http://hdl.handle.net/10397/952 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/10397/5497 Title: Low-voltage organic thin film transistors based on high-k gate insulators<br/><br/>Authors: Li, Jinhua<br/><br/>Abstract: Organic thin film transistor (OTFT) as a basic unit of organic integrated circuits has attracted much attention recently. In the past ten years, great progress has been made in this field. Many novel organic semiconductors were developed with the performance that can rival the amorphous silicon, which pave the way to practical applications of flexible OTFTs. However, OTFTs are encountering great challenges in high operating voltages due to the lack of suitable gate dielectrics. OTFTs with high operating voltage will not only result in high energy consumption but also be incompatible with many emerging applications, including wearable electronics, disposable sensors, and radio frequency identification (RFID) tags. In principle, low-voltage OTFTs can be realized by decreasing the thickness of the gate insulator layer or using high-k gate dielectrics. It will be more convenient to fabricate low-voltage OTFTs with high-k gate dielectrics because only a gate insulator with enough thickness (> 100 nm) is compatible with solution process. Therefore we systematically studied the application of high-k gate dielectrics in OTFTs in the thesis. Relaxor ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) ( P(VDF-TrFE-CFE) ) 56/36.5/7.5 mol% exhibits high relative dielectric constant (~ 60) that is much bigger than those of the organic insulators used in OTFTs before. We successfully used the P(VDF-TrFE-CFE) high-k polymer in low-voltage and solution processible OTFTs as the gate insulator for the first time. Both n-channel and p-channel OTFTs based on conjugated polymers were fabricated and showed the carrier mobilities equal to or higher than 0.1 cm²/Vs at the operating voltage of 3 V, suggesting that P(VDF-TrFE-CFE) is an excellent high-k gate dielectric material for OTFTs. This work paves a way for developing various high-k gate dielectrics based on relaxor ferroelectric polymers for low-voltage transistors.; OTFTs based on a high mobility n-type semiconductor poly{[n,n9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) and different gate dielectrics were fabricated. The average carrier mobility decreases from 0.76 to 0.08 cm²/Vs with the increase of the gate dielectric constant from 2.6 to 7.8. We find that the P(NDI2OD-T2) polymer film shows unconventional face-on molecular packing, which results in short distance between carriers and gate dielectric and pronounced interactions between them. Therefore the decrease of the electron mobility with the increasing dielectric constant is attributed to the Frohlich polaron effect for the interaction between electrons in the channel and ionic polarization cloud in the gate dielectric. Flexible low-voltage OTFTs based on the important small-molecule organic semiconductor pentacene were fabricated by using the high-k relaxor ferroelectric polymer (P(VDF-TrFE-CFE)) as gate dielectrics. P(VDF-TrFE-CFE) films were modified with different thin polymer films (few nanometers) by solution process, which can optimize the dielectric/semiconductor interface and the performance of pentacene OTFTs. The low-voltage OTFTs fabricated on plastic substrates show the carrier mobility up to 0.6 cm²/Vs at the operating voltage of 4 V and a good stability after 1000 times bending tests. It is expected that the high-k terpolymer gate dielectric is suitable for low-voltage OTFTs based on various small-molecule organic semiconductors since the dielectrics surface can be easily modified with different thin polymer films by solution process. We have successfully fabricated low-voltage nonvolatile floating gate memories based on OTFTs with the n-typed semiconducting polymer P(NDI2OD-T2) and high-k gate dielectric P(VDF-TrFE-CFE) on flexible substrates by solution process. A thin layer of Au nano-particles was embedded in the gate insulator layer, which can keep charge and result in a memory effect. The floating gate memory demonstrated low programming/erasing voltages of ±6 V, little degradation after 10⁵ programming/erasing cycles and good retention after 10⁴ s, which suggested great promise in the application of the memory devices in flexible electronics.<br/><br/>Description: xx, 175 leaves : col. ill. ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P AP 2012 Li http://hdl.handle.net/10397/5496 Title: Optical properties and modulated luminescence of metal ion doped phosphors<br/><br/>Authors: Zhang, Yang<br/><br/>Abstract: Metal ions (including rare-earth and transition metal ions) doped phosphors have draw much attention due to their applications in light-emitting displays, lasers, optoelectronic devices and biomedicine. Among them, rare-earth doped upconversion phosphors are unique and attractive for fundamental research and great potential applications. The ability to manipulate the spectral properties of phosphors is highly desirable for understanding physical processes of energy transition and widespread applications. It is of considerable interest to find new approaches to rationally modulate the luminescence of phosphors. To date, modification of luminescence in phosphors excited by a given excitation source can normally be achieved through a conventional chemical approach, i.e., changing the composition of host materials and/or doping ions, thus limiting our understanding of the detailed process of luminescence and applications. Until now, there are no reports of an established approach to modulate upconversion emission in an in-situ and real-time way. The unique crystal structure of ferroelectric materials provides us an opportunity to couple variables including electric field and temperature to crystal symmetry in a single compound. In this thesis, optical properties of rare-earth-doped ferroelectrics are investigated, and the influences of factors such as temperature and site substitution on the phosphors are observed. We also present a new approach to enhance and modulate upconversion emission through applying a relatively low bias voltage to the rare-earth-doped BaTiO₃ (BTO) thin films. We investigated the luminescence properties of rare-earth-doped perovskite type hosts. Upconversion PL of Er³⁺ -doped BTO with perovskite ABO₃ structure was studies in terms of Er³⁺ substitutions for Ba (A-) and Ti (B-site) with different Er³⁺ doping concentrations. Photoluminescence (PL) quenching with an increasing Er3+ doping concentration was investigated based on the structural change and energy transfer of cross-relaxation process in BTO: Er, i.e. ²H₁₁/₂ + ⁴I₁₅/₂ → ⁴I₉/₂ + 4I₁₃/₂. Temperature dependence of the PL in BTO: Er was revealed, which was associated with phase transitions of BTO host. The results imply that the emission from substituted Er³⁺ ions may be used as structural probes for the ferroelectric titanates.; Significant tuning and enhancement of upconversion PL represent a great challenge. In contrast conventional chemical and plasmonic approaches, we firstly showed an enhancement and modulation of upconversion emission by applying low bias voltage to the rare-earth-doped BTO thin films on different substrates. The emission intensity at 523 nm of ITO/BTO:Yb/Er/SrRuO₃/SrTiO₃ heterostructure under the bias voltage at 10 V is almost 2.7 times as that of the unbiased one. Moreover, the PL intensity of the BTO:Yb/Er can be modulated with ac electric field. This work provides a real-time and dynamic way to control PL under an electric field. The phenomena of electroluminescence (EL) have attracted an ever-increasing demand for applications throughout the illumination and display industries. In this thesis, we reported on the fabrication and characteristic of strain-induced piezoelectric potential stimulated luminescence from ZnS:Mn film on Pb(Mg₁/₃Nb₂/₃)O₃-xPbTiO₃ (PMN-PT) substrate. The light emission of the ZnS:Mn arises from the piezoelectric potential, resulting from the converse piezoelectric effect of PMN-PT substrate. In contrast to conventional electroluminescence devices including LEDs, such novel light source can be controlled by high-frequency electric-field up to MHz. Moreover, a high-frequency ultrasonic transducer was also realized. The observed phenomena can be used to develop a dual-modal source combing light and ultrasonic signal. In conclusion, we have utilized the unique characteristics of ferroelectrics and piezoelectrics to rationally modulate the light emission from metal ions-doped phosphors. These findings will aid further investigation of fundamental research and the widespread applications of the phosphors.<br/><br/>Description: xxi, 177 leaves : ill. (chiefly col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P AP 2012 Zhang