Electronic Structure Of Nanocrystals Research Articles

Unraveling the atomic and electronic structure of nanocrystals on superconducting Nb(110): Impact of the oxygen monolayer

The niobium surface is almost always covered by a native oxide layer which greatly influences the performance of superconducting devices. Here we investigate the highly stable niobium oxide overlayer of Nb(110), which is characterized by its distinctive nanocrystal structure as observed by scanning tunneling microscopy (STM). Our ab initio density functional theory (DFT) calculations show that a subtle reconstruction in the surface niobium atoms gives rise to rows of fourfold coordinated oxygen separated by regions of threefold coordinated oxygen. The fourfold oxygen rows are determined to be the source of the nanocrystal pattern observed in STM and the two chemical states of oxygen observed in core-level x-ray photoelectron spectroscopy (XPS) are ascribed to the threefold and fourfold oxygens. Furthermore, we find excellent agreement between the DFT calculated electronic structure with scanning tunneling spectroscopy and valence XPS measurements.

Ab initio calculations of the electronic structure of silicon nanocrystals doped with shallow donors (Li, P)

The results of ab initio calculations of the electronic structure of Si nanocrystals doped with shallow donors (Li, P) are reported. It is shown that phosphorus introduces much more significant distortions into the electronic structure of the nanocrystal than lithium, which is due to the stronger central cell potential of the phosphorus ion. It is found that the Li-induced splitting of the ground state in the conduction band of the nanocrystal into the singlet, doublet, and triplet retains its inverse structure typical for bulk silicon.

Controlling the Optical Properties of Inorganic Nanoparticles

AbstractThe sophistication with which we can now prepare and characterize inorganic nanoparticles has inspired new areas of research into the fundamental properties and applications of these fascinating nanoscale systems. In this article some of the recent ideas concerning control of their optical properties are examined and explained, focusing on semiconductor nanocrystals. It is known that the optical properties of nanocrystals can be size‐tunable, but it is less obvious how shape matters. To explain how size as well as shape matters, the electronic structure of nanocrystals is sketched in relatively simple terms, leading to an introduction to deeper concepts at the heart of spectroscopy such as the exciton fine structure. The exciton fine structure states, although obscured by inhomogeneous line broadening, dictate selection rules for optical excitation. These viewpoints are compared and contrasted to well‐established principles in molecular spectroscopy that provide inspiration as well as perspective. The control of optical poperties is founded on our ability to prepare good quality colloidal particles. Recent advances in nanocrystal shape control are described. The current status of heterostructures is examined, with an emphasis on charge separation in CdSe–CdTe nanorods.

Formation and electronic structure of germanium nanocrystals formed by ion beam synthesis

Germanium nanocrystals embedded in SiO2 have been formed by ion implantation and subsequent thermal annealing. Ge74 ions were implanted utilising ion energy/dose combinations to yield a uniform Ge excess of ∼5at.% over the implanted layer. Subsequent thermal annealing at 1000°C in a 95% N2+5% H2 ambient for as little as 10s was sufficient to form nanocrystals approximately 2nm in diameter. Size distributions corresponding to differing annealing times were determined by TEM, with longer annealing times leading to the formation of larger nanocrystals. The electronic structure of the nanocrystal was studied by XAS at the Ge L3 edge, and shifts of the absorption edge position to higher energies with increasing nanocrystal size were observed. We discuss the latter in relation to quantum confinement effects.