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Manna, S.

Publications and source records attributed to Manna, S..

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Bragg coherent diffractive imaging of ferromagnetic nickel nanoparticles

We synthesized Ni nanoparticles with an atmospheric-thermal chemical vapor deposition method and investigated their lattice distortions and defects using Bragg coherent diffractive imaging. Because the strain can alter the magnetic properties of ferromagnetic materials, it is important to study the lattice distortion and structural imperfection. As a result of the Bragg coherent diffractive imaging, we found that the Ni nanocubes have a deformation-free zone in the middle. In addition, the defects such as dislocation and twin domain defects were detected in other Ni nanoparticles based on the Bragg diffraction images and patterns. The imaging of nanoparticles can contribute to a better understanding of their growth mechanisms and improve the design and synthesis of nanoparticles.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Thermodynamic Stability of Molybdenum Oxycarbides Formed from Orthorhombic Mo 2 C in Oxygen-Rich Environments

Molybdenum carbide (Mo 2 C) nanoparticles and thin films are particularly suitable catalysts for catalytic fast pyrolysis (CFP) as they are effective for deoxygenation and can catalyze certain reactions that typically occur on noble metals. Oxygen deposited during deoxygenation reactions may alter the carbide structure, leading to the formation of oxycarbides, which can determine changes in catalytic activity or selectivity. Despite emerging spectroscopic evidence of bulk oxycarbides, so far there have been no reports of their precise atomic structure or their relative stability with respect to orthorhombic Mo 2 C. This knowledge is essential for assessing the catalytic properties of molybdenum (oxy)carbides for CFP. In this article, we use density functional theory (DFT) calculations to (a) describe the thermodynamic stability of surface and subsurface configurations of oxygen and carbon atoms for a commonly studied Mo-terminated surface of orthorhombic Mo 2 C and (b) determine atomic structures for oxycarbides with a Mo:C ratio of 2:1. The surface calculations suggest that oxygen atoms are not stable under the top Mo layer of the Mo 2 C(100) surface. Coupling DFT calculations with a polymorph sampling method, we determine (Mo 2 C) x O y oxycarbide structures for a wide range of oxygen compositions. Oxycarbides with lower oxygen content (y/x = 2) adopt layered structures reminiscent of the parent carbide phase, with flat Mo layers separated by layers of oxygen and carbon; for higher oxygen content, our results suggest the formation of amorphous phases, as the atomic layers lose their planarity with increasing oxygen content. We characterize the oxidation states of Mo in the oxycarbide structures determined computationally, and simulate their X-ray diffraction (XRD) patterns in order to facilitate comparisons with experiments. Our study may provide a platform for large-scale investigations of the catalytic properties of oxycarbides and their surfaces and for tailoring the catalytic properties for different desired reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Observation of x-ray radiation pressure effects on nanocrystals

Bragg coherent diffractive imaging is a powerful technique that can be used to explore the internal structure and strain of nanoscale crystalline objects. During the data collection process, the Bragg peak typically stays within a small range of pixels on the x-ray sensitive area detector. Here, we report abrupt and irreversible Bragg peak movement during the coherent x-ray data collection process for both Pd nanocubes and a Ni nanowire. Here, we report that this phenomenon can be attributed to x-ray momentum transfer, also known as radiation pressure, to the nanocrystals. Understanding this effect is crucial given the anticipated coherent flux increases at next-generation synchrotron sources.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Direct band gap optical emission from Ge islands grown on relaxed Si{sub 0.5}Ge{sub 0.5}/Si (100) substrate

Strained Ge islands have been grown on fully relaxed Si{sub 0.5}Ge{sub 0.5} substrate by pulsed laser ablation technique. The formation of strained Ge islands has been found for film with higher thickness following Stranski–Krastanov growth mechanism. The variation of strain with changing Ge layer thickness has been analyzed using Raman spectroscopy and high-resolution X-ray diffraction techniques. X-ray photoelectron spectra have shown the absence of any Si-Ge intermixing and oxidation of Ge films. A strong no-phonon photoluminescence emission from Ge islands has been observed, showing the superior optical characteristics of the islands grown on relaxed substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Direct band gap optical emission from compressively strained Ge films grown on relaxed Si{sub 0.5}Ge{sub 0.5} substrate

Compressively strained Ge films have been grown on relaxed Si{sub 0.5}Ge{sub 0.5} virtual substrate in ultra high vacuum using molecular beam epitaxy. Structural characterization has shown that the Ge films are compressively strained with partial strain relaxation in a film thicker than 3.0 nm, due to onset of island nucleation. Photoluminescence spectra exhibit the splitting of degenerate Ge valence band into heavy hole and light hole bands with a broad direct band gap emission peak around 0.81 eV. Temperature and excitation power dependent emission characteristics have been studied to investigate the mechanism of luminescence quenching at high temperatures and the role of non-radiative recombination centers.

36 MATERIALS SCIENCE↗