Browsing by Subject "Density functionals."

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Open Access
Electronic structure analysis and density functional study of an alternating donor/acceptor polymer
(2001) Köse, Muhammet Erkan
Conducting polymers have attracted great attention for the last two decades because of their potential applications in many fields. One of the major goals for the scientists who are studying in this field is to synthesize conducting polymers with extremely low band gap. Such polymers would be intrinsically conducting, and thus eliminating the need for doping. Several powerful approaches are proposed to synthesize low band gap polymers. One of them is the donor/acceptor concept, which was proposed by Havinga and his co-workers. In their model, they attached regularly alternating electron withdrawing and electron donating groups to the carbon backbone. By connecting these groups, they aimed to decrease the lowest unoccupied molecular orbital (LUMO) level, and to increase the highest occupied molecular orbital (HOMO) level of the polymer respectively. By this way they claimed that one could obtain very low band gap conducting polymers by introducing the alternating donor/acceptor groups, where the electronegativity difference between these groups is highest. Huang and Pickup synthesized donor/acceptor copolymers of 3,4- ethylenedioxythiophene (EDOT) which is considered as electron donating group and 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b’]dithiophene (CDM) as electron withdrawing group. By changing the monomer ratio in the copolymers, they decreased the band gap to less than 0.16 eV, however, the intrinsic conductivity was about 10-3 S/cm. Although a very low band gap was achieved, the conductivities of copolymers were still low. To understand this behaviour and also to check the validity of donor/acceptor concept, we performed theoretical studies for these systems. Monomer through hexamer of EDOT, oligomers for 1:1 copolymer of EDOT and CDM were optimized using density functional theory. Ionization potentials, electron affinities, energy gaps, and band widths of the polymers were obtained by extrapolation. The lowest band gaps are calculated for copolymer poly-CDM-EDOT and poly-CDM. Band gaps of the polymers are found to agree well with the experimental values. When we combine two monomers (EDOT: electron-rich, CDM: electrondeficient) in one to one ratio and in different ratios in order to form co-oligomers, the highest occupied molecular orbitals of co-oligomers are averaged between the HOMO levels of parent homo-oligomers. Lowest unoccupied molecular orbitals of cooligomers are also averaged between the LUMO levels of parent homo-oligomers; however, averaged LUMO levels of co-oligomers are more close in energy to the LUMO level of CDM oligomer. We also found that poly-CDM and poly-CDMEDOT have very flat conduction bands. These narrow bands are attributed to the localized states, which decrease the mobility of n-type carriers in these bands consistent with the experimental findings. Overall, we concluded that donor/acceptor concept can be used to decrease the band gap while sacrificing the dispersion of valence and conduction bands. However, band broadening does not occur in alternating donor/acceptor polymers as donor/acceptor approach suggested.
Open Access
A study of adsorption of single atoms on carbon nanotubes
(2003) Durgun, Engin
The adsorption of individual atoms on the semiconducting and metallic singlewall carbon nanotubes (SWNT) have been investigated by using first-principles pseudopotential plane wave method within Density Functional Theory. The stable adsorption geometry and binding energy have been determined for a large number of foreign atoms ranging from alkali and simple metals to the transition metals and group IV elements. We have found that the character of the bonding and associated physical properties strongly depend on the type of adsorbed atoms, in particular on their valence electron structure. Our results indicate that the properties of SWNTs can be modified by the adsorbed foreign atoms. While the atoms of good conducting metals, such as Zn, Cu, Ag and Au, form very weak bonds, transition metal atoms, such as Ti, Sc, Nb and Ta, and group IV elements C and Si are adsorbed with relatively high binding energy. Owing to the curvature effect, these binding energies are larger than the binding energies of the same atoms on the graphite surface. We have showed that the adatom carbon can form strong and directional bonds between two SWNTs so that the tubes are connected. These connects can be used to produce nanotube networks or grids. Most of the adsorbed transition metal atoms excluding Ni, Pd and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as one-dimensional nanomagnets or nanoconductors and conducting connects etc.
Open Access
Theoretical investigation of approaches for obtaining narrow band gaps in conducting polymers
(2004) Durdağı, Serdar
Over the last few years, there has been a great deal of research interest in developing organic conjugated polymers with narrow energy band gaps. Narrow band gap polymers would be intrinsically conducting, and thus eliminating the need for doping. There are several approaches for the construction of low band gap systems. Copolymerization of aromatic and o-quinoid heterocycles, minimization of bond length alternation, copolymerisation of donor and acceptor moieties might be most important factors for the lowering the band gap. The main aim of this work is to determine the reasons for low band gaps and to analyse the major effects, separately. Recently a number of low band gap systems were synthesized. These systems consist of aromatic donors and quinoid acceptors. To analyse the behaviour of donor/acceptor systems, we performed theoretical studies for these systems. We chose thieno [3,4-b] pyrazine (ThP), F-substituted thieno [3,4-b] pyrazine (F-ThP) and quinoxaline (Qx) as an acceptor and thiophene (Th) and pyrrole (Py) as donor units. Monomer through octamer of ThP, monomer through tetramer of F-ThP and Qx, were optimised by using density functional theory (DFT). All calculations were performed with the Gaussian 98 program. For ThP-Th and ThP-Py co-oligomers, energy levels were calculated for monomer through tetramer and, for F-ThP-Py, monomer through trimer data were used. Ionization Potentials (IPs), electron affinities (EAs), band gaps, and band widths of polymers were obtained by extrapolation. IPs and EAs are taken as negative HOMO and LUMO energies. The order of the band gaps of the homo-polymers is found: F-PThP (1.1 eV) < PThP(1.4 eV) < PQx.(2.4 eV). Band gaps of the systems are found to agree well with the experimental results. We increased the acceptor strength of PThP by adding fluorine units. The band gap is decreased by adding fluorine, however, the valence band width decreased, too. According to the donor-acceptor concept, we predict narrow band gaps and wider band widths for donor-acceptor co-polymers. However, for ThP-Th and ThP-Py copolymers, the band gaps are smaller than those of the homo polymers, but there is no increase on conduction band width. ThP-Th and ThP-Py have the same band gap (1.2 eV). The stronger donor pyrrole does not lead to a smaller band gap and wider band widths than the weaker donor thiophene. Thus, there is a contradiction with donor-acceptor concept qualitatively, and also quantitatively. The results show that, there is no certain correlation between band gaps and inner bond lengths. The bond length alternation (BLA) data are not consistent with the band gap results. Due to our results, the geometrical mismatch between aromatic and quinoid repeat units forces geometry distortions that impose smaller band gaps. Overall, we concluded that the situation is more complicated than simple concepts imply and further studies are required before a final conclusion can be reached.