Semiconductor Nanostructures

The dynamics of photonic evolution for the eigenstate of a generic semiconductor-nanointerface is studied in terms of its two-dimensional electron gas (2DEG) fundamental-sublevel eigenenergy's correlation with respective 2DEG areal density, versus instantaneous cumulative photonic intake. Application of this treatment to the experimental photoresponse of typical modulation – doped optoelectronic nanodevices leads to a realistic tracing of the nanostructure pertinent photodynamics.

Low-temperature near-field scanning optical microscopy was used to study the dependence of the emission spectra of single self-organized InAs on GaAs, InAs on AlAs and InP on GaInP quantum dots (QDs) on contact pressure exerted by a near-field optical fiber tip (nanoindentation). A large energy shift (up to 150 meV), broadening (up to ten meV), and intensity increase (up to one order of magnitude) of single QD emission lines have been observed at tip compressions up to 70 nm. Ground state energy shift rates from 0.5 to 3.5 meV/nm have been measured for different aperture types (rounded and flat, metal coated and uncoated) and sizes (50-300 nm) in agreement with numerical calculations using Picus-Bir orbital-strain Hamiltonian. A reduction of the hydrostatic pressure coefficient due to a nonuniform In distribution in self-organized QDs has been observed. Anomalously strong lateral inhomogeneity of the local stress field has been observed. 1 Contents Introduction I. Experimental details A. QD samples B. Near-field photoluminescence and indentation C. Fiber probes II. Calculation of indentation-induced energy shift of QDs A. Pressure-induced energy shift in III-V QDs B. Finite element modeling of the local stress and energy shift rate for a QD. III. Near-field PL spectroscopy of single QDs under nanoindentation A. NPL spectra B. Energy shift of QD emission lines C. Lateral distribution of local strain in QD structures D. Effect of aperture size on the energy shift of QDs Acknowledgments IV. Appendix: Characterization of the QDs using near-field magneto-PL spectroscopy A. InAs/AlAs and InAs/GaAs QDs B. InP QDs References

This thesis presents a fine insight into many aspects of nanostructures and their applications in photocatalysis. Au/Ag -Cadmium sulfide (CdS) nanostructures viz., nanospheres, nanorods and nanowires of different dimensions have been synthesized to investigate the effect of size, shape, nature and spatial orientations of co-catalyst onto the change in photophysical, (absorbance, photoluminescence, relaxation lifetimes) and photocatalytic properties to achieve the best information in CdS nanostructures. The present thesis is divided into eight chapters:

Introducing defect vacancies into nanostructures is a straightforward yet powerful technique employed by scientists to manipulate their properties. In this study, we used a tight-binding model to investigate the eects of an external electric eld and the creation of double vacancies at dierent sites on penta-graphene nanoribbons on the electronic and transport properties. Understanding the eects of external electric elds and vacancy creation on the electronic and transport properties of penta-graphene nanoribbons is important for the advancement of nanoelectronics and the development of innovative applications. After calculating the formation energy for all vacancy structures, it was determined that the maximum stability is achieved when the second vacancy is at the C2 site. By creating vacancies in the structure, in addition to tuning the energy gap, an indirecttodirect bandgap transition can be achieved in penta-graphene nanoribbons. The presence of a direct gap in the electronic properties of penta-graphene nanoribbons has signicant implications. Direct bandgap materials can absorb and emit photons with energies close to the bandgap energy and may be better suited for optical devices. It was also observed that the creation of vacancies in penta-graphene nanoribbons leads to a phase transition from a semiconductor to a metal. Next, the eect of these vacancies on the transport properties of penta-graphene nanoribbons was investigated. The results clearly show that the maximum current and threshold voltage can be controlled by creating vacancies at various sites on the nanoribbon. In general, by creating double vacancies in the structure or applying an external electric eld, an indirect transition to a direct band gap and a semiconductortometal phase transition have been observed. Additionally, when a double vacancy is introduced into the system and an external electric eld is applied simultaneously, at bands are observed in the band structure, as are the tunable band gap, semiconductortometal phase transition, and indirecttodirect bandgap transition. Additionally, to explore the cause of the change in electronic and transport properties with the creation of a vacancy in the structure, the charge density distribution of carbon atoms was analyzed using density functional theory calculations. Due to the dierence in charge density between the penta-graphene nanoribbon sites, signicant charge transfer occurs in the structure after a double vacancy is created in the structure. This charge transfer leads to the generation of an electric current in the nanoribbon. Because of these unique characteristics, penta-graphene nanoribbons are promising candidates for the development of solar cells.

