W. W. Li

Temperature dependence of electronic transitions and optical properties in multiferroic BiFeO3 nanocrystalline film determined from transmittance spectra

The ultraviolet-infrared transmittance spectra of BiFeO 3 nanocrystalline film have been studied in the temperature range 5.3–300 K. A redshift trend of the absorption edge and optical constants with increasing the temperature can be observed. Four interband electronic transitions can be uniquely assigned and strongly depend on the temperature. Moreover, two magnetic transitions located at about 150 and 200 K have been observed and can be interpreted as spin-reorientation transitions. It was found that the optical band gap decreases from 2.69 ± 0.01 to 2.65 ± 0.01 eV with increasing the temperature due to the modification of the electron-phonon interactions.

Optical properties of pulsed laser deposited rutile titanium dioxide films on quartz substrates determined by Raman scattering and transmittance spectra

Optical response of rutile TiO2 films grown under different laser energy by pulsed laser deposition has been investigated by Raman scattering and spectral transmittance. Dielectric functions in the photon energy range of 1.24–6.5 eV have been extracted by fitting the experimental data with the Adachi’s model [S. Adachi, Phys. Rev. B 35, 7454 (1987)]. The refractive index dispersion in the transparent region is mainly ascribed to the higher A1-A2 electronic transitions for the rutile TiO2 films. Owing to slightly different crystalline structures and film densities, the optical band gap linearly increases with increasing packing density. The phenomena were confirmed by different theoretical evaluation methods.

Intrinsic evolutions of optical functions, band gap, and higher-energy electronic transitions in VO(2) film near the metal-insulator transition region

Transmittance spectra of (011) vanadium dioxide (VO2) film have been studied in the temperature range of 45–80 °C. Owing to increasing carrier concentration, the near-infrared extinction coefficient and optical conductivity around metal-insulator transition (MIT) rapidly increase with the temperature. Moreover, three electronic transitions can be uniquely assigned and show the hysteresis behavior near the MIT region. It was found that the optical band gap decreases from 0.457 to 0.042 eV before the MIT, then reduces to zero for the metal state. This confirms the fact that the a 1 g and e g π bands are moved close and finally overlap with the temperature.

Temperature dependence of phonon modes, dielectric functions, and interband electronic transitions in Cu2ZnSnS4 semiconductor films

The quaternary semiconductor Cu(2)ZnSnS(4) (CZTS) has attracted a lot of attention as a possible absorber material for solar cells due to its direct bandgap and high absorption coefficient. In this study, photovoltaic CZTS nanocrystalline film with a grain size of about 10 nm has been grown on a c-plane sapphire substrate by radio-frequency magnetron sputtering. With increasing the temperature from 86 to 323 K, the A(1) phonon mode shows a red shift of about 9 cm(-1) due to the combined effects of thermal expansion and the anharmonic coupling to the other phonons. Optical and electronic properties of the CZTS film have been investigated by transmittance spectra in the temperature range of 8-300 K. Near-infrared-ultraviolet dielectric functions have been extracted with the Tauc-Lorentz dispersion model. The fundamental band gap E(0), and higher-energy critical points E(1) and E(2) are located at 1.5, 3.6, and 4.2 eV, respectively. Owing to the influences of electron-phonon interaction and the lattice expansion, the three interband transitions present a red shift trend with increasing temperature. It was found that the absorption coefficient in the visible region increases due to the modifications of electronic band structures. The detailed study of the optical properties of CZTS film can provide an experimental basis for CZTS-based solar cell applications.

Oxygen pressure manipulations on the metal-insulator transition characteristics of highly (0 1 1)-oriented vanadium dioxide films grown by magnetron sputtering

The metal–insulator transition behaviour of vanadium dioxide (VO2) films grown at different oxygen pressures is investigated. With the aid of temperature-dependent electrical and infrared transmittance experiments, it is found that the transition temperature in the heating process goes up with increasing argon–oxygen ratio, whereas the one in the cooling process shows an inverse variation trend. It is found that the hysteresis width of the phase transition is narrowed at a lower argon–oxygen ratio because the defects introduced by excess oxygen lower the energy requirement of transformation. Furthermore, the defects reduce the forbidden gap of the VO2 system due to the generation of a V5+ ion. The present results are valuable for the achievement of VO2-based optoelectronic devices.

Abnormal temperature dependence of interband electronic transitions in relaxor-based ferroelectric (1―x)Pb(Mg1/3Nb2/3)O3―xPbTiO3 (x=0.24 and 0.31) single crystals

The transmittance spectra of ( 1 − x ) Pb ( Mg 1 / 3 Nb 2 / 3 ) O 3 − x PbTiO 3 ( x = 0.24 and 0.31) single crystals have been studied in the temperature range of 5.3–300 K. It was found that the direct band gap E g d is 3.150 ± 0.016 eV , indirect band gap E g i is 2.939 ± 0.014 eV , and the phonon energy E p is 0.098 ± 0.014 eV for the PMN-0.24PT crystal at 300 K. With increasing the temperature, the E g d of the PMN-0.24PT crystal decreases from 3.263 ± 0.017 to 3.150 ± 0.016 eV while the E g d of the PMN-0.31PT crystal increases from 3.050 ± 0.015 to 3.101 ± 0.016 eV . The peculiar characteristic can be ascribed to the monoclinic and rhombohedral multiphase coexistence in the PMN-0.31PT crystal.

