|Abstract: ||Nanostructured metal sulfides (MS) have attracted great interest in
recent years because of the possibility to synthesize nanoparticles from
solution and tuning their optoelectronic properties through quantum
confinement phenomena. A large number of applications have been
reported in the field of solar cells, light-emitting diodes, lithium-ion
batteries, thermoelectric devices, sensors, fuel cells and nonvolatile
memory devices. Among the large family of semiconductor sulfides,
the present Thesis is focussed on bismuthinite. The choice was motivated
by a combination of factors. Its non-toxicity, low cost synthesis
and high absorption properties make Bi2S3 a promising material for
several application, such as solid-state semiconductor-sensitized solar
cells. The intrisic anisotropy in the crystal structure of this material
facilitates the formation of elongated nanostructures, in particular
nanorods, nanoribbons, and nanowires. These structures find important
applications in many nanodevices, for example field emitters,
solar cells, and lithium-ion batteries.
On the other hand, researchers are still far from a complete understanding
of Bi2S3 properties. The colloidal synthesis of bismuthinite
nanostructures, although cheap and environmentally friendly, does
not allow a perfect control on stoichiometry and surface passivation.
The large majority of the experimental studies does not report photoluminescence
of the nanocrystals, which indicates the presence of
trap states and a low defect tolerance of the material. These facts
cause a lower efficiency of the devices based on Bi2S3 nanoparticles
with respect to anologous system based on other nanomaterials (e.g
Sb2S3 in solid-state sensitized solar cells). The absence of linear optical
data makes more difficult to investigate the electronic structure of
the material by means of spectroscopic techniques. Ab initio atomistic
simulations represent a valid alternative to get an insight on the
optoelectronic properties of bismuth sulfide. Despite some computational
work on bulk Bi2S3 is already present in literature, there are
currently no ab initio studies concerning nanostructures of this material.
Such lack of information motivates the work of the present
thesis that focuses on the investigation of morphology and electronic
properties of Bi2S3 nanocrystals.
The thesis is organized as follows. In the first chapter the main differences
and advantages of nanostructured materials over the bulk counterpart
are presented. I put the accent on metal sulfide nanocrystals,
in particular those of the Bi2S3 family (pnictogen chalcogenide) and
their application in several fields of physics, environmental science,
and engineering. A section is reserved to report the development
of elongated semiconductor nanostructures and their peculiarity with
respect to nanocrystals with lower aspect ratio.
The second chapter describes the computational and experimental
methods used in this study. The basic concepts of density functional
theory and its implementation in quantum-chemistry codes are reported.
A description of the synthesis and spectroscopic methods
used to check the validity of the theoretical predictions is also given.
For the detailed list of the basis sets, pseudopotentials, and exchangecorrelation
functionals used in each calculation I refer to the end of
Chapter 3 and 4.
Chapter 3 deals with the bulk properties of Bi2S3. Atomic and crystal
cell relaxation are performed. Also I investigated electronic properties
from the calculation of the band structure, density of states, and
efficient mass. These simulations are an important preliminary to
the study of Bi2S3 nanostructures. By comparing my results with
the previous studies present in literature it is possible to validate
the method (functionals, pseudopotentials, etc) and proceed with the
study of unexplored systems.
Chapter 4 investigates the properties of Bi2S3 nanostructures. First,
I focus on elongated nanoribbons (that are the building blocks of the
crystal structure) and study saturated and unsaturated nanocrystals
of finite size in comparison with one-dimensional infinite ones. By
means of (time-dependent) density functional theory calculations it
is demonstrated that the optical gap can be tuned through quantum
confinement with sizable effects for nanoribbons smaller than three
nanometers. A comparison with Sb2S3, shown that Bi2S3 nanostructures
have similar tunability of the band gap and a better tendency of
passivating defects at the (010) surfaces through local reconstruction.
Then, the focus shifts over ultrathin nanowires formed by the aggregation
of a small number of nanoribbons, with lateral sizes as small as
3 nm as in fact observed by transmission electron microscopy. Their
electronic properties are investigated finding that surfaces induce peculiar
1D-like electronic states on the nanowire edges that are lo- cated
300 meV above the valence band. Sulfur vacancies are also responsible
for localized states a few hundreds meV below the conduction
band. The possibility to remove the surface-induced intragap states
is further investigated by passivating the surfaces of the nanowires
with carboxylic and amine groups that are commonly employed in
colloidal synthesis. The small methylamine and acetic acid molecules
are expected to fully passivate the surfaces of the nanowires removing
the edge states and restoring a clean band gap.
Conclusions are finally reported in Chapter 5. The results of the
present Thesis provide a characterization of the energetics and optoelectronic
properties of bismuth sulfide nanostructures showing the
relevance of surface defects and suggesting a possible route for improving
optoelectronic properties of Bi2S3 nanostructures by tuning
the size of the ligand molecules.|