William J. Bowman

Measuring bandgap states in individual non-stoichiometric oxide nanoparticles using monochromated STEM EELS: The Praseodymium-ceria case.

We describe a method to perform high spatial resolution measurement of the position and density of inter-band impurity states in non-stoichiometric oxides using ultra-high energy resolution electron energy-loss spectroscopy (EELS). This can be employed to study optical and electronic properties of atomic and nanoscale defects in electrically-conducting and optically-active oxides. We employ a monochromated scanning transmission electron microscope with subnanometer diameter electron probe, making this technique suitable for correlating spectroscopic information with high spatial resolution images from small objects such as nanoparticles, surfaces or interfaces. The specific experimental approach outlined here provides direct measurement of the Pr inter-band impurity states in Pr0.1Ce0.9O2-δ via valence-loss EELS, which is interpreted with valence-loss spectral simulation based on density of states data to determine the energy level and character of the inter-band state. Additionally, observation of optical color change upon chemically-induced oxygen non-stoichiometry indicates that the population of the inter-band state is accompanied by an energy level shift within the bandgap.

Challenges and Opportunities in Materials Science with Next Generation Monochromated EELS

The recent development of monochromated scanning transmission electron microscopes (STEM) offering energy resolutions of better than 20 meV and electron probes of 1 A in size provides a completely new tool to materials characterization. Unique opportunities opened by access to ultra-high energy resolution low loss EELS include determination of optical properties in the IR, bandgap mapping, detection of defect interband states and localized vibrational spectroscopy. While techniques such as Raman and IR spectroscopy have been routinely used to characterize phonon modes in solids for many years, the vibrational excitations probed with focused fast electron beams is largely unexplored. At ASU we are currently applying ultra-high energy resolution low-loss EELS to a variety of materials that are important in fields such as energy, environmental science and information technology. Here we show representative initial results on carbons, oxides and metal hydrides. All data were acquired on a newly installed Nion UltraSTEM equipped with a probe corrector and monochromator [1].

Interfacial Strain Mapping and Chemical Analysis of Strained-Interface Heterostructures by Nanodiffraction and Electron Energy-Loss Spectroscopy

Strain engineering is a relevant optimization route to introduce and/or optimize defects for mixed ionicelectronic conducting oxides. Interfacial strain control of electrical conductivity [1] and resistive switching [2] was reported for sideways-contacted Gd0.1Ce0.9O2-δ|Er2O3 (GCO|ERO) ‘microdot’ heterostructures with alternating monolayers of insulating ERO and mixed-conducting GCO, whose lattice mismatch yielded compressive strain in the GCO layers. Here we explore these and other GCO heterostructures with alternative straining oxides that impart varying degrees of tensile strain on GCO, such as Bi4NbO8.5|GCO (BNO|GCO). We apply local strain and chemical mapping, along with high resolution imaging in the TEM and scanning TEM (STEM) to provide nanoscale insights regarding strained heterostructure design.

Local Mapping of Bandgap Electronic State in PrxCe1-xCh2-δ: Elucidating Enhancement and Mechanism of Grain Boundary Electrical Conductivity

Grain boundary engineering presents an interesting route to optimize the properties of polycrystalline materials. For instance, electrical conductivity of polycrystalline mixed ionic-electronic conducting oxides can be modulated by tuning local charge carrier concentration (ionic and/or electronic). We previously reported a factor 100 enhancement of grain boundary electrical conductivity in a polycrystalline GdxCe1-xO2-δ solid solution resulting from the addition of Pr solute cations, i.e. (Pr,Gd)xCe1-xO2-δ [1]. It was speculated that modification of grain boundary electrical properties is related to enhanced electronic conductivity stemming from the population and depopulation of the interbandgap Pr 4f electronic level [2] localized within nanometers of grain boundary cores. Here, we apply monochromated EELS to model Pr0.1Ce0.9O2-δ (PCO) thin film specimens to map this bandgap state at the nanometer level to elucidate grain boundary transport mechanisms.

Correlated Electron Microscopy across Length Scales to Elucidate Structural, Electrical and Chemical Properties of Oxide Grain Boundaries

Because of their favorable ionic and/or electronic conductivity, non-stoichiometric oxides are utilized for energy storage, energy conversion, sensing, catalysis, gas separation, and information technologies, both potential and commercialized. Charge transport in these materials is influenced strongly by grain boundaries, which exhibit fluctuations in composition, chemistry and atomic structure within Ångstroms or nanometers [1-3]. Here, studies are presented that elucidate the interplay between macroscopic electrical conductivity, microscopic character, and local composition and electronic structure of grain boundaries in polycrystalline CeO2-based solid solutions. Electron energy-loss spectroscopy (EELS) in the aberration-correction scanning transmission electron microscope (AC-STEM) is used to quantify local composition and electronic structure. Electron diffraction orientation imaging microscopy is employed to assess microscopic grain boundary character, and links macroand nanoscopic techniques. These correlated experimental approaches provide unique insights into fundamental GB science, and highlights how novel aspects of nanoscale GB engineering may be manipulated to control ion transport properties in electroceramics.

