Per Erik Vullum

Positioning effects on quantum dot solar cells grown by molecular beam epitaxy

We report current-voltage and spectral response characteristics of high density InAs/GaAs quantum dot (QD) solar cells with different positions where dots are located. The short circuit current density (Jsc), open circuit voltage (Voc), and external quantum efficiency of these cells under air mass 1.5 are presented and compared with a GaAs reference cell. An extended photoresponse in contrast to the GaAs reference cell was confirmed for all these cells. The effect of inserting QD layers into emitter and base region on device performance is shown. The Jsc is reduced, while the Voc is maintained. The cell with QDs located toward the base side shows better performance, confirmed by both current-voltage and spectral response measurements.

Structural phases driven by oxygen vacancies at the La0.7Sr0.3MnO3/SrTiO3 hetero-interface

An oxygen vacancy driven structural response at the epitaxial interface between La0.7Sr0.3MnO3 films and SrTiO3 substrates is reported. A combined scanning transmission electron microscopy and electron energy loss spectroscopy study reveal the presence of an elongated out-of-plane lattice parameter, coupled to oxygen vacancies and reduced manganese oxidation state at the La0.7Sr0.3MnO3 side of the interface. Density functional theory calculations support that the measured interface structure is a disordered oxygen deficient brownmillerite structure. The effect of oxygen vacancy mobility is assessed, revealing an ordering of the vacancies with time.

TEM observations of rhombohedral and monoclinic domains in LaCoO3-based ceramics

Twin structures in rhombohedrally distorted polycrystalline LaCoO3, La0.8Ca0.2CoO3-δ and La0.5Sr0.5Co0.5Fe0.5O3-δ have been studied by transmission electron microscopy. Normal {100} and {110} twinning was predominant in LaCoO3 and La0.8Ca0.2CoO3-δ. In addition, ferroelastic domains with a one-dimensional superstructure corresponding to a tripling of the pseudocubic lattice parameter, a c, were found in some grains. The monoclinic superstructure vanished over time and was sensitive to external mechanical stress. In La0.5Sr0.5Co0.5Fe0.5O3-δ no normal twinning was observed, but domains with a one-dimensional superstructure of two times a c were observed. The superstructures have been interpreted as atomic scale twinning, which reduce the crystal symmetry from rhombohedral to monoclinic.

Surface stability of epitaxial La0.7Sr0.3MnO3 thin films on (111)-oriented SrTiO3

We report on the stability of the La0.7Sr0.3MnO3 thin film surface when deposited on (111)-oriented SrTiO3. For ultrathin La0.7Sr0.3MnO3 films, an initial 3-dimensional morphology is observed, which becomes 2-dimensional with increasing film thickness. For even thicker samples, we show that the surface morphology evolves from 2-dimensional to 3-dimensional and that this observation is consistent with an Asaro-Tiller-Grinfeld instability, which can be controlled by the deposition temperature. This allows for synthesis of films with step-and-terrace surfaces over a wide range of thicknesses. Structural characterization by x-ray diffraction and transmission electron microscopy shows that the films are strained to the SrTiO3 substrate and reveals the presence of an elongated out-of-plane lattice parameter at the interface with SrTiO3.

3D aligned-carbon-nanotubes@Li2FeSiO4 arrays as high rate capability cathodes for Li-ion batteries

3D aligned-carbon-nanotubes (ACNTs)@Li2FeSiO4 nanocomposite arrays on Al foil were developed as cathode materials for Li-ion batteries. The ACNTs were grown directly on an Al foil by a chemical vapor deposition method to achieve a 3D current collector structure for direct charge transport. Li2FeSiO4 nanoparticles were deposited on the surface of the ACNTs by a polyvinylalcohol (PVA)-assisted sol-gel method. The 3D samples showed a high degree of alignment of nanotubes with a favorable pore morphology before and after cycling. According to electrochemical measurements, the 3D sample with optimized mass ratio of ACNTs and Li2FeSiO4 (2:1) showed excellent rate capability and capacity retention, delivering a discharge specific capacity of 142 mAh g(-1) at a rate of 0.5 C (C = 160 mAg(-1)) and maintaining 99% of the initial discharge capacity after 50 cycles at 24 ° C. Up to 20 C, the delivered charge/discharge capacity was 94 mAh g(-1) after 172 cycles, which is 54% of the value obtained at C/20 (175 mAh g(-1)). In comparison, carbon coated nanoporous Li2FeSiO4 obtained under analogous conditions by a PVA-assisted sol-gel method can only deliver a capacity of 80 mAh g(-1) and showed poor rate capability. In addition, despite amorphization, dissolution and chemical composition changes occurring in the 3D samples upon extended cycling, the 3D samples showed good long-term cycling stability at a high current density (5 C), maintaining ~80% of the initial discharge capacity after 1000 cycles and ~70% after 2000 cycles.

