Contribution of spatially and spectrally resolved cathodoluminescence to study crack-tip phenomena in silica glass

Observation of cavitation governing fracture in glasses

Crack propagation is the major vehicle for material failure, but the mechanisms by which cracks propagate remain longstanding riddles, especially for glassy materials with a long-range disordered atomic structure. Recently, cavitation was proposed as an underlying mechanism governing the fracture of glasses, but experimental determination of the cavitation behavior of fracture is still lacking. Here, we present unambiguous experimental evidence to firmly establish the cavitation mechanism in the fracture of glasses. We show that crack propagation in various glasses is dominated by the self-organized nucleation, growth, and coalescence of nanocavities, eventually resulting in the nanopatterns on the fracture surfaces. The revealed cavitation-induced nanostructured fracture morphologies thus confirm the presence of nanoscale ductility in the fracture of nominally brittle glasses, which has been debated for decades. Our observations would aid a fundamental understanding of the failure of disordered systems and have implications for designing tougher glasses with excellent ductility.

Oxygen Vacancy-Related Cathodoluminescence Quenching and Polarons in CeO2

We used cathodoluminescence (CL) spectroscopy to characterize the oxygen vacancies (V_O) in ceria (CeO₂). The effects of the processing atmosphere and thermal quenching temperature on the nature and distribution of the intrinsic defects and on the spectroscopic behavior were investigated. The presence of polarons and associates of the polarons with the oxygen vacancies such as (V_O ^••-Ce_Ce ^')^• is demonstrated. CL intensity quenching above a critical concentration of V_O has been shown. Even though the emission centers in all samples are the same, their concentration changes with the oxygen partial pressure of the processing atmosphere. Deconvolution of the observed CL spectra shows that the emissions originating from the F⁰ centers prevail over those of F⁺ centers of V_O when the defect concentration is high.

Quantitative defect analysis in MOCVD GaN-on-GaN using cathodoluminescence

Cathodoluminescence (CL) is used as a quantitative characterization technique to probe impurities at the metal-organic chemical vapor deposition (MOCVD) grown GaN-on-GaN homoepitaxial interfaces. CL intensity contrast shows a strong correlation with the interfacial impurity concentrations. Based on the analysis of recombination mechanisms of electron beam induced non-equilibrium carriers, an analytical model is proposed to quantitatively determine the impurity concentrations from CL intensity. The extracted interfacial impurity concentrations from the analytical model show a good agreement with the compensation levels obtained from capacitance-voltage measurement, signifying the potential of CL for probing the quantitative impurity levels in GaN-on-GaN structures. This approach can also be extended to be applied in other material systems.

Analysis of defect luminescence in Ga-doped ZnO nanoparticles

We applied cathodoluminescence (CL) spectroscopy to evaluate the defect-induced luminescence within ZnO and Ga-doped ZnO (GZO) nanoparticles. The observed emissions from defect sites present in the GZO lattice exhibited a strong dependence on both dopant content and synthesis methods. The strong and broad defect-induced emissions and inhomogeneous population of intrinsic defects in nano-sized ZnO particles could effectively be suppressed by Ga doping, although large dopant amounts caused the generation of negatively-charged defects, VZn and Oi, with a subsequent increase of the luminescence. Upon deconvolution of the retrieved CL spectra into individual sub-bands, the physical origin of all the sub-bands could be clarified, and related to sample composition and synthesis protocol. This study lays the foundation of quantitative CL evaluation of defects to assess the quality of GZO optoelectronic devices.

Oxygen hole states in zirconia lattices: quantitative aspects of their cathodoluminescence emission

Systematic assessments of cathodoluminescence (CL) spectroscopy, Raman spectroscopy (RS), and X-ray diffraction (XRD) are presented for pure zirconia and for a series of Y-doped zirconia powders (henceforth, simply referred to as undoped ZrO2 and YSZ powders, respectively) synthesized according to a coprecipitation method of Zr and Y chlorides. Emphasis is placed here on spectral emissions related to oxygen-vacancy sites (i.e., oxygen hole states) equally detected from undoped and Y-doped ZrO2 samples, either as intrinsic defects or, extrinsically induced, by means of cathodoluminescence. Most counterintuitively, the undoped ZrO2 sample (i.e., the one with presumably the lowest amount of oxygen vacancies) experienced the strongest CL emission. A progressive "quenching" effect on CL emission with increasing the fraction of Y(3+) dopant could also be observed because the intrinsic vacancies present in the undoped lattice are the most efficient since they can trap two electrons to gain electrical neutrality. However, as soon as Y(3+) ions are introduced in the system, those intrinsic vacancies migrate to Y-sites in next-nearest-neighbor locations, namely in a less efficient lattice location. This phenomenon is tentatively referred to as "delocalization" of vacancy sites. Moreover, the fact that Y-doped zirconia series presents quite similar CL spectra compared to the undoped zirconia could be an evidence that the radiative centers of undoped and Y-doped ZrO2 are basically the same. A fitting procedure has been made aiming to give a rational description of the variation of the spectra morphology, and a parameter able to describe the monoclinic to tetragonal phase transformation has been found. This parameter and the overall set of CL data enabled us to quantitatively assess polymorphic phase fractions by CL spectroscopy in the scanning electron microscope.

Towards ultrastrong glasses

The development of new glassy materials is key for addressing major global challenges in energy, medicine, and advanced communications systems. For example, thin, flexible, and large-area glass substrates will play an enabling role in the development of flexible displays, roll-to-roll processing of solar cells, next-generation touch-screen devices, and encapsulation of organic semiconductors. The main drawback of glass and its limitation for these applications is its brittle fracture behavior, especially in the presence of surface flaws, which can significantly reduce the practical strength of a glass product. Hence, the design of new ultrastrong glassy materials and strengthening techniques is of crucial importance. The main issues regarding glass strength are discussed, with an emphasis on the underlying microscopic mechanisms that are responsible for mechanical properties. The relationship among elastic properties and fracture behavior is also addressed, focusing on both oxide and metallic glasses. From a theoretical perspective, atomistic modeling of mechanical properties of glassy materials is considered. The topological origin of these properties is also discussed, including its relation to structural and chemical heterogeneities. Finally, comments are given on several toughening strategies for increasing the damage resistance of glass products.