Bandgap State Mapping via Valence-Loss EELS at Grain Boundaries in Non-Stoichiometric Pr x Ce 1-x O 2-δ

We recently reported the enhancement of grain boundary electrical conductivity in polycrystalline GdxCe1xO2-δ solid solution resulting from the addition of Pr [1]. The Pr concentration at grain boundaries was measured via STEM EELS to be approximately three times greater than adjacent grains. Thus it is believed that the modification of grain boundary electrical properties is related to the population and depopulation of a bandgap electronic state associated with the Pr 4f level [2] localized to within 2 nm – 3 nm of the boundaries. To elucidate the nature of this bandgap state, here we employ monochromated valence-loss EELS using a Nion UltraSTEM to spatially map—and correlate with composition—the position and occupancy of the Pr 4f level at grain boundaries in PrxCe1-xO2-δ.

Nanocharacterization of Strontium Titanate Thin Films and Oxide-Electrode Interfaces in Resistive Switching Devices

Thin film resistive switching devices based on perovskite SrTiO3 (STO) are the subject of recent studies focused on defect kinetics [1] and electrochemical switching mechanisms [2]—work which has addressed critical aspects of material performance and device design [3]. However, nanoand atomic-scale understanding of switching mechanisms, and the role of device fabrication parameters on switching behavior and device performance is an ongoing area of research [2]. Further, multi-bit architectures offer higher device density, so guidelines for design, fabrication and characterization of these devices is desired. We investigate nanoand atomic-scale aspects of singleand stacked multi-bit thin film resistive switching devices with varying electrode materials and thin film stacking schemes. From a materials perspective, we focus on oxide microstructure, potential highly defective zones, and their interfaces with the different electrodes employed.

Nanoscale Strain and Composition Mapping in Ionic Thin Film Heterostructures for Resistive Switching Devices

Interfacial strain control of electrical conductivity and resistive switching behavior in multilayer thin film oxide heterostructures has recently been reported for Gd0.1Ce0.9O2-δ|Er2O3 (GCO|ERO) heterostructures fabricated using pulsed laser deposition [1]. Strain control was achieved by growing alternating layers of insulating and conducting oxides (fig. 1a,b) whose dissimilar lattice constants result in interfacial lattice mismatch. Total strain in the conducting layers (i.e. GCO) is modulated via choice of insulating oxide and/or total interface count at constant device thickness (fig. 1a,b shows 275 nm tall devices with 6 and 60 interfaces, respectively). Furthermore, depending on the composition of the insulating phase, tensile or compressive strain may be imparted on the conducting GCO. Here we employ precession-enhanced nanodiffraction (PEND) and electron energy-loss spectroscopy (EELS) in an aberration-corrected STEM to assess the nanoscale strain and compositional distribution in layers of a strained heterostructure microdevice.

Oxygen Vacancies at Grain Boundaries in Doubly-Doped Ceria Determined using EELS

In oxygen conducting ceramics such as cerium-based oxides, O 2diffusion occurs via vacancy hopping. The vacancy concentration can be modulated through doping with aliovalent cations such as Gd 3+ or Pr 3+ . Sluggish ionic conductivity in these electrolytes has been attributed to various defects which increase the activation energy for anion migration. The association of mobile oxygen vacancies with dopant cations, and the presence of highly resistive grain boundaries in polycrystalline electrolytes are well-accepted mechanisms which degrade total ionic conductivity [1]. The predominant explanation for high grain boundary resistivity in ceramics of high purity is the space charge double layer (SCDL) which results in vacancy-depleted regions emanating from grain boundary cores [2]. Recently it was shown that addition of 0 – 2 at% transition metal (TM) ions such as Cr, Fe, Ni and Cu to high purity gadolinium-doped ceria (GDC) enhanced grain boundary electrical conductivity by as much as 15 times by reducing the SCDL potential barrier [3]. Here we use a combination of impedance spectroscopy (EIS) and electron energy-loss spectroscopy (EELS) to characterize the electrical conductivity and vacancy concentration of grain boundaries in ceria doubly-doped with Gd and Pr.

Nanocharacterization and Electrical Properties of Grain Boundaries in Gd/Pr Doubly-Doped Ceria

Grain boundaries in doped ceria electrolytes have a deleterious effect on the total ionic conductivity especially at intermediate temperatures (300 550 °C) [1,2]. The high resistivity of grain boundaries has been attributed to a space charge double layer which is believed to create a vacancy depletion region emanating from grain interfaces. Other factors may also contribute to high grain boundary resistance. For example, recent high resolution elemental analysis in the transmission electron microscope (TEM) of 20% Gd-doped ceria (GDC) by our group and others shows significant Gd segregation to grain boundaries yielding enrichment zones of approximately 60 at% Gd, far exceeding the optimal Gd concentration (10 20 at%) for maximum ionic conductivity.