Browsing by Subject "Charge carriers"

Now showing 1 - 8 of 8

Open Access
Collective modes in a quasi-one-dimensional, two-component electron liquid
(Pergamon Press, 1994) Tanatar, Bilal
Under favorable conditions, a new collective mode besides the usual plasmons may exist in degenerate electron-hole liquids. We calculate the dispersion and damping of this new mode (called the acoustic plasmon mode) in a quasi-one-dimensional, two-component electron liquid. We carry out our calculations first within the random-phase approximation, then include the effects of local-field corrections using a Hubbard-like approximation. The latter decreases the acoustic plasmon dispersion. © 1994.
Open Access
Investigation of charge carriers in doped thiophene oligomers through theoretical modeling of their UV/Vis spectra
(2008) Salzner, U.
The nature of the charge carriers in conducting organic polymers (COPs) is a long standing problem. Polythiophene is one of the prototypes of COPs and intensively studied. Because doping leads to changes in UV/vis spectra that are characteristic of the absorbing species, UV/vis spectra of charged thiophene oligomers with up to 25 rings were calculated with time-dependent density functional theory. The credibility of the method was established by comparing the results with a variety of theoretical levels and with experiment. Effects due to counterions (Cl3-) and solvent (CH 2Cl2) were examined. It was found that TDDFT employing hybrid functionals is accurate enough to distinguish the absorbing species. The findings offer an explanation for the experimentally observed difference in UV-spectra of medium-sized and long oligomers upon doping. As chain lengths of the oligomers increase and energy levels get closer, configuration interaction leads to additional absorption peaks in the high energy sub-band region (at around 1.5-2.5 eV). Thus, long oligomers do not behave differently from medium-sized ones upon doping, only their spectra are different. At low doping levels radical cations (polarons) are produced. At higher doping levels, dications that harbor weakly interacting polaron pairs are formed. Bipolarons are predicted only on very short chains or at high doping levels. There is no bipolaron binding energy and disproportionation of monocations into dications and neutral species is energetically unfavorable. © 2008 American Chemical Society.
Open Access
Low-temperature photoluminescence study of GaS0.5Se0.5 layered crystals
(Elsevier Science, 2001) Aydnl, A.; Gasanly, N. M.; Gökşen, K.
Photoluminescence (PL) spectra of GaS0.5Se0.5 layered single crystals have been studied in the wavelength range of 565-860 nm and in the temperature range of 15-170 K. Two PL bands centered at 585 nm (2.120 eV) and 640 nm (1.938 eV) were observed at T = 15 K. Variations of both bands were studied as a function of excitation laser intensity in the range from 10-3 to 15.9 W cm-2. These bands are attributed to recombination of charge carriers through donor-acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located at 0.029 and 0.040 eV below the bottom of the conduction band to deep acceptor levels located 0.185 and 0.356 eV above the top of the valence band are suggested to be responsible for the observed A- and B-bands in the PL spectra, respectively. A simple energy level diagram explaining the recombination process is proposed.
Open Access
Magnetic-field dependence of low-temperature mobility in quasi-one-dimensional electron systems
(Institute of Physics Publishing Ltd., 1994) Tanatar, Bilal; Constantinou, N. C.
We study the mobility of a quasi-one-dimensional (Q1D) electron system in the presence of an axial magnetic field at low temperatures. We consider the mobility limits for remote-impurity scattering, homogeneous-background scattering, interface-roughness scattering, and alloy-disorder scattering mechanisms. For a system in which all carriers are in the lowest subband, the electron-impurity interaction is modelled for the above cases, and analytic expressions are derived. Calculations appropriate for a GaAs Q1D structure are presented for typical wire radius R, electron density N, impurity density Ni, and applied magnetic field B.
Open Access
Nature of charge carriers in long doped oligothiophenes: the effect of counterions
(2008) Zamoshchik, N.; Salzner, U.; Bendikov, M.
A series of oligothiophene dications doped with Cl3 - ions were studied using density functional theory (DFT) at the B3LYP/6-31G(d) level. The balance between the bipolaron and polaron pair states was addressed by studying the closed-shell singlet, open-shell singlet, and triplet states of oligothiophene divalent salts using the relative energies of the different isomers, differences between the singlet and triplet states, bond length alternation analysis, charge distribution analysis, and isodesmic reactions. We found that contribution of polaron pair state to the electronic structure of the oligothiophene divalent salts is not observed in 8T2+(Cl3́ -)2 (short oligothiophene salts), appears in 12T 2+(Cl3 -)2 (medium-sized oligothiophene salts), and becomes the dominant in 20T2+(Cl 3 -)2 (long oligothiophene salts). Bipolarons are intrinsically unstable with respect to dissociation into polaron pairs regardless of the presence of counterions. Thus, even in the presence of counterions, we did not observe any bipolaron stabilization energy, however, doping ions localize polarons. The singlet and triplet states are energetically degenerate for long oligothiophene divalent salts, such as 20T 2+(Cl3 -)2. © 2008 American Chemical Society.
Open Access
Raman studies of doped polycrystalline silicon from laser-annealed, doped a-Si:H
(Pergamon Press, 1994) Compaan, A.; Savage, M. E.; Aydınlı, Atilla; Azfar, T.
We have used Raman scattering to follow the progress of multiple-pulse (sub-melt-threshold) laser annealing in doped hydrogenated amorphous silicon films (a-Si:H) on glass. In phosphorous-doped a-Si:H the Raman signal shows that recrystallization begins with the first laser pulse but the multiple pulses are needed to generate the highest hole concentrations of ∼6×1020 cm-3. In boron-doped a-Si:H the electron concentration reaches ∼1×1021 cm-3 after laser anneal which produces a dip rather than a peak near the phonon line as a consequence of a negative Fano-interference parameter, q. The results show that Raman scattering can be used to obtain carrier concentrations in poly-silicon provided that wavelength-dependent Fano interference effects are properly included. © 1994.
Open Access
Short-range correlations in coupled quantum-wire systems
(TÜBİTAK, 2002) Yurtsever, A.; Tanatar, Bilal
We study the contact values of the inter-wire pair-correlation function in electron-electron and electron-hole double-wire systems. For this purpose the ladder approximation as generalized to multicomponent systems is used. The ladder approximation yields positive values for the inter-wire gee(0) and geh(0) for all values of the density parameter rs and distance d between the wires. This allows us to infer possible instabilities in the system more reliably compared to other approaches. We also investigate the effects of quantum-wire width and screening on the inter-wire pair-correlation functions.
Open Access
Strongly Scale-Dependent Charge Transport From Interconnections of Silicon Quantum Dots and Nanowires
(Cambridge University Press, 2017) Ilday, S.
We present the first characterization of strongly scale-dependent charge transport of a unique, hierarchical complex topology: an interconnected random network of silicon quantum dots (QDs) and nanowires. We show that this specific topology has different charge transport characteristics on the nanoscale and the microscale: photogenerated charge carriers tend to be confined inside the QDs and externally injected charge carriers flow preferably along the nanowires. The former enables expression of quantum confinement properties, and the latter mainly contributes to the good electrical conduction on the microscale. Our findings strongly suggest that this multifunctionality can be controlled and used in photovoltaic device applications.