Atomap: a new software tool for the automated analysis of atomic resolution images using two-dimensional Gaussian fitting

Scanning transmission electron microscopy (STEM) data with atomic resolution can contain a large amount of information about the structure of a crystalline material. Often, this information is hard to extract, due to the large number of atomic columns and large differences in intensity from sublattices consisting of different elements. In this work, we present a free and open source software tool for analysing both the position and shapes of atomic columns in STEM-images, using 2-D elliptical Gaussian distributions. The software is tested on variants of the perovskite oxide structure. By first fitting the most intense atomic columns and then subtracting them, information on all the projected sublattices can be obtained. From this, we can extract changes in the lattice parameters and shape of A-cation columns from annular dark field images of perovskite oxide heterostructures. Using annular bright field images, shifts in oxygen column positions are also quantified in the same heterostructure. The precision of determining the position of atomic columns is compared between STEM data acquired using standard acquisition, and STEM-images obtained as an image stack averaged after using non-rigid registration.

Assessing electron beam sensitivity for SrTiO3 and La0.7Sr0.3MnO3 using electron energy loss spectroscopy

Thresholds for beam damage have been assessed for La0.7Sr0.3MnO3 and SrTiO3 as a function of electron probe current and exposure time at 80 and 200kV acceleration voltage. The materials were exposed to an intense electron probe by aberration corrected scanning transmission electron microscopy (STEM) with simultaneous acquisition of electron energy loss spectroscopy (EELS) data. Electron beam damage was identified by changes of the core loss fine structure after quantification by a refined and improved model based approach. At 200kV acceleration voltage, damage in SrTiO3 was identified by changes both in the EEL fine structure and by contrast changes in the STEM images. However, the changes in the STEM image contrast as introduced by minor damage can be difficult to detect under several common experimental conditions. No damage was observed in SrTiO3 at 80kV acceleration voltage, independent of probe current and exposure time. In La0.7Sr0.3MnO3, beam damage was observed at both 80 and 200kV acceleration voltages. This damage was observed by large changes in the EEL fine structure, but not by any detectable changes in the STEM images. The typical method to validate if damage has been introduced during acquisitions is to compare STEM images prior to and after spectroscopy. Quantifications in this work show that this method possibly can result in misinterpretation of beam damage as changes of material properties.

Effect of Sb Segregation on Conductance and Catalytic Activity at Pt/Sb-Doped SnO2 Interface: A Synergetic Computational and Experimental Study

Antimony-doped tin dioxide (ATO) is considered a promising support material for Pt-based fuel cell cathodes, displaying enhanced stability over carbon-based supports. In this work, the effect of Sb segregation on the conductance and catalytic activity at Pt/ATO interface was investigated through a combined computational and experimental study. It was found that Sb-dopant atoms prefer to segregate toward the ATO/Pt interface. The deposited Pt catalysts, interestingly, not only promote Sb segregation, but also suppress the occurrence of Sb(3+) species, a charge carrier neutralizer at the interface. The conductivity of ATO was found to increase, to a magnitude close to that of activated carbon, with an increment of Sb concentration before reaching a saturation point around 10%, and then decrease, indicating that Sb enrichment at the ATO surface may not always favor an increment of the electric current. In addition, the calculation results show that the presence of Sb dopants in ATO has little effect on the catalytic activity of deposited three-layer Pt toward the oxygen reduction reaction, although subsequent alloying of Pt and Sb could lower the corresponding catalytic activity. These findings help to support future applications of ATO/Pt-based materials as possible cathodes for proton exchange membrane fuel cell applications with enhanced durability under practical applications.

Quantitative strain analysis of InAs/GaAs quantum dot materials

Geometric phase analysis has been applied to high resolution aberration corrected (scanning) transmission electron microscopy images of InAs/GaAs quantum dot (QD) materials. We show quantitatively how the lattice mismatch induced strain varies on the atomic scale and tetragonally distorts the lattice in a wide region that extends several nm into the GaAs spacer layer below and above the QDs. Finally, we show how V-shaped dislocations originating at the QD/GaAs interface efficiently remove most of the lattice mismatch induced tetragonal distortions in and around the QD.

Magnetic domain configuration of (111)-oriented LaFeO3 epitaxial thin films

In antiferromagnetic spintronics control of the domains and corresponding spin axis orientation is crucial for devices. Here we investigate the antiferromagnetic axis in (111)-oriented LaFeO3/SrTiO3, which is coupled to structural twin domains. The structural domains have either the orthorhombic a- or b-axis along the in-plane ⟨11¯0⟩ cubic directions of the substrate, and the corresponding magnetic domains have the antiferromagnetic axis in the sample plane. Six degenerate antiferromagnetic axes are found corresponding to the ⟨11¯0⟩ and ⟨112¯⟩ in-plane directions. This is in contrast to the biaxial anisotropy in (001)-oriented films and reflects how crystal orientation can be used to control magnetic anisotropy in antiferromagnets.