In this paper, the study of the formation of ZnMnO nanocrystals (NCs) embedded in an alumina matrix starting from ZnMnO/Al 2 O 3 multilayers grown on a Si substrate by pulsed laser deposition with subsequent annealing is presented. The correlation between the morphology of grown NCs with the ratio R of the thickness of the ZnMnO layer to that of the Al 2 O 3 layer in the as-grown multilayer structure was observed. The results show the possibility of controlling the strain, size, and distribution of grown NCs by setting the ratio R.

Here, the proposed device is a negative capacitance based ferroelectric gate stack, raised buried oxide pocket doped SOI TFET (RBOX-SOI-Fe TFET). The architecture of the device is carefully designed with proper optimization of the pocket and ferroelectric thickness to boost the on-state current, on-off current ratio, and to improve subthreshold swing. At, first the polarization versus electric field curve of the proposed device is studied. Next, the DC and the analog/RF, linearity performance analysis are illustrated. The various analog/RF performance parameters like gate-to-source capacitance (Cgs-1.7 × 10-16 F/µm), gate-to-drain capacitance (Cgd-2.3 × 10-17 F/µm), total capacitance (Cgg-1.96 × 10-16 F/µm), transconductance (7.35 × 10-3 S/µm), output conductance, corner frequency (7.1 × 10 13 Hz), the gain bandwidth product (1.2 × 10 13 Hz), transit time (1.4 × 10-15 s), transistor frequency product (3.1 × 10 15 Hz/V), and intrinsic gain (6.91 × 10 3) have been investigated. The proposed device parameters have been compared with the existing devices in literature and RBOX-SOI-Fe TFET evinces an improved result in terms of both DC and Analog/RF parameters. Device simulations have been performed by Sentaurus TCAD 2D-simulator.

The physical properties of solid structures depending on the boundary conditions in topologically ordered nanocluster systems are discussed. These systems are synthesized using various laser methods in the form of metal/semiconductor nanostructures/nanoclusters and thin films on solid substrates. A particular attention is paid to the possible mechanisms and tendency to high-temperature superconductivity in these thin-film nanocluster structures.

The dielectric continuum model of optical phonons and the elastic continuum model of acoustic phonons provide tractable models of dimensionally-confined phonons and they provide a convenient basis for modeling phonon effects in nanostructures. This talk will highlight recent applications of approximate continuum models to a variety of device applications and potential device applications.

We have employed reflection high energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HREM) to study Cu films grown on hydrogen terminated Si( 100) and Si( 111) substrates by molecular beam epitaxy. X-ray diffraction and RHEED studies indicate ( 100) Cu growth on Si( 100) and (11 l)Cu growth on Si( 111). HREM reveals orientation relationships of [OOl]o,~~ [Ol l]si, (OlO),l] (0ll)si and [i12]c,[j [Ol l]si, (220),,11 ( 1 li), for Si( 100) and Si( 1 1 1 ), respectively. A copper silicide layer forms on Si( 100) with deposition and appears to aid in proper lattice matching. No significant interdiffused region was detected in the films deposited on Si( 11 l), however, distinct orientational variants were observed in this case.

