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Koirala, Pratik

Publications and source records attributed to Koirala, Pratik.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Structure of the (110) Ln ScO 3 ( Ln =Gd,Tb,Dy) surfaces

The surface reconstruction on the (110) surface of LnScO 3 (Ln=Gd,Tb,Dy) has been investigated using x-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron diffraction coupled with higher-level density-functional theory methods. The experimental techniques generate constraints on the surface chemistry and structure that are used to inform the theoretical comparison of a number of potential surface structures. The resulting structure is then compared to experimental aberration-corrected transmission electron microscopy results using multislice simulation. The surfaces of both single-crystalline substrates and hydrosauna synthesized nanoparticles exhibit a Sc-rich double layer with a Sc 3 O 4 termination.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Al rich (111) and (110) surfaces of LaAlO 3

Lanthanum aluminate (LaAlO 3 ) has garnered significant interests since the discovery of a 2D electron gas at the LaAlO 3 /SrTiO 3 interface. Here in this work, the (110) surface of LaAlO 3 was studied using a combination of transmission electron microscopy plan and profile imaging, image simulation, electron diffraction, X-ray photoelectron spectroscopy, and density functional theory calculations. Profile imaging of the (110) surface was performed on a (111) oriented single crystalline sample with an Al rich surface. Our results indicate that high annealing temperatures (1000–1300°C) commonly used in preparing well ordered, flat surfaces of LaAlO 3 result in Al rich surfaces. The (110) surface was found to be single layered AlO x with a (2 × 1) reconstruction. Angle resolved X-ray photoelectron spectroscopy indicated that the (2 × 1) surface structure was not hydrated. Convex hull construction of surface enthalpies as a function of excess AlO 1.5 on the (110) surface was done using density functional theory calculations.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Direct Observation of Large Flexoelectric Bending at the Nanoscale in Lanthanide Scandates

There is a growing interest in the flexoelectric effect, since at the nanoscale it is predicted to be very large. However, there have been no direct observations of flexoelectric bending consistent with current theoretical work that implies strains comparable to or exceeding the yield strains of typical materials. In this work, we show a direct observation of extraordinarily large, two-dimensional reversible bending at the nanoscale in dysprosium scandate due to the converse flexoelectric effect, with similar results for terbium and gadolinium scandate. Within a transmission electron microscope, thin features bend up to 90° with radii of curvature of about 1 μm, corresponding to very large nominal strains. Analysis including independent experimental determination of the flexoelectric coefficient is semiquantitatively consistent with interpreting the results as due to flexoelectricity. These results experimentally demonstrate large flexoelectric bending at the nanoscale.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Electronic structure of lanthanide scandates

Here, x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and density functional theory calculations were used to study the electronic structure of three lanthanide scandates: GdScO 3 , TbScO 3 , and DyScO 3 . X-ray photoelectron spectra simulated from first-principles calculations using a combination of on-site hybrid and GGA + U methods were found to be in good agreement with experimental x-ray photoelectron spectra. The hybrid method was used to model the ground state electronic structure and the GGA + U method accounted for the shift of valence state energies due to photoelectron emission via a Slater-Janak transition state approach. From these results, the lanthanide scandate valence bands were determined to be composed of Ln4 f , O2 p , and Sc3 d states, in agreement with previous work. However, contrary to previous work the minority Ln4 f states were found to be located closer to, and in some cases at, the valence band maximum. This suggests that minority Ln4 f electrons may play a larger role in lanthanide scandate properties than previously thought.

36 MATERIALS SCIENCE↗
DOE OSTI2017

ALD synthesis of platinum nanoparticles on single-crystal SrTiO 3 pretreated with wet chemical etching

Formic acid-hydrogen peroxide and buffered hydrogen fluoride-hydrogen peroxide solutions were used to etch single-crystal strontium titanate to remove carbon contamination and increase hydroxyl group density to improve atomic layer deposition onto these materials. X-ray photoelectron spectroscopy indicated that both are effective for carbon contamination removal. However, for increasing hydroxyl group density on the strontium titanate surface, the buffered hydrogen fluoride-hydrogen peroxide is more effective. Transmission electron microscopy and x-ray photoemission show enhanced platinum deposition on the etched strontium titanate surface. Here these results provide the basis for optimizing atomic layer deposition for other technologically relevant materials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

When does atomic resolution plan view imaging of surfaces work?

We present that surface structures that are different from the corresponding bulk, reconstructions, are exceedingly difficult to characterize with most experimental methods. Scanning tunneling microscopy, the workhorse for imaging complex surface structures of metals and semiconductors, is not as effective for oxides and other insulating materials. This paper details the use of transmission electron microscopy plan view imaging in conjunction with image processing for solving complex surface structures. We address the issue of extracting the surface structure from a weak signal with a large bulk contribution. This method requires the sample to be thin enough for kinematical assumptions to be valid. The analysis was performed on two sets of data, c(6×2) on the (100) surface and (3×3) on the (111) surface of SrTiO 3 , and was unsuccessful in the latter due to the thickness of the sample and a lack of inversion symmetry. In conclusion, the limits and the functionality of this method are discussed.

36 MATERIALS SCIENCE↗