Ultraviolet-infrared dielectric functions and electronic band structures of monoclinic VO2 nanocrystalline film: Temperature-dependent spectral transmittance

Nanocrystalline vanadium dioxide (VO2) film on c-plane sapphire substrate has been prepared by direct-current magnetron sputter deposition. The electronic band structures of the VO2film with monoclinic phase have been investigated by ultraviolet-infrared transmittance spectra in the temperature range of 5.3–300 K. It was found that the transmittance decreases while the dielectric functions slightly increase with the temperature. The optical bandgap decreases from 0 . 839 ± 0 . 003 to 0 . 788 ± 0 . 003 eV with increasing the temperature due to the variations of lattice constant and V d -O p hybridization. Moreover, three higher-order interband electronic transitions can be uniquely distinguished and the temperature effects on the higher-order transition energy become much weaker.

Electronic properties of nanocrystalline LaNiO3 and La0.5Sr0.5CoO3 conductive films grown on silicon substrates determined by infrared to ultraviolet reflectance spectra

Electronic band structures of nanostructured LaNiO3 (LNO) and La0.5Sr0.5CoO3 (LSCO) films have been investigated by near-normal incident optical reflectance at room temperature. Dielectric constants of the conductive films in the photon energy range of 0.47–6.5 eV have been extracted with the Drude–Lorentz function. It is found that four interband electronic transitions can be uniquely assigned for the perovskite-type metallic oxides. Moreover, optical conductivity is approximately varied from 100 to 450 Ω−1 cm−1 and shows a different variation trend for the LNO and LSCO layers. The discrepancy could be ascribed to diverse electronic structure, grain size, and crystalline formation.

Phonon mode and phase transition behaviors of (1-x)PbSc1/2Ta1/2O3-xPbHfO3 relaxor ferroelectric ceramics determined by temperature-dependent Raman spectra

The composition dependence of phase transition temperature in (1-x)PbSc1/2 Ta 1/2O3-xPbHfO3 (PSTH) ceramics ( 0 ≤ x ≤ 0 . 2 ) has been investigated by Raman spectra. From the typical phonon mode variations, the PSTH ceramics unambiguously undergoes three structural transformations with increasing the temperature from 82 to 673 K. It was found that the F 2 g phonon mode disappears above the Curie temperature. Moreover, the PSTH ceramics exhibited the paraelectric to ferroelectric phase transitions at 287, 293, 313, 320, and 330 K with the composition. The phenomena can be ascribed to the enhanced length of Pb-O-Ta bonds induced by the incorporation of Hf4+ ions.

Diversity of electronic transitions and photoluminescence properties in nanocrystalline Mn/Fe-doped tin dioxide semiconductor films: An effect from oxygen pressure

Transition metal (TM: Mn or Fe) doped tin dioxide (SnO2) films with the compositions of 5% (Sn0.95TM0.05O2) have been deposited on sapphire substrates by pulsed laser deposition under oxygen pressure (Po) varied from 10−4 to 1 Pa. The x-ray diffraction, scanning electron microscopy, and infrared spectra analysis show that different TM dopants can affect the variations of crystallization and lattice distortion. Moreover, x-ray photoelectron spectroscopies indicate that the effective Po during the growth does not change the valence state of Sn4+ in the Sn0.95TM0.05O2 films. The spectral behaviors of the films have been investigated in the photon energy range of 0.47-6.5 eV (2650-190 nm). From transmittance spectra, the shoulder structures become more prominent for the Sn0.95Fe0.05O2 film than those for the Sn0.95Mn0.05O2 film due to the Fe repelling effect of a stronger p-d hybridization. The refractive index values for the Sn0.95Mn0.05O2 film are found to be larger than those for the Sn0.95Fe0.05O2 film at the photon energy of 0.47 eV. The main peaks at about 1.9 and 2.2 eV in photoluminescence (PL) emission spectra for both Sn0.95Mn0.05O2 and Sn0.95Fe0.05O2 films can be observed, and it could be explained by the fact that the electrons in the conduction band of SnO2 relax to defect states and then radiatively recombine with the holes. From direct comparison of PL and transmittance results for the films, the electronic transition energies, the emission peaks’ intensities and positions are shown to present the Po dependent behavior. The distinct trends indicate that the incorporation of Mn and Fe elements can provide a significant difference in the crystalline and electronic band structure. It can be concluded that the oxygen pressure and dopant contributions are responsible for the adjustment of electronic band structures and result in different optical response behaviors for the Sn0.95TM0.05O2 films.