In this study, we focused on the origin on the selective deposition of rutile and anatase TiO 2 thin films during the sputtering process. The observation on microstructural evolution of the TiO 2 films by transmission electron microscopy revealed the coexistence of rutile and anatase TiO 2 phases in the initial stage under the preferential growth conditions for the anatase TiO 2 ; the observations further revealed that the anatase phase gradually dominated the crystal structure with increasing film thickness. These results suggest that the bombardment during the sputtering deposition did not obviously affect the TiO 2 crystal structure, and this was also confirmed by off-axis magnetron sputtering experiments. We also investigated the mechanism of the effect of Sn impurity doping on the crystal structure using first-principles calculations. It is found that the formation energy of Sn-doped rutile TiO 2 is lower than that of Sn-doped anatase TiO 2 ; this suggests that the Sn-doped TiO 2 favours the rutile phase. These results offer a guideline for the utilization of selective deposition of rutile and anatase TiO 2 thin films in various industrial applications.

Many thanks to Songyan Hou and Aurélien Olivier for all the insightful discussions and experiments we shared. I am thankful to all the staff in NTU for their help maintaining and fixing the equipments so I can work efficiently. A great thank to the Nano-fabrication fabrication center team -Mohammad Shamsul, Mak Foo Wah, Zulkiflee Bin Abdullah, Irene, Chong Gang Yih, Yang Xiaohong, Ngo Ling Ling and Chung Kwok Fai -the builders of Nanoelectronics lab -Hasman Bon Hassan and Debbie -the Characterisation lab's team -Seet Lye Ping, Muhd Fauzi Bin Abdullah and Guo Xin Tina at SiCOE -and Leng-Low Poh Chee at Photonics lab. To my colleagues from CINTRA -Xuan Quyen, Christophe Couteau -and of course, my fellow PhD students and friends, from discussions during lunch and coffee breaks to experiments and travels -thanks to

In this paper, systematic study of structural, vibrational, and optical properties of undoped and 1-10 at.% Fe doped ZnO nanostructures, synthesized adopting chemical precipitation route, has been reported. Prepared nanostructures were characterized employing an assortment of microscopic and spectroscopic techniques, namely Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX) Spectroscopy, X-ray Diffraction
(XRD), Fourier Transform Infrared (FTIR), Micro-Raman Spectroscopy (lRS), and UV-visible and Photoluminescence (PL) spectroscopy. With Fe incorporation, a gradual morphological transformation of nanostructures is demonstrated vividly through SEM/TEM characterizations. Interestingly, the morphology of nanostructures evolves with 1–10 at. % Fe doping concentration in ZnO. Nanoparticles obtained with 1 at. % Fe evolve to nanorods for 3 at. % Fe; nanorods transform to nanocones (for 5 at. % and 7 at. % Fe) and finally nanocones transform to nanoflakes at 10 at. % Fe. However, at all these stages, concurrence of primary hexagonal phase of Zn1-xFexO along with the secondary phases of cubic ZnFe2O4 and rhombohedric Fe2O3, is revealed through XRD analysis. Based on collective XRD, SEM, TEM, and EDX interpretations, a model for morphological evolution of nanostructures was proposed and the pivotal role of Fe dopant was deciphered. Furthermore, vibrational properties analyzed through Raman and FTIR spectroscopies unravel the intricacies of formation and gradual enhancement of secondary phases with increased Fe concentration. UV-visible and PL spectroscopic analyses provided further insight of optical processes altering with Fe incorporation. The blue shift and gradual quenching of visible photoluminescence with Fe doping was found in accordance with structural and vibrational analyses and explicated accordingly.

In this paper gas sensing properties of 0.5-3% polyaniline (PAni) doped SnO 2 thin¯lms sensors prepared by chemical route have been studied towards the trace level detection of NO 2 gas. The structural, optical and surface morphological properties of the PAni doped SnO 2 thin¯lms were investigated by performing X-ray di®raction (XRD), Transmission electron microscopy (TEM) and Raman spectroscopy measurements. A good correlation has been identi¯ed between the microstructural and gas sensing properties of these prepared sensors. Out of these¯lms, 1% PAni doped SnO 2 sensor showed high sensitivity towards NO 2 gas along with a sensitivity of 3:01 Â 10 2 at 40 C for 10 ppm of gas. On exposure to NO 2 gas, resistance of all sensors increased to a large extent, even greater than three orders of magnitude. These changes in resistance upon removal of NO 2 gas are found to be reversible in nature and the prepared composite¯lm sensors showed good sensitivity with relatively faster response/recovery speeds.

