a hard X-ray nanoprobe is applied. The simultaneous acquisition provides insights into compositional and functional variations at the nanoscale demonstrating the extraordinary capabilities of these combined techniques. The findings on cobalt doped zinc oxide nanowires show an anticorrelation between the band edge emission of the zinc oxide host and the intra-3d cobalt luminescence, indicating two competing recombination paths. Moreover, time-resolved XEOL measurements reveal two exponential decays of the cobalt luminescence. The fast and newly observed one can be attributed to a recombination cascade within the cobalt atom, resulting from direct excitation. Thus, this opens a new fast timescale for potential devices based on cobalt color centers in ZnO nanowires in photonic circuits.

A computational and spectroscopic study of Dy3+ doped BaAl2O4 phosphors

Computational and experimental methods are employed to study the optical properties of Dy-doped BaAl2O4 matrix. Atomistic modelling is used to make predictions of Dy doping sites and charge compensation schemes. The symmetry predicts from atomistic modeling was used to calculate crystal field parameters and to obtain the energies of the electronic transitions of the Dy3+ ion. Dy-doped BaAl2O4 was prepared via a sol-gel proteic technique. The optical properties were studied using X-ray excited optical luminescence (XEOL) measurements. The X-ray absorption near-edge structure (XANES) at the Ba and Eu LIII-edges exhibits typical absorption spectra. The area under the XEOL spectra increases as the photon energy increases in the region around the Ba and Dy LIII-edges. The XEOL spectra showed typical Dy lines. Experimental and calculated values for the transition energies are compared. Computational and experimental methods are employed to study of Dy-doped BaAl2O4 matrix. Atomistic modelling is used to make predictions of Dy doping sites. The optical properties were studied using XEOL measurements.

Relationship between scintillation properties and crystallite sizes in Y2O3:Eu3+

This work describes the relation between crystallite size and scintillation properties of Eu3+-doped yttrium oxide. Nanocrystalline Y2O3:Eu3+ 1.0 mol% powders were prepared by Pechini method and thermal treated at 700–1100 °C. The thermal treatment temperature has influence on the crystallite sizes of Y2O3:Eu3+ nanocrystals and, consequently, on the structural and electronic defects concentration, which contributes to nonradiative recombination routes in scintillation materials. This relationship between the crystallite sizes and scintillation properties was investigated by X-ray diffraction and X-ray excited optical luminescence measurements.

Structural and optical characterizations of Ca2Al2SiO7:Ce3+, Mn2+ nanoparticles produced via a hybrid route

Pure, Ce3+ doped and Ce3+ and Mn2+ co-doped Ca2Al2SiO7 ceramic powders were prepared by two different methodologies which are the proteic sol–gel process and a new hybrid route combining the proteic sol–gel with solid state reaction processes. The second one is an eco-friendly method because it uses natural raw materials in replacement of the metal alkoxides used in the traditional sol–gel routes. X-ray diffraction showed that Ca2Al2SiO7 crystalline phase was obtained for both preparations. Differential thermal analysis indicated that the exothermic event around 850°C, for sample produced by proteic sol–gel method, and around 927°C, for ceramics prepared by hybrid synthesis, can be associated to crystallization of Ca2Al2SiO7. Transmission electron microscope indicates that regular and spherical nanoparticles were obtained with average sizes of about 12nm. The Scherrer’s method was used to determine the average crystallite sizes and it was shown that nanometric crystallites were obtained of about 74nm for samples produced via hybrid route. For all the single phase samples, the crystallite sizes are about the same and that agrees with TEM results. Diffuse optical reflectance measurements were used to estimate the Ca2Al2SiO7 optical band gap and the obtained value is about 6eV, photoluminescence (PL) spectra presented typical emissions of Ce3+ and Mn2+ ions. Upon excitation at 352nm the emission spectra showed a broad band centered at 415nm due to the Ce3+ 4f1→5d1 typical transition. This emission is resonant with Mn2+ excitation and it transfers energy to Mn ions generating a second broad emission band centered at 620nm due to the Mn2+. The PL results were used to obtain, as a fist approach, the Ce3+ energy levels diagram and, using the Tanabe–Sugano diagrams, the transitions due to the Mn2+ were calculated. X-ray excited optical luminescence measurements showed the same emission spectra as the PL emission spectra. Luminescence lifetime decay constants were measured for Ce and Mn co-doped and for Ce doped samples and the results indicate Ca2Al2SiO7:Ce3+, Mn2+ showed quite fast responses with main time constants below 30ns.

Luminescence properties of defects in nanocrystalline ZnO

Formation of intrinsic acceptor defects in ZnO is rare due to the low formation energy of donors. Understanding this phenomenon is of interest for the fabrication of high quality light emitting diodes. Herein, we examine the temperature dependent formation of defects in nanocrystalline ZnO through a combination of X-ray excited optical luminescence (XEOL) together with X-ray absorption near edge structures (XANES) and electron spin resonance (ESR). Certain defects are shown to form under low temperature and are unstable above 700 °C. These defects have high g-values characteristic of acceptors and short spin-lattice relaxation times. XEOL measurements show that acceptor defects with a characteristic red luminescence are also formed under these conditions. Low g-value (donor) defects forming at temperatures >700 °C are shown to have spin-lattice relaxation characteristic of nonradiative recombination centers.

