46079-G10
Molecular Dynamics Simulations of Time-Dependent and Transport Processes
In the first year of this project our goal was to develop a molecular dynamics code that will allow us to perform simulations to describe the microscopic response of nano-objects (quantum dots, nanowires, and nanotubes) to nonequilibrium electronic distributions. These nonequilibrium situations are encountered when matter is subjected to ultrafast laser pulses or to external applied voltages. This goal was accomplished: we amended the well-established ground-state molecular dynamics formalism, as implemented in the tight-binding molecular dynamics code Trocadero, to account for the fraction of electrons populating the excited states. This new computational capability has several distinct features. Specifically, molecular dynamics can be now performed in the presence of an external electromagnetic field, which was coupled by us to the atomic orbitals in a way that includes the wave polarization. Moreover, the resulting ion dynamics is highly accurate because it includes the correction due to the motion of atomic-orbital basis and because of transferable non-orthogonal tight-binding description of the solid. |new paragraph| In addition to completing the code development, we have also performed a study on the thermodynamic stability and optical properties of Si quantum dots (QD) and nanowires (NW). After extensive literature review, this study appeared necessary before the planned investigation of stability under nonequilibrium electronic distributions. The main findings are summarized next. Regarding the Si QDs, it is very important to first understand their optical absorption in order to next tune the laser pulse at the appropriate frequencies. While it was well known that QD's properties depend on size due to the quantum confinement effect, it was not clear if the quantum confinement (and the optical absorption) is modified by QD's shape changes. A very early theoretical investigation indicated that a Si QD shape does not have a notable effect on optical properties and this instilled a belief in the community that this effect is unimportant. Depending on the core structure and the arrangement of facets, QDs can have different symmetries. Our important finding was that the QD symmetry is an important player in controlling optical properties and in highly symmetric QDs, even small shape changes can dramatically affect the optical response. This is demonstrated in the shown toc figure, which summarizes our microscopic calculations carried out on two same-size Si QDs with slightly different symmetries/shapes: On the right we show the structure, frontier orbitals, energy levels, and optical spectrum for a Si QD with maximal Ih symmetry. On the left, we show a same size Si QD that has the central pentagonal rings (shaded in blue) of the Ih QD replaced with hexagonal ones. This “defect” leads to a “less perfect” D6d QD with a slightly oblate shape (~1.1 aspect ratio). Notably, the symmetry lowering does not affect the QD stability. However, it drives selected frontier orbitals into the gap (HOMO, LUMO, LUMO+1), while the other levels become dense. Optical properties are dramatically different. This is evidenced in the absorption spectrum, showing significant absorption in the visible range for the D6d QD but not for the Ih QD one. We attribute this extreme shape-sensitivity to the special spatial distribution of certain orbitals, which directly couples to the QD's width, shape, and height. |new paragraph| The study of Si NWs was directed towards understanding their structure, which is a prerequisite for the comprehending their optic response. Unlike carbon, which forms nanotubules with a hexagonal bonding network even at diameters extending down to a few nanometers, Si forms NWs. In spite of a large body of experimental and theoretical research, the ground state Si NW structure at the lowest diameters (less than 10 nm in diameter) is not known. Relying on thermodynamic arguments, one conjectures that in relatively thick NWs arrangements with bulk-like cores are more likely. However, as the diameter is decreased, surfaces and edges are becoming increasingly important in the NW energetic balance, and quasi-one-dimensional organizations with non-cubic core structures but low surface and edge energies are possible. Our calculations were able to identify the ground state Si NW structure, from the several candidate structures proposed in literature. |new paragraph| For the next year, we plan to use our unique code to perform a series of simulations to explore (i) the potential of electromagnetic fields (laser pulses) to induce transformations at the nanoscale, and (ii) the structural stability of current-carrying nanostructures. Specifically, by way of simulations we will be describing: The response of bare and hydrogen-functionalized carbon nanotubes to laser pulses. If any type of selectivity could be achieved, it will stimulate research towards methods for nanotube sample purification or reversible hydrogen chemisorption. (ii) The electrical and thermal stability of silicon dots, silicon nanowires, and carbon nanotubes. If nanostructures were to be employed in new sensors for improving exploration, our simulations will identify the thinnest wires capable to sustain current-induced forces under hostile high temperatures.
