Computational Physics at Carthage

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Computational Physics at Carthage (Kevin Crosby)

Molecular dynamics (MD) is a computation tool to approximate the interactions between atoms in a variety of materials. These interactions can be quite complex, and the number of atoms in the "computational cell" is typically very large. For these reasons, MD is usually implemented on very powerful computers or on a cluster of linked computers. At Carthage my reseach students have applied MD code to study a variety of problems in material science. We use a cluster of 12 computers shared with the Computer Science Department to run both classical and quantum-ab initio MD code.

Strain Modulation of Band Gaps in Carbon Nanotubes

Carbon nanotube (CNT) structures represent a recently discovered (1991) phase of carbon. The CNT structure is best described as a single graphene sheet rolled into a seamless tube. These single-walled CNTs (SWCNT) can be classified into three categories depending on the geometry of the graphene sheet edge: zig-zag, armchair, and chiral.

The electronic structure of a SWCNT is determined by its geometric properties. Approximately 2/3 of all SWCNTs are semiconducting, while the remainder are insulating. SWCNTs are increasingly studied for their potential uses as ultra-fast transistors and switches in next-generation computing and display technologies. We are studying the modulation of the band gap in SWCNTs with applied tensile strain. The ability to tune the band gap in SWCNT-based devices opens up exciting new possibilities with respect to scale and speed of computing and display technologies.

Fermi Distribution

 
Ballistic Deposition of Carbon-60 (buckyballs) Molecules onto Si-C Substrates

The mechanical properties of C-60 molecules make them exciting candidates for a variety of technological applications including drug delivery, high temperature lubricants, wear-protective coatings, opto-electronic devices, and other nanotechnologies. In many of these applications, it is desirable to produce thin films of C-60 molecules adhered to solid substrates. To study the dynamics of C-60 adhesion to a substrate of Si-C, we built and deployed MD code to simulate the ballistic deposition of C-60 molecules on an Si-C substrate over a range of impact parameters. This work resulted in a phase diagram describing the adhesion kinetics as a function of temperature and beam energy.
Impact of C-60 molecule on Si substrate
Structural Phase Transitions in Carbon-60 thin films

Thin films of C-60 molecules exhibit FCC lattice structures at low temperatures. At higher tempertures, different crystal geometries are produced. In this ongoing project, students explore the nature of the FCC to BCC structural phase transition of C-60 thin films. We are interested in determining the role of lattice defects in this transition.
C-60 Molecules deposited onto an Si-C substrate
Grain Boundary Diffusion in Nano-crystalline Copper

Thermal self-diffusion in metallic interconnects is a significant technological obstacle to further scale-reduction of integrated circuitry. Diffusion near gain boundaries and other crystal defects is greatly enhanced over bulk diffusion. Using molecular dynamics, we study the processes associated with diffusion near grain boundaries in nano-crystalline copper under tensile stress. A paper describing the essential results of this work is available here.

 

A section of a copper computational cell showing a coherent twist grain boundary on the central (111) crystallographic plane. The copper crystal is under uniaxial tensile stress applied at the (111) faces.
Molecular Dynamics Studies of Stress Voids in Copper Thin Films

Integrated circuitry relies on the integrity of thin metallic films adhesively bonded to solid substrates. These films, which provide the conducting pathways between transistor elements within the circuit, are increasingly driven to new scales of miniaturization. Typically these films are under very high tensile stress applied by a rigid substrate. These stresses arise from deposition conditions and can be in excess of hundreds of MPa for aluminum and copper films on silicon substrates. High internal stresses are undesirable in most applications and can result in the mechanical failure of the film through the growth of “stress voids” nucleated within the film. Atomic migration is enhanced near such voids leading to open-circuit failure of a current-carrying interconnect. I have a variety of projects related to the investigation of diffusion near such defects under mechanical stress.