Systematic investigations of structural properties of 1-10% Fe doped ZnO nanostructure (Fe:ZnO NS) prepared via chemical precipitation method have been reported. Structural properties were probed thoroughly employing scanning electron microscope (SEM) and transmission electron microscope (TEM), energy dispersive X-ray (EDAX) analysis and X-ray diffraction (XRD). Morphological transformation of nanostructures (NS) with Fe incorporation is evident in SEM/TEM images. Nanoparticles (NP) obtained with 1% Fe, evolve to nanorods (NR) for 3% Fe; NR transform to nanocones (NC) (for 5% and 7% Fe) and finally NC transform to nanoflakes (NF) at 10% Fe. Morover, primary phase of Zn 1-x Fe x O along with secondary phases of ZnFe 2 O 4 and Fe 2 O 3 were also revealed through XRD measurements. Based on collective XRD, SEM, TEM, and EDAX interpretations, a model for morphological evolution of NS was proposed and the pivotal role of Fe dopant was deciphered.

In this article, we explore potential of Exploding Wire Technique (EWT) to synthesize the copper nanopar-ticles using the copper metal in a plate and wire geometry. Rietveld refinement of X-ray diffraction(XRD) pattern of prepared material indicates presence of mixed phases of copper (Cu) and copperoxide (Cu2O). Agglomerates of copper and copper oxide comprised of ∼20 nm average size nanoparticlesobserved through high resolution transmission electron microscope (HRTEM) and energy dispersive x-ray(EDX) spectroscopy. Micro-Raman (R) and Fourier transform infrared (FTIR) spectroscopies of preparednanoparticles reveal existence of additional minority CuO phase, not determined earlier through XRDand TEM analysis. R investigations vividly reveal cubic Cu2O and monoclinic CuO phases based on thedifference of space group symmetries. In good agreement with Raman analysis, FTIR stretching modescorresponding to Cu2-O and Cu-O were also distinguished. Investigations of R and FTIR vibrationalmodes are in accordance and affirm concurrence of CuO phases besides predominant Cu and Cu2O phase.Quantum confinement effects along with increase of band gaps for direct and indirect optical transitionsof Cu/Cu2O/CuO nanoparticles are reflected through UV–vis (UV–vis) spectroscopy. Photoluminescence(PL) spectroscopy spots the electronic levels of each phase and optical transitions processes occurringtherein. Iterative X-ray photoelectron spectroscopy (XPS) fitting of core level spectra of Cu (2p3/2) andO (1s), divulges presence of Cu2+and Cu+in the lattice with an interesting evidence of O deficiency inthe lattice structure and surface adsorption. Magnetic analysis illustrates that the prepared nanomaterialdemonstrates ferromagnetic behaviour at room temperature.

Water driven stabilization of cadmium sulphide (CdS) nanoparticles, synthesized through a novel and facile electro-explosion of wire (EEW) technique, is reported by us. The transformation of prepared material was visually evident; as greenish black colour of the colloidal as well as the powder particles, obtained just after the synthesis, changed to orange colour after one month. Cubic Hawleyite phase CdS nanoparticles with 2.3-13.4 nm average crystallite size were ascertained by XRD analysis. HRTEM and AFM analysis collectively confirmed the formation of stable CdS nanoparticles. The crucial S-H vibrational mode, a signature interaction of CdS nanoparticles with surrounding water molecules, was revealed by FTIR analysis. The composition of prepared nanoparticles was accessed through XPS analysis. Not only structural but optical properties of nanoparticles also altered due to aging of nanoparticles. Enhanced band gap of CdS nanoparticles and gradual prominence of absorption energy with aging were demonstrated through UV-vis analysis. Complementary to this, PL spectroscopic analysis revealed the photophysics of CdS nanoparticles by providing details of radiative recombination channels. Thus, intricacies of CdS nanoparticles stabilization in aqueous environment were unravelled on the basis of variations in crystallinity, local chemical environment, alterations in electronic structure and optical processes occurring therein.

