Browsing by Subject "band gap"

Now showing 1 - 4 of 4

Open Access
A desity functional study on narrow band gap donor-acceptor type conducting polymers
(2004) Karaltı, Ozan
The band gap is one of the most important factors for controlling the physical properties. The search for polymers having narrow band gaps is a current topic. Tuning of the band gap by structural modification is possible. There are some approaches used for designing narrow band gap polymers. One of the approaches used for designing low band gap polymers is the donor acceptor concept where it is thought that regularly alternating conjugated donor and acceptor like moieties in a conjugated chain will induce a small band gap and at the same time will lead to widening of the valence and conduction bands. Forcing the polymers to adopt unfavorable structures and decreasing the bond length alternation along the conjugated backbone are other two methods used for synthesizing narrow band gap polymers. Yamashita et al. synthesized copolymers composed of benzobis(1,2,5- thiadiazole) and [1,2,5] thiadiazolo[3,4-b]thieno[3,4-e] pyrazine units as acceptor and thiophene and pyrrole units as donors. These synthesized copolymers have very narrow band gaps. The success with designing these systems were attributed to the donor-acceptor concept. We intended to understand the reasons for narrow band gaps and to determine whether donor acceptor concept is valid. Density functional theory (DFT) calculations were performed for homo and co-oligomers (having 1:1 and 1:2 acceptor to donor ratios) of thiophene (Th), pyrrole (Py), benzo[1,2-c;3,4- c']bis[1,2,5]-thidiazole (BBT) and [1,2,5] thiadiazolo[3,4-b]thieno[3,4-e] pyrazine (TTP) We estimated the band gaps of polymers by extrapolating the HOMO-LUMO gaps of the oligomers, using second degree polynomial fit, at the B3P86-30% /CEP-31g* level of theory. Theoretical analysis showed that the main reason for the band gap reduction is not the donor-acceptor concept and the prediction of the band width widening is not valid. Influence of the quinoid structures and reduce in the bond length alternation are the resons for the band gap reduction.
Open Access
Electronic structure of graphene nano-ribbons
(2008) Şen, Hüseyin Şener
Graphite is a known material to human kind for centuries as the lead of a pencil. Graphene as a two dimensional material, is the single layer of graphite. Many theoretical works have been done about it so far, however, it newer took attention as it takes nowadays. In 2004, Novoselov et al. was able to produce graphene in 2D. Now that, making experiments on graphene is possible scientists have to renew their theoretical knowledge about systems in two dimension because graphene, due to its electronic structure, is able to prove the ideas in quantum relativistic phenomena. Indeed, recent theoretical studies were able to show that, electrons and holes behave as if they are massless fermions moving at a speed about 106m/s (c/300, c being speed of light) due to the linear electronic band dispersion near K points in the brillouin zone which was observed experimentally as well. Having zero band gap, graphene cannot be used directly in applications as a semiconductor. Graphene Nano-Ribbons (GNRs) are finite sized graphenes. They can have band gaps differing from graphene, so they are one of the new candidates for band gap engineering applications such as field effect transistors. This work presents theoretical calculation of the band structures of Graphene NanoRibbons in both one (infinite in one dimension) and zero dimensions (finite in both dimensions) with the help of tight binding method. The calculations were made for Zigzag, Armchair and Chiral Graphene Nano-Ribbons (ZGNR,AGNR,CGNR) in both 1D and 0D. Graphene nano-ribbons with zero band gap (ZGNR and AGNR) are observed in the calculations as well as the ribbons with finite band gaps (AGNR and CGNR) which increase with the decrease in the size of the ribbon making them much more suitable and strong candidate to replace silicon as a semiconductor.
Open Access
First-principles investigation of graphitic nanostructures
(2013) Şen, Hüseyin Şener
In this thesis, first-principles investigations of several graphene related nanosystems based on density functional theory are presented. First, the electronic structure of several graphene nano-ribbons both in 1D and 0D (up to systems with more than 1000 atoms) including all types (armchair, zigzag and chiral) are discussed using tight binding calculations. We observed that the band gap of the ribbons depend both on the length of the ribbon and the angle of chirality. Second, the effect of phosphorus and sulfur during the growth of carbon nanotubes is investigated from ab-initio density functional theory based calculations. To this end, we present the binding chemistry of phosphorus and sulfur atoms on graphene with and without vacancies and kink like defect structures. Consequently, the difference between the bindings of these two atoms is discussed in order to understand the reason behind their effects on the growth mechanism. The details of the phosphorus or sulfur binding are important in order to understand the occurrence of Y-junctions and kinks in carbon nanotubes as well. Third, we focus on the interaction of bilayer graphite and multi-walled carbon nanotubes with the Li atom since these materials are prime candidates for the electrodes for battery applications. The need for rechargeable batteries with high capacity increased enormously by the invention of electronic devices like cell phones or MP3 players. Hence, there is a huge effort to develop and improve Li-ion batteries. Therefore, we have investigated interaction of Li with graphene and Li intercalation to bilayer graphene and multi-walled carbon nanotubes from planewave pseudo potential calculations. Finally, super-periodic graphitic structures observed through scanning tunnelling microscope are described and investigated from density functional calculations. The difference between the observed and actual periodicity and the occurrence of the so-called Moire patterns are explained in terms of geometrical calculations and the charge density of these systems.
Open Access
Quantum entanglement and light propagation through Bose-Einstein condensate (BEC)
(2009) Taşgın, Mehmet Emre
We investigate the optical response of coherent media, a Bose-Einstein condensate (BEC), to intense laser pump stimulations and weak probe pulse propagation. First, we adopt the coherence in sequential superradiance (SR) as a tool for continuous-variable (CV) quantum entanglement of two counter-propagating pulses from the two end-fire modes. In the first-sequence the end-fire and side mode are CV entangled. In the second sequence of SR, this entanglement is swapped in between the two opposite end-fire modes. Second, we investigate the photonic bands of an atomic BEC with a triangular vortex lattice. Index contrast between the vortex cores and the bulk of the condensate is achieved through the enhancement of the index via atomic coherence. Frequency dependent dielectric function is used in the calculations of the bands. We adopt a Poynting vector method to distinguish the photonic band gaps from absorption/gain regimes.