http://hdl.handle.net/10397/11 Search the Channel search http://repository.lib.polyu.edu.hk/jspui/simple-search http://hdl.handle.net/10397/5676 Title: Ferromagnetism in carbon-doped ZnO thin films and nanostructures<br/><br/>Authors: Wei, Changsong<br/><br/>Abstract: The ZnO-based diluted magnetic semiconductors (DMS) have attracted scientific interest due to their unique properties and multifunctionality in ferromagnetic and ferroelectric. Transition metals have been chosen as dopants for ZnO-based DMS. However, the origin of the ferromagnetism is controversial as it is suspected that the ferromagnetism is induced by transition metal dopant clusters or secondary phases. In order to avoid the ambiguous ferromagnetism in ZnO-based DMS, carbon, as a non-magnetic element, has been suggested to be an appropriate dopant for ZnO-based DMS. Hence, in this thesis, the possibility of using carbon as a dopant to produce ZnO-based DMS is explored. The ZnO thin films were prepared firstly by the filtered cathodic vacuum arc technique. Optimization of the growth parameters, such as substrates temperature and oxygen pressure, were achieved in relation to the structural, electrical and optical properties of the ZnO thin films. The fabrication of the carbon-doped ZnO (ZnO:C) thin films and nanostructures were performed by the ion beam irradiation (IBI) technique. The two vital parameters of IBI, ion beam energy and irradiation time, were investigated systematically to determine the effective carbon doping parameters for the formation of ferromagnetic ZnO:C thin films and nanostructures.; The structural, electrical and magnetic properties of the ZnO:C thin films and nanoneedles were characterized. The ZnO:C thin films and nanoneedles all showed the hexagonal wurtzite structure with high crystallinity. The ferromagnetism in the ZnO:C system with Curie temperature (TC) higher than 330 K was found. The saturated magnetization (Ms) was enhanced with the increase of the carbon concentration. The anomalous Hall effect and negative magnetoresistance with p-type conduction were detected in the ZnO:C system. These unusual transport behaviors in the ZnO:C system are regarded as the effect correlating to the magnetic inhomogeneous and magnetic phase transition. The ferromagnetism in the ZnO:C system is stable in ambient air for more than one year. In addition, the magnetic anisotropy was detected as a signature of intrinsic ferromagnetism in the ZnO:C system. As for the ZnO:C nanoneedles, there is another dominated effect due to the strong dipoledipole magnetic interaction among the nanoneedles resulted in the magnetic anisotropy. Several characterization methods were employed to investigate the origin of ferromagnetism in the ZnO:C system. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) analysis suggest that the carbon doping introduced some defects in the ZnO samples leading to the decrease of grain size of the ZnO:C with increasing the carbon concentration. X-ray diffraction (XRD) spectra show the reduction of lattice constant c with carbon doping which is expected as smaller C ions substituted into the O sites of ZnO lattices. The X-ray absorption near-edge structure (XANES) spectra indicate a strong hybridization of O 2p orbitals with C 2p states in ZnO:C system. This p-p interaction leads to an indirect ferromagnetic coupling of C atoms. It is speculated that the substitution of C ions into O sites is related to the origin of the ferromagnetism in the ZnO:C system.<br/><br/>Description: xvii, 121 leaves : ill. (some col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P AP 2012 Wei http://hdl.handle.net/10397/5675 Title: Modeling the dielectric behaviors of perovskites : ferroelectrics and incipient ferroelectrics<br/><br/>Authors: Deng, Haiyao<br/><br/>Abstract: This dissertation is devoted to an atomistic understanding of the dielectric properties of oxide perovskites. I shall concentrate on the long-known barium titanate (BTO) and strontium titanate (STO), although other related materials will also be discussed. The BTO is known as a ferroelectric, which undergoes a series of polar structural phase transitions as temperature goes down. In the work presented here, the focus is placed on the tetragonal-cubic transition that occurs about 410K. A lot of theoretical work have been invested to clarify the transition mechanism, which pertains basically to the behaviors of the static dielectric constant and hence to the lowest frequency phonon mode. Those work were largely based on First-priciples computations or models whose origin need be revealed. Despite their usefulness, they