Bi Nanolines Research Articles

The geometry of Bi nanolines on Si(0 0 1)

A study of the Bi nanoline geometry on Si(0 0 1) has been performed using a combination of ab initio theoretical technique and scanning tunnelling microscopy (STM). Our calculations demonstrate decisively that the recently proposed Haiku geometry is a lower energy configuration than any of the previously proposed line geometries. Furthermore, we have made comparisons between STM constant-current topographs of the lines and Tersoff–Haman STM simulations. Although the Haiku and the Miki geometries both reproduce the main features of the constant-current topographs, the simulated STM images of the Miki geometry have a dark stripe between the dimer rows that does not correspond well with experiment.

Short range and long range strain fields of Bi nanoline

The practical realisation of nanoscale devices requires the development of practical nanofabrication techniques. The Bi nanolines which self-assemble on Si(001) are promising templates for atomic scale wires, and also have a fascinating subsurface reconstruction. Elastic interactions are often responsible for the limited dimensionality of epitaxial nanoscale structures. The present work examines the elastic strain field around the Bi nanoline in terms of its interaction with other surface features. A short range tensile strain around the Bi nanoline may be identified by the effect on the electronic structure of the neighbouring dimers. A longer range elastic interaction is exhibited in a repulsive interaction between the nanoline and defects and steps. Atomic resolution variable bias scanning tunnelling microscopy (STM) and first principles electronic structure calculations have been used to elucidate these effects, and excellent agreement has been found between experimental observations and theoretical results.

Interaction between electronic structure and strain in Bi nanolines on Si(0 0 1)

Heteroepitaxial strain can be a controlling factor in the lateral dimensions of 1-D nanostructures. Bi nanolines on Si(0 0 1) have an atomic structure which involves a large sub-surface reconstruction, resulting in a strong elastic coupling to the surrounding silicon. We present variable-bias STM and first principles electronic structure calculations of the Bi nanolines, and show that the strain associated with the nanolines affects the atomic and electronic structure of at least two neighbouring Si dimers, and identify the mechanism behind this. We also present partial charge densities (projected by energy) for the nanoline with clean and hydrogenated surroundings and contrast them to the clean Si(0 0 1) surface.

Structure of Bi nanolines: using tight binding to search parameter space

We describe how we have used tight-binding calculations as a quick, efficient toolto search for possible structures of Bi nanolines on Si(001). After identifyingpromising candidate structures, we have concentrated on these with ab initioelectronic structure techniques. The energetics of the tight binding areshown to be in good agreement with the density functional calculationsand with experimental observations, and have proved invaluable in thesearch for a structure, validating the use of tight binding as a search tool.

Stress relief as the driving force for self-assembled bi nanolines.

Bi nanolines self-assemble on Si(001) and are remarkable for their straightness and length-they are often more than 400 nm long, and a kink in a nanoline has never been observed. Through electronic structure calculations, we have found an energetically favorable structure for these nanolines that agrees with our scanning tunneling microscopy and photoemission experiments; the structure has an extremely unusual subsurface structure, comprising a double core of seven-membered rings of silicon. Our proposed structure explains all the observed features of the nanolines, and shows that surface stress resulting from the mismatch between the Bi and the Si substrates is responsible for their self-assembly. This has wider implications for the controlled growth of nanostructures on semiconductor surfaces.

Bi nanoline passivity to attack by radical hydrogen or oxygen

Nanolines of Bi 1.5 nm wide and more than 400 nm long way be grown on Si(001). Although they are not conducting themselves, they may be used as templates for nanowires of other metals, such as Al or Ga. To this end, we have found that these nanolines are stable against exposure to both atomic hydrogen and ozone. Hydrogen will passivate the silicon dimers, acting as an atomic-scale mask. Oxidation of the silicon surrounding the nanoline would allow for electrical isolation of the wire from the substrate, reducing or eliminating leakage currents into the substrate.