The study of mixed phase Cu/Cu 2 O/CuO nanoparticles synthesized by Exploding Wire Technique has been recently reported by us. Aiming to achieve single phase CuO nanoparticles, the mixed phase Cu/Cu 2 O/CuO nanoparticles were subjected to annealing at different temperature and time durations in oxygen environment. In this article, we discussed two samples; two phase Cu 2 O/CuO and single phase pure CuO nanoparticles obtained by annealing at 500°C and 900°C for 10 h. Rietveld refinement and Williamson-Hall analyses revealed formation of pure phase of CuO at 900°C with an average crsytallite size of 27.6 nm. Irregular shape of nanoparticles with average size of~8 nm was observed by Transmission Electron Microscopy. Selected Area Electron Diffraction pattern matches with standard interplanar distance of CuO. Fourier Transform Infrared and Micro-Raman (µR) spectra exhibit broadening of vibrational modes; indicative of pure phase CuO at 900°C. Extensive X-ray Photoelectron Spectroscopy analysis revealed that the percentage contributions of Cu 1+ and oxygen vacancy (V O) decreases whereas; Cu 2+ and interstitial oxygen (O i) enhances on increasing the annealing temperature from 500°C to 900°C and thus, resulting the pure phase formation of CuO nanoparticles. Notably, through our analyses we propose an electronic band structure diagram on the basis of valance band maximum, as obtained by XPS and the band gap energy as estimated via UV-visible spectroscopy for mixed phase of Cu 2 O/ CuO (1.6 ± 0.02 eV) and pure phase of CuO (1.3 ± 0.02 eV) nanoparticles.

Pressure is shown to have a drastic effect on the annealing characteristics of ptype, nitrogen-doped ZnSe. Samples annealed in vacuum show decreased carrier concentrations and simultaneous formation of deep-donor-related luminescence, while samples annealed under pressure show suppression of this compensating donor. The results are interpreted as an increase in the formation energy of the compensating deep donor under pressure. In addition the samples annealed under pressure show emergence of a new, intense, green luminescence band centered at 2.44 eV. The magnitude of the shift of this peak under applied stress suggests that it results from a recombination involving a deep acceptor.

For the sake of people's health and the safety of the environment, more efforts should be directed towards the fabrication of gas sensors that can operate effectively at room temperature (RT). In this context, increased attention has been paid to developing gas sensors based on rare-earth (RE)doped transparent conducting oxides (TCO). In this report, lanthanum-doped zinc oxide (La-doped ZnO) films were fabricated by sol-gel and spin-coating techniques. XRD analysis revealed the hexagonal structure of the ZnO films, with preferred growth along the (002) direction. The crystallite size was decreased from 33.21 to 26.41 nm with increasing La content to 4.0 at.%. The UV-vis-NIR indicating that the films are highly transparent (˃ 80%), La-doping increased the UV blocking ability of the films and narrowed the optical band gap (Eg) from 3.275 to 3.125 eV. Additionally, La-doping has influenced the refractive index of the samples. Gas sensing measurements were performed at ambient temperature (30 °C) and a relative humidity (RH) of 30%, employing different flow rates of carbon dioxide (CO 2) gas used synthetically with air. Among the evaluated sensors, the ZnO: 4.0 at.% La sensor exhibited the most significant gas response, with a value of 114.22%. This response was observed when the sensor was subjected to a flow rate of 200 SCCM of CO 2 gas. Additionally, the sensor revealed a response time of 24.4 s and a recovery time of 44 s. The exceptional performance exhibited by the sensor makes it very appropriate for a wide range of industrial applications. Additionally, we assessed the effect of humidity, selectivity, reusability, repeatability, detection limit, and limit of quantification.

