RESUMO
This study employs first-principles calculations to investigate how introducing Yb into aluminum nitride (AlN) leads to a large enhancement in the material's piezoelectric response (d33). The maximum d33 is calculated to be over 100 pC/N, which is 20 times higher than that of AlN. One reason for such a significant improvement in d33 is the elastic-softening effect, which is indicated by a decrease in the elastic constant, C33. The strain sensitivity (du/dε) of the internal parameter, u, is also an important factor for improving the piezoelectric stress constant, e33. On the basis of mixing enthalpy calculations, YbxAl1-xN is predicted to be more stable as a wurtzite phase than as a rock salt phase at composition up to x ≈ 0.7. These results suggest that Yb can be doped into AlN at high concentrations. It was also observed that the dielectric constant, ε33, generally increases with increasing Yb concentrations. However, the electromechanical coupling coefficient, k332, only increases up to x = 0.778, which is likely because of the relatively lower values of ε33 within this range.
RESUMO
Polarity is among the critical characteristics that could governs the functionality of piezoelectric materials. In this study, the polarity of aluminum nitride (AlN) thin films was inverted from Al-polar to N-polar by doping Si into AlN in the range of 1-15 at.%. Polarity inversion from Al-polar to N-polar also occurred when MgSi was codoped into AlN with Mg to Si ratio was less than 1. However, the polarity can be reversed from N-polar to Al-polar when the ratio of Mg and Si was greater than 1. The effect of Si and MgSi addition was investigated with regards to their crystal structure, lattice parameters, polarity distribution and the oxidation state of each elements. Furthermore, the effect of intermediate layer as well as the presence of point defect (i.e. aluminum vacancy) were investigated and how these factors influence the polarity of the thin films are discussed in this report.
RESUMO
The enhancement mechanism of piezoelectric properties by codoping Mg + X (X = Nb, Ti, Zr, Hf) into aluminum nitride (AlN) was investigated by first-principles calculations. Theoretically, the piezoelectric constant (d 33) can be increased when the elastic constant (C 33) is decreased and the piezoelectric stress constant (e 33) is increased. All components of e 33, which consists of the clamped e 33, the Born effective charge (Z 33), and the strain sensitivity (du/dε) of the internal parameter, were improved by the addition of Mg + X into AlN. The decrease in C 33 and the increase in du/dε that were observed in Mg + X-codoped AlN indicate the occurrence of elastic softening which was considered to be influenced by changes in the interatomic bond in the wurtzite structure. The bonding analysis of metal-nitrogen (Me-N) pairs in the Mg + X-codoped AlN system which was carried out by crystal orbital Hamilton populations showed that the covalent bonding (Me-N) was weaker than in pure AlN. Therefore, this weaker covalent bond is considered to be one of the origins of the elastic softening. Similar phenomena were also found for Sc-doped AlN which has higher piezoelectric response than that of pure AlN.
RESUMO
A highly efficient dye-sensitized solar cell (DSC) was fabricated using a nanocrystalline nitrogen-doped titania electrode. The properties of the nitrogen-doped titania powder, film, and solar cell were investigated. The substitution of oxygen sites with nitrogen atoms in the titania structure was confirmed by X-ray photoemission spectroscopy (XPS). The UV-vis spectrum of the nitrogen-doped powder and film showed a visible light absorption in the wavelength range from 400 to 535 nm. An enhancement of the incident photon-to-current conversion efficiency (IPCE) in the range of 380-520 nm and 550-750 nm was observed. An 8% overall conversion efficiency has been achieved. The results of the stability test indicated that the solar cell fabricated by the nitrogen-doped titania exhibited great stability.