shed little light upon the modes other than the lowest frequency one and their inter-relations. This situation may be partly due to the difficulty in establishing an atomistic model that is amenable to intuitive analysis. In this dissertation, I try to fill this gap and show how such a model can actually be achieved. In doing so, insights into other problems are obtained.; A central issue concerns the identity of the observed peaks in the imaginary part of the dielectric function. The assignment of these peaks are accomplished by comparison with the said model. Accordingly, the peaks around 180 cm⁻¹ and 510 cm⁻¹ are considered to correspond to O vibrations along and normal to the Ti-O-Ti bonds, respectively. The perceived anistropy is found to arise from the strong Ti-O covalent bonding. All remaining peaks are considered to correspond to Ti motions. These motions are highly anharmonic. As a result, these peaks are prone to temperature changes and spectrual weight transfer has to happen between them. Based on the model, one can explain (1) the huge ratio εa/εc and (2) the polar chain formation in the cubic phase. STO, on the contrary, is only an incipient ferroelectric. The model is equally applicable to this compound. The O vibrations are found at similar but slightly shifted frequencies. Such shifts are comprehensible by this model. In this compound, a key issue relates to the correlations between a zone boundary mode and a zone center mode. Both display softening behaviors. However, the softening with the latter is incomplete. The physics behind this incompleteness has been a matter for debate. In this dissertation, it is attributed to the inter-relations between these two modes. I explore this idea from both a vibronic and a phenomenological point of view. A fingerprint is found. Ferroelectricity can be induced in STO by replacing O¹ ⁶ with O¹⁸. The isotope effect is remarkable: the transition temperature can be as high as 24K. Different views have appeared to explain the effects. Based on the aforementioned model, I put forth a different idea, which states that, the main effects of the isotope substitution are to suppress the Ti zero-point fluctuations. This is signified by a decrease in the Ti tunneling frequency. In addition, I discuss the implications for cuprate superconductors.<br/><br/>Description: xvi, 186 p. : ill. ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P AP 2012 Deng http://hdl.handle.net/10397/5655 Title: Study of optical Tamm states based on the phase properties of one-dimensional photonic crystals<br/><br/>Authors: Chen, Zefeng; Han, Peng; Leung, C. W.; Wang, Yu; Hu, Mingzhe; Chen, Yihang<br/><br/>Abstract: We demonstrate the physical mechanism of optical Tamm states using the phase properties of the photonic crystals. Based on such mechanism, we propose an efficient way that can precisely produce optical Tamm states at specific frequencies. Moreover, we show that dielectric photonic crystals and single-negative materials can be effectively connected through their reflection phase. Two kinds of one-dimensional dielectric photonic crystals with different single-negative characteristics are designed and repeated alternately to construct a superlattice structure. The band structures and the transmission spectra of this superlattice show that multiple optical Tamm states arise and these Tamm states are coupled with each other to form transmission bands. A special zero-effective-phase gap can also be observed in such superlattice.<br/><br/>Description: DOI: 10.1364/OE.20.021618 http://hdl.handle.net/10397/5654 Title: Superbroadband near-infrared emission and energy transfer in Pr³⁺-Er³⁺ codoped fluorotellurite glasses<br/><br/>Authors: Zhou, Bo; Tao, Lili; Tsang, Yuen Hong; Jin, Wei; Pun, Edwin Yue-Bun<br/><br/>Abstract: We report the first demonstration of superbroadband emission extending from 1.30 to 1.68 μm in praseodymium(Pr³⁺)-erbium(Er³⁺) codoped fluorotellurite glasses under 488 nm excitation. This superbroad near-infrared emission is contributed by the Pr³⁺: ¹D₂→ ¹G₄ and Er³⁺: ⁴I ₁₃ ̷ ₂→ ⁴I ₁₅ ̷ ₂ transitions which lead to emissions located at 1.48 and 1.53 μm, respectively. The quenching of the Pr³⁺ emission resulted from the cross relaxation [ ¹D₂, ³H₄]→[ ¹G₄, ³F₃,₄] was effectively compensated by the codoping of Er³⁺. The results suggest that, other than the heavy-metal and transition-metal elements of active bismuth (Bi), nickel (Ni), chromium (Cr), etc., Pr³⁺-Er³⁺ codoped system is a promising alternative for the broadband near-infrared emission covering the expanded low-loss window.<br/><br/>Description: DOI: 10.1364/OE.20.012205