Manyalibo J. Matthews

Bio: Manyalibo J. Matthews is an academic researcher from Lawrence Livermore National Laboratory. The author has contributed to research in topics: Laser & Raman spectroscopy. The author has an hindex of 40, co-authored 199 publications receiving 7341 citations. Previous affiliations of Manyalibo J. Matthews include Alcatel-Lucent & Massachusetts Institute of Technology.

Additively manufactured hierarchical stainless steels with high strength and ductility

Abstract: Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.

Origin of dispersive effects of the Raman D band in carbon materials

Abstract: The origin and dispersion of the anomalous disorder-induced Raman band $(D$ band) observed in all ${\mathrm{sp}}^{2}$ hybridized disordered carbon materials near 1350 ${\mathrm{cm}}^{\ensuremath{-}1}$ is investigated as a function of incident laser energy. This effect is explained in terms of the coupling between electrons and phonons with the same wave vector near the K point of the Brillouin zone. The high dispersion is ascribed to the coupling between the optic phonons associated with the D band and the transverse acoustic branch. The large Raman cross section is due to the breathing motion of these particular phonons near the K point. Our model challenges the idea that the Raman D peak is due to laser-energy-independent features in the phonon density of states, but rather is due to a resonant Raman process.

Raman and resonance Raman investigation of MoS 2 nanoparticles

Abstract: Raman and resonance Raman spectra of ${\mathrm{MoS}}_{2}$ nanoparticles, in the form of inorganic fullerenelike nanoparticles with diameters ranging from 200 to 2000 \AA{} in size, and platelets ranging from 50 to 5000 \AA{} in size, are presented. Off resonance, the first-order Raman bands are broadened, but not significantly shifted, and no additional bands are observed, indicating that the atomic structure is preserved, at least locally, in the nanoparticles. The broadening effect is assigned to phonon confinement by facet boundaries. In the resonance Raman spectra of the nanoparticles, several additional first-order peaks are observed. The electron-phonon coupling responsible for the strong-resonance conditions is identified through dynamic band calculations. Using temperature-dependent resonance Raman measurements, we assign these peaks to zone-boundary phonons activated by disorder and finite-size effects. By analyzing the position of the dispersive peak at 429 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ under resonance conditions, it was possible to probe the softening of modes propagating in the c-axis direction.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Raman spectroscopy as a versatile tool for studying the properties of graphene

Abstract: Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.