الفهرس 
Chapter 1 IntroductionOnly 14 pages are availabe for public view
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Abstract
Organic molecules could play an important role in the development
of the emerging field of molecular electronics due to their design simplicity
and structural flexibility. Many factors could be used to tune the electrical
properties of organic molecules. Substitution, anchors, and isomerization,
could be used to acquire desired functionalities such as switching, wiring,
rectifying, etc. In this project we focused on substitution and isomerization
as means for structure modification.
We investigated the effect of gradual increment of nitrogen atoms on
the I-V characteristics of benzene-1,4-dithiol which appeared as one of the
most investigated molecules since the emergence of molecular electronic.
Also we reported the effect of twisting of the middle dihedral on the I-V
characteristics of a promising molecular diode namely Dipyrimidinyl-
Dithiophene.
All electronic structure calculations have been carried out using the
Gaussian 09 program package. Structures of the investigated organic
molecules we optimized at B3LYP/6-31G (d) level. The I-V properties of the
studied molecules we performed by using Atomistic tool kit (ATK 2008.10)
This thesis consists of three main chapters:
CHAPTER I: (Introduction)
It gives a general introduction about molecular electronics and survey
on the subject of this project. We presented a general overview on different
types of molecular electronic components such wires, rectifiers, diodes, and
transistors with emphasize on the molecular diodes and molecular wires.
CHAPTER II: (Computational Methods and Details)
This chapter revised a brief background about quantum mechanics for
electronic structure calculations. Firstly, Hartree-Fock (HF) models were
addressed, with focus on density functional theory (DFT). We used DFT in
conjugation with non equilibrium green functions formalism (NEGF) in
order to investigate the I-V properties of benzene-1,4-dithiol (M0) with
systematic insertion of nitrogen atoms in the benzene ring and constitutional
isomery of diazabenzene. Furthermore, Landuar Buttiker formula for
electron transport calculations was also presented.
CHAPTER III: (Results and Discussion)
This chapter collects the results and discussion. It is divided into two
sections as the follows:
Tuning Electrical Conductivity of Benzene-1, 4- Dithiol.
Our findings demonstrated that the increment of number of nitrogen
atoms significantly increases the current by about 47% especially at high
bias voltages. The sequential substitute of nitrogen atoms shifts the main
transmission peak to the bias window which leads to sharp increase in the
current pass through the relevant molecular system. The molecular projected
self-Hamiltonian (MPSH) eigenstates indicated that the increment of
nitrogen atoms increases orbital density on the anchors which strengthens
coupling between molecule and two electrodes. The results also showed that
constitutional isomery significantly affects the I-V behavior of the
diazabenzenes at all applied voltages, at a given bias the current varies by
30-40%.
The effect of Conformational Isomerization on Dipyrimidinyl-
Dithiophene Dithiol as a Diblock Molecular Diode.
To investigate the effect of conformational isomerization on the
electrical properties of dipyrimidinyl-dithiophene dithiol, seven different
conformers with different torsion angle are optimized at B3LYP/6-31+G (d,
p) level. The torsion angle varied from 0° to 180° with steps of 30°. Based
on optimized structures I-V curve and other electrical properties like
rectification ratio were calculated.
The obtained results demonstrated that changing the torsional angle
between the diblock subunits significantly affects the passed current. On
going from 0° to 90° the passed current decreased dramatically till it reached
its minimum limit, at 90° (5.42 μA- 0.088 μA). Further increase of torsion
angle dramatically increases the transmitted current until it reach its
maximum limit (6.47 μA) at 180°, which means that changing torsion angle
hinders the passing current by about 75 %.We analyzed transmission spectra
in order to know the driving force behind this significant change in the I-V
properties. The changing of the torsion angle from 0° to 90° switch off the π
electron delocalization and this leads to shifting off the main transmission
peak out of the bias window producing dramatic decrease in the passed
current through molecular system. In order to link the chemical properties of
organic molecules with its electronic functions, rectification ratio, dipole
moment, energies of frontier molecular orbitals (FMOs), energy gap (HLG)
between the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO), and spatial orientation of FMOs
were also analyzed. The results indicated that changing mid torsional angle
dramatically increases rectification ratio to reaches its maximum limit of
(4.7 μA) at 90°, matching the change of the energies of HOMO, LUMO, and
HLG, which refers to the HLG may be the driving force behind this sharp
increase in the rectification in this new category of molecular diodes. Also
changing torsion angle localized the spatial orientation of HOMO and
LUMO on the donor and acceptor parts respectively.
Based on the reported results, we propose dipyrimidinyldithiophene
dithiol as a molecular switch with two conformations (planar
and perpendicular). The planar conformation records high conductivity ‘ON’
while perpendicular structure displays weak conduction ‘OFF’. The results
reported in this work shed light on the importance of geometrical parameters
and their impact in the electrical properties of molecular systems and how
could they used in tuning the function of these systems.