Carla Molteni

Pressure-induced structural transformations in a medium-sized silicon nanocrystal by tight-binding molecular dynamics

We use a recently developed constant-pressure molecular dynamics method for nonperiodic systems to study pressure-induced structural transformations in medium-sized silicon nanocrystals, where the kinetics is experimentally known to be bulk rather than surface dominated, choosing Si705 as a representative example. Pressure is applied and tuned through a liquid described by a classical potential, while the nanocrystal is treated within a tight-binding scheme. Upon pressurization the nanocrystal undergoes a structural transformation which starts at the surface and gradually propagates into the bulk core. The high-pressure structure is disordered and metallic, with an x-ray diffraction pattern compatible with both the ideal β-tin and simple hexagonal structures. Strong similarities with a recently calculated high-pressure phase of bulk amorphous silicon are evident. Upon pressure release, the original diamond structure is not recovered and a high degree of disorder persists.

Material design from first principles: the case of boron nitride polymers

In recent years, first-principles quantum-mechanical simulations have become established as a complementary tool to experiments in the design and characterization of new materials. Here we illustrate this in the case of boron nitride (BN) analogues of conjugated organic polymers which offer a cheap alternative to inorganic semiconductors in the manufacture of electronic devices. By analogy with heterostructures, such as quantum wells and superlattices, currently used by the conventional semiconductor industry, we show how copolymers consisting of sections of carbon and BN can be designed to tune the electronic properties of these new materials.

Temperature and strain rate effects in grain boundary sliding

The authors have performed total energy density functional theory calculations to investigate the sliding process at the {Sigma} = 5 (001) twist grain boundary in germanium. The accurate quantum mechanical description of the interatomic bonding provides valuable insights into the mechanisms of bond breaking and remaking that occur during the sliding. In this paper the authors show how total energy calculations can be used to describe finite temperature and strain rate effects in this grain boundary.