Colloquium
Dept of Physics & Astronomy
Presents
a candidate for the
Nanophysics position
in Physics & Astronomy
Electronic
Structure of Semiconductor Nanocrystals:
Influence of the Surface
For a
thorough understanding of nanocrystalline material
systems, knowledge of band gap widening and band alignment as a function of
particle size, chemical doping, and surface termination is critical to rational
design and utilization of these novel materials. Tailoring the electronic properties of
nanostructures is often achieved by confining electrons to dimensions
comparable to their wavelength, which leads to quantum well states that modify
the density of states. Until recently, the effect of the surface layer on the
electronic and structural properties of the quantum dots has been neglected due
to the inherent difficulty in both modeling and measuring this region. In this presentation, I will discuss efforts
into understating the impact the surface region of nanomaterials
has on the overall properties of the crystal.
Soft
x-ray probes such as x-ray absorption spectroscopy, photoemission spectroscopy,
and soft x-ray fluorescence experiments are used to determine the band edge
shifts and doping effects on silicon, germanium, and cadmium selenide nanomaterials in order
to better understand the electronic properties.
Clean Si and Ge nanocrystals films exhibit strong quantum confinement
effects at the band edges with sub-monolayer coverages,
but show a loss in band edge shifts at higher coverages. This is due to the particle-particle
interaction which can be controlled using an in situ surface passivation scheme.
Introduction of surface ligands to the Si and Ge systems induce strong
changes in the observed band edge shifts.
For comparison, band edge shifts in CdSe nanocrystal films were investigated. It was found that Cd
s states located at the conduction band minimum (CBM) show drastic size
dependencies while the Cd hybridized p levels
located a few electron volts up from the CBM show more localized behavior. Interestingly, changing the surface ligand has very little effect on the shift of the band
edges, but does affect the local bonding significantly. These changes manifest themselves in induced
magnetism in the undoped CdSe
nanocrystal system.
Lastly, the impact that chemical dopants have
on the nanocrystal electronic structure is
discussed. In CdSe,
the dopants can be used as a local probe for
identifying changes in nanocrystal surface
structure. In addition, the chemical
environment of Er in Si nanoparticles is probed and the band edge shifts as a
function of both particle size and Er concentration
are measured. It is found that the 1.5
mm infrared luminescence can be achieved with proper passivation
of the particles. Overall, these
experiments all show that the surface physics of nanostructured
materials are complex and cannot be ignored for a complete understanding of the
nanoparticle electronic structure.
Arthur St. John Hill Auditorium