2D XAFS–XEOL Mapping of Ga1–xZnxN1–xOx Nanostructured Solid Solutions

We have applied two-dimensional X-ray absorption fine structure–X-ray-excited optical luminescence (2D XAFS–XEOL) and X-ray absorption near-edge structure (XANES) spectroscopy to study the nature of the X-ray-excited optical luminescence process in GaN–ZnO solid-solution nanostructures, both indium-doped (GIZNO) and undoped (GZNO). XAFS–XEOL analysis of GZNO reveals that X-ray-excited optical luminescence (XEOL) is a strong function of the excitation energy, and hence penetration depth, as well as the turn-on of any shallow core levels. Indium K-edge XANES results reveal that indium in GIZNO forms a surface oxide but has limited solubility in the solid solution. This observation is consistent with the morphology change from nanoparticles in GZNO to nanosheets in GIZNO induced by the presence of a surface In layer, which ultimately becomes oxidized to form In2O3. XAFS–XEOL measurements suggest that the XEOL from GIZNO is primarily due to oxygen defects in the indium oxide surface layer and effective energy transfer from the GaN–ZnO solid solution.

Effect of Indium Doping on the Growth and Physical Properties of Ultrathin Nanosheets of GaInN/ZnO Solid Solution

We have prepared single-crystal, thin GaInN/ZnO solid-solution nanosheets by nitration of a single source Ga−In−Zn−O gel-like homogeneous precursor. The thickness of these nanosheets can be as low as about 2 nm. The In doping ratio typically was 1−2%. It is found that while indium plays an important role in the formation of the nanosheets and contributes to their narrow bandgap energy of about 2.55 eV, the impact of Zn doping on the bandgap shrinkage is higher than that of indium. An X-ray absorption near-edge structures (XANES) study confirms the validity of In doping in the solid solution, revealing that it increases chemical disorder, which is evident by the corresponding distortion of the Ga L3,2-edge and the N K-edge spectra. From the XANES, we observe that Zn atoms are displaced from the GaN lattice as In is incorporated into the solid solution. X-ray excited optical luminescence measurements show that indium doping reduces zinc acceptors in the GaN lattice, resulting in a blue shift and narrowing of the material’s emission band.

Molecular orientation and optical luminescence properties of soluble star shaped oligothiophene molecules for organic electronic applications

Angle resolved near-edge absorption fine structure measurements of the C 1s edge of a film of the star-shaped molecule 4(HPBT) show a high degree of order, with the molecules maintaining a largely planar geometry in an upright configuration. X-ray excited optical luminescence measurements showed a dependence of the luminescence spectrum on the excitation energy, with (0–0), (0–1), and (0–2) vibrational modes being clearly visible when exciting off-resonance; the appearance of these features under off-resonance is attributed to a scattering process that is quenched when the excitation approaches a core absorption resonance. The HOMO–LUMO gap is estimated from the luminescence measurements (2.28 eV), and this value is confirmed by the comparison of the C 1s absorption and C Kα emission spectra.

X-ray excited optical luminescence from crystalline silicon

Synchrotron based X-ray excited optical luminescence (XEOL) has been measured with many direct bandgap semiconductors. We present XEOL measurements on crystalline silicon (Si), obtained despite of its indirect bandgap and the consequently low luminescence efficiency. Spectra of monocrystalline and multicrystalline (mc) Si at room temperature are compared to theoretical spectra. A possible application in the synchrotron-based research on mc-Si is exemplified by combining XEOL, X-ray fluorescence (XRF) spectroscopy, photoluminescence (PL) spectroscopy, and microscope images of grain boundaries. This approach can be utilized to investigate the recombination activity of metal precipitates, to analyze areas of different lifetimes on mc-Si samples and to correlate additional material parameters to XRF measurements. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temporal- and Site-Specific Determination of the Origin of the Luminescent Bands in Silicon Nanowires

We have performed temporally resolved X-ray excited optical luminescence measurements on Si nanowires. By performing Si K edge X-ray excitation measurements we have determined that the short-lifetime components have primarily Si character, while the long-lifetime component stems from the SiO2 shell and SiO2−Si interface. The lifetime of the short-lifetime component decreases with increasing luminescence energy, i.e., as the size of the nanosilicon species decreases.

New EXAFS Measurements by XEOL and TEY on Porous Silicon

Results of an EXAFS investigation on porous Silicon carried out by X-ray Excited Optical Luminescence (XEOL) and Total Electron Yield (TEY) techniques, at the Si K absorption edge, are reported. For the first time XEOL spectra of porous silicon have been recorded in a wide energy range (1800–2500 eV) and EXAFS signals have been singled out from them. Simultaneous TEY and XEOL measurements yield to different results: in particular TEY-EXAFS is sensitive up to the third coordination shell of Si, while XEOL-EXAFS reveals only the contributions of the first two coordination shells; moreover they show a different dependence on changes of the etching parameters. This evidences the sensitivity of XEOL technique to the local structure of the quantum confined luminescent sites. The dependence of the light emission properties on the main preparation parameters and their influence on the short-range structure of red and yellow porous silicon samples are also investigated.