Theoretical methods and results for electronic-structure investigations of amorphous carbon
Uwe Stephan Dissertation This work is concerned with methods and results for the calculation of electronic properties of amorphous carbon models (a-C). These investigations are based upon a very efficient non-selfconsistent ab-initio procedure for the evaluation of electronic states of extended sy...
| Summary: | Uwe Stephan
Dissertation
This work is concerned with methods and results for the calculation of
electronic properties of amorphous carbon models (a-C). These investigations
are based upon a very efficient non-selfconsistent ab-initio procedure for
the evaluation of electronic states of extended systems using modified self-
consistent DFT-LDA states and potentials of neutral atoms.
Starting from the LCAO matrices constructed in this method, the electronic
densities of states (DOS) of model systems are calculated by diagonalization
or with use of the recursion method. Both techniques and, in particular,
several versions of the recursion method will be investigated and compared
with respect to their numerical efficiency and practical applicability. For
DOS calculations in carbon systems a modification of the atomic SCF routine
will be proposed and tested in application to the crystalline carbon
allotropes diamond and graphite.
In this work, the investigation of a-C structures is based on various
structural models which have been generated in the author's research group by
means of molecular-dynamics simulations using the empirical Tersoff potential
as well as the just mentioned DFT-LDA approach. The total energy in this
latter procedure is calculated as the sum of the band-structure energy and an
empirical repulsive pair potential; contrary to the purely empirical approach,
this scheme therefore includes pi-bonding effects and gives rise to a superior
description of defect states in these models.
As suggested by an analysis of the localization properties of the eigenstates,
the defect structure in a-C models depends primarily on the ability of pi- and
weak-sigma-bonded undercoordinated atoms to cluster. To investigate these
clustering effects, a pi-bonding analysis will be proposed which enables the
quantification and classification of the defect states and the estimation of
gaps between pi bands. This procedure, which will be justified by local DOS
calculations, provides essential structure-property correlations in dependence
on the mass densities of the models. Within predominantly fourfold-coordinated
models, the occurrence of a certain fraction of threefold-coordinated atoms
turns out to stabilize the network by achieving optimum stress and defect
minimization due to the preferred formation of pi-bonded atom pairs. Such
models exhibit mass densities and pi gaps of about 3.0 g/cm^3 and 2.4 eV,
respectively, in close agreement with recent experimental results. |
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