The first-principles with pseudopotentials method based on the density functional theory was applied to calculate the
electronic structure of ZnO doped with carbon. It optimized the wurtzite structure of ZnO when carbon was on the
substitutional O site (CO), the substitutional Zn site (CZn) or in the interstitial site (Ci) respectively. The calculated conclusions show that the CZn and Ci behave as donor. However, CO results in acceptor. It is illuminated that CO is helpful to achieve p-type ZnO.
We have investigated the geometrical structures, the electronic properties, and the formation energies of nN-Mg codoped
ZnO in neutral state by adopting the first-principles calculations based on the density function theory (DFT). The
calculated results indicate that N atoms prefer to occupy the substitution O site and Mg substitutes the Zn site of the
nearest site of N, which act as an acceptor. Compared with the formation energies of various configurations in neutral
state, it is found that 4N-Mg complex has the lowest formation energy using NO as dopant resource under Zn-rich
condition, indicating that 4N-Mg codoping can enhance the N dopant solubility under this condition. Meanwhile, the Znrich
condition is better for p-type doping than the O-rich condition. It demonstrates that 4N-Mg complex is in favor of
achieving p-type conduction in ZnO. Simultaneously, analysis of density of states (DOS) of nN-Mg complex find that
the valence band maximum (VBM) has a little raise near the Fermi energy level, indicating that the complexes are the
typical p-type characteristic. However, for 4N-Mg complex, the Fermi level is located near the top of valence band.
Furthermore, from the band structure and PDOS of 4N-Mg complex, it is observed that the complex produces an
additional impurity band at the top of the valence band. Meanwhile, the PDOS value of 4N-Mg complex at the Fermi
level is relatively large. In addition, 4N-Mg complex has much lower ionization energy of 0.167eV than that of other
complexes. Therefore, better quality p-type conductivity is achieved by codoping 4N-Mg in ZnO.
Ag monodoped, N monodoped and (nN, Ag) codoped ZnO have been investigated by the first-principles calculations,
where the formation energies and ionization energies of various complexes and the electronic structure for 3N-Ag
complex are studied. The calculated results are that N prefers to substitute O site, and Ag substitutes Zn site under the
most growth condition, which indicate NO and AgZn all act as acceptors. Meanwhile, it's shown that N-Ag, 2N-Ag
complex contribute little to p-type conduction because of the relatively higher ionization energy. However, 3N-Ag
complex may have the lowest ionization energy among various complexes, while the formation energy of 3N-Ag is
lower than that of N monodoped, Ag monodoped, N-Ag and 2N-Ag complex under the Zn-rich condition, which
indicates that 3N-Ag complex is energetically favorable for the formation of p-type ZnO. Furthermore, by studying the
electronic structure of 3N-Ag complex, it may generate an additional impurity band above the valence band maximum of
ZnO. It is found that NO generated holes around the top of the valence band, and at the same time, N 2p states hybridized
with 4d states of AgZn at the Fermi energy, and the hybridization lowered the repulsive interaction between the two dual
acceptors, which enhance the concentration of impurities and the stability of the system, indicating that the dual
acceptors evidently improve p-type conductivity of ZnO. Thus, it is found that 3N-Ag complex is the better dopant
configuration. That can gain a better quality p-type ZnO under the Zn-rich condition. Our theoretical results are
consistent with the experiment results.
With density functional theory (DFT), the structural and electronic properties of both neutral and different
negatively charged (TiO2)n clusters with n=1-5 have been investigated. The HOMO-LUMO (highest occupied molecular
orbital- lowest unoccupied molecular orbital) energy gaps as a function of the charge have been presented. The
calculated results suggest that the least change of energy gap between (TiO2)4 and (TiO2)42- happens, and the following is
that of (TiO2)3 clusters. Distribution of extra electrons in (TiO2)n- and (TiO2)n2- has also been performed. The calculated
results indicate that in both these two kinds of negative clusters, the excess charge is localized around the titanium with
the lowest coordinated number. These theoretical results may be helpful for understanding the influence of electron
transport on the small (TiO2)n clusters better.
With density functional theory (DFT), the structures and stability of Bn clusters with n=2-8 have been studied. By
using the all electron basis, all the geometries have been globally optimized without any symmetry constraint. It is found
that all the small Bn (n=2-8) clusters prefer to form planar structures with sp2 bonds, which are in good agreement with
others' related studies. Bn and Bn- are also compared. In contrast with the neutral Bn clusters, although B-B distances in
Bn
- have slight differences, but addition of one electron does not change their structures significantly. As for energies, all
the anions are lying lower than their corresponding neutral clusters. In addition, calculations of energetic and electronic
properties for all the neutral clusters have been presented. Both of these two properties show that in Bn (n=2-8), B3 and
B5 are more stable than others. Vibrational spectra of Bn (n=3-8) clusters have also been discussed. In each spectrum,
intensity peaks which are associated with the vibration of boron clusters related to B-B bond stretching can be observed
and they are highest. However, among all the Bn clusters, such peaks of B3 and B5 show lower intensity than others. This
results suggest that B3 and B5 are relatively more stable, which further demonstrates the conclusion above.
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