We prepared ZnO nanostructures using chemical and thermal evaporation methods. The properties of the fabricated nanostructures were studied using scanning electron microscopy, x-ray diffraction, photoluminescence, and electron paramagnetic resonance (EPR) spectroscopy. It was found that the luminescence in the visible region has different peak positions in samples prepared by chemical and evaporation methods. The samples fabricated by evaporation exhibited green luminescence due to surface centers, while the samples fabricated by chemical methods exhibited yellow luminescence which was not affected by the surface modification. No relationship was found between green emission and g ϳ 1.96 EPR signal, while the sample with yellow emission exhibited strong EPR signal.

The green emission band of ZnO has been investigated by both experimental and theoretical means. Two sets of equally separated fine structures with the same periodicity (close to the longitudinal optical (LO) phonon energy of ZnO) are well resolved in the low-temperature broad green emission spectra. As the temperature increases, the fine structures gradually fade out and the whole green emission band becomes smooth at room temperature. An attempt to quantitatively reproduce the variable-temperature green emission spectra using the underdamped multimode Brownian oscillator model taking into account the quantum dissipation effect of the phonon bath is done. Results show that the two electronic transitions strongly coupled to lattice vibrations of ZnO lead to the observed broad emission band with fine structures. Excellent agreement between theory and experiment for the entire temperature range enables us to determine the dimensionless Huang-Rhys factor characterizing the strength of electron-LO phonon coupling and the coupling coefficient of the LO and bath modes.Historically, zinc oxide (ZnO) is a technologically important material thanks to its piezoelectric characteristics and other unique properties such as its transparency up to the near ultraviolet (UV). It is also known that ZnO is a semiconductor with a wide band gap (∼3.37 eV) and an extremely large exciton binding energy (as high as 60 meV). 1 Recently, it has attracted renewed research interest due to its newly-found application potential in exciton-type short-wavelength optoelectronic devices that are functional at room temperature or above. 1-3 Despite a long history of industrial applications, a clear understanding of some fundamental properties of ZnO still remains elusive. 1,2,4-7 For example, contention still surrounds the microstructural origin. 4,8 To date, very different defect origins, such as the substitutional Cu 2+ on the zinc site, 9 oxygen vacancy (V O ), 10 zinc vacancy (V Zn ), 11 and interstitial zinc (Zn i ), 12 have been suggested to be responsible for the green band of ZnO. Among them, the substitutional Cu 2+ model proposed first by Dingle 9 has received much attention due to the distinct spectral features of a sharp zero-phonon line (ZPL) and a broad longitudinal optical (LO) phonon sideband at low temperature. [13][14][15] Taking into account only the coupling between one LO phonon mode and one electronic transition, Kuhnert and Helbig 13 employed a Poission distribution, I n ) S n e -S /n!, to fit the line shape of the green emission band and then obtained a Huang-Rhys factor of S ) 6.5. It is well-known that the Poission distribution simply gives only a backbone of the absorption or luminescence line shape of the electron-LO phonon coupling system. Broadening due to acoustic-phonon-bath dissipation and the temperature effect cannot be accounted for in the model. Moreover, in addition to the first set of structures, the second set of structures with the same periodicity was also observed but its origin is not yet understood. 9,...

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We report on a photoluminescence observation of robust excitonic polarons due to resonant coupling of exciton and longitudinal optical (LO) phonon as well as Fano-type interference in high quality ZnO crystal. At low enough temperatures, resonant coupling of excitons and LO phonons leads to not only traditional Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides the zero-phonon line (ZPL). The SLs and ASLs are found to be not mirror symmetric with respect to the ZPL, strongly suggesting that they are from different coupling states of exciton and phonons. Besides these spectral features showing the quasiparticle properties of exciton-phonon coupling system, the first-order SL is found to exhibit characteristic Fano lineshape, caused by quantum interference between the LO components of excitonic polarons and the continuous phonon bath. These findings lead to a new insight into fundamental effects of exciton-phonon interactions.

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.

Polycrystalline anatase Ti1−xCoxO2 (x=0–0.06) films have been fabricated by sol-gel spin coating. The effects of Co doping on the structural, optical, and magnetic properties are investigated. It is shown that oxygen vacancies and/or defects in the films are generated during thermal treatment in vacuum. Co doping reduces crystal quality and inhibits crystalline grain growth. Due to the introduction of Co, photoluminescence (PL) spectra become weak and the band gap emission has a blueshift. PL spectra reveal that the solubility of Co is lower than 0.02. At 300 K, the saturated magnetization is around 1.8 μB/Co, which is independent of the concentration of Co. This value is approximately equivalent to the average magnetic moment of bulk metallic cobalt (1.75 μB/Co). Zero-field-cooling/field-cooling measurements indicate that room temperature ferromagnetism in Co-doped TiO2 films is not an intrinsic property of the material. The presence of Co metal is identified by x-ray photoelectron spectroscopy and scanning electron microscopy.

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