The search for new cost-effective electrolyte materials for IT-SOFC towards its mass scale commercialization has gained momentum in recent years. The Ca-doped ceria having composition Ce0.91Ca0.09O2 was prepared using the facile conventional solid-state method. The structural and electrical properties of low sintered ceramic samples have been characterized by X-ray diffraction (XRD), UV-VIS diffuse reflectance spectroscopy (DRS) and A.C. impedance technique respectively. The oxide ion conductivity was measured between the temperatures 573 K−973 K in air. The obtained results showed that total conductivity is mainly dependent on the grain boundary effect. The nanocrystalline Ce0.91Ca0.09O2 exhibited the high total ionic conductivity of 7.36  10 3 S cm 1 at 973 K with a lower activation energy of 0.96 eV. The obtained results highlight the use of cost-effective dopant in ceria lattice to develop commercially viable electrolyte materials for IT-SOFC.

Towards the development of green energy devices, it is necessary to focus on commercial electrolyte materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). Ca-doped ceria (CDC) samples having a composition of Ce (1Àx) Ca x O 2Àd (0.03 r x r 0.1) were synthesized by a facile solid-state route and sintered at a lower temperature (1473 K). X-ray diffraction, Raman, X-ray photoelectron, Fouriertransform infrared, UV-VIS diffuse reflectance, field emission scanning electron microscopy-energy dispersive X-ray with elemental mapping, and electrochemical impedance spectroscopy techniques were used for the characterization of these CDC samples. The 0.10 CDC showed high oxide ion conductivity of 8.01 Â 10 À3 S cm À1 at 973 K with a lower activation energy of 0.78 eV. The 0.03 CDC, 0.05 CDC, and 0.07 CDC samples exhibited ionic conductivities of 1.66 Â 10 À4 , 4.42 Â 10 À3 , and 5.76 Â 10 À3 S cm À1 at 973 K with activation energies of 1.65, 1.01, and 0.92 eV, respectively. The present work aims to develop Ca-doped ceria as economically viable electrolytes for IT-SOFCs.

Low-cost semiconductors have emerged as potential electrolyte materials for intermediate temperature solid oxide fuel cells (IT-SOFC). The present work describes the synthesis and characterization of Ti 0.95 M 0.05 O 2-d (M = Ni, Cu, Zn) followed by the determination of ionic conductivity using a.c. impedance technique. The formation of the solid solution was confirmed by XRD, Raman, FTIR, DR-UV-Vis, PL, BET, FE-SEM, and EDX. The obtained ionic conductivities (in S cm-1) for TiO 2 (1.92 9 10-5), Ti 0.95 Ni 0.05 O 2-d (2.78 9 10-5), Ti 0.95 Zn 0.05 O 2-d (2.16 9 10-5) at 973 K and for Ti 0.95 Cu 0.05 O 2-d (2.39 9 10-3) at 1023 K are comparable to commercially available YSZ operating at high temperature and doped ceria electrolytes. The ionic conductivities were found to increase linearly in the temperature range of 673-1023 K. The lower activation energy of 1.08, 1.41, and 1.17 eV was obtained for TiO 2 , Ti 0.95 Ni 0.05 O 2-d, and Ti 0.95 Zn 0.05 O 2-d, respectively. Thus, low-cost Ni, Cu, Zn-doped TiO 2 solid solutions may be regarded as plausible electrolyte materials for SOFCs.

La-doped CeO 2 nanoparticles of composition Ce 1−x La x O 2−δ (0 ≤ x ≤ 0.1) have been studied here as prospective electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFCs). They were synthesized by autocombustion method and the powder samples were calcined at 700 °C to get ultrafine nanocrystalline particles. They were characterized by XRD, Raman, FTIR, XPS, DRS, FESEM/EDX, particle size analyzer and ac-impedance techniques. Ionic conductivity was measured from 350 − 750 °C. The Ce 0.90 La 0.1 O 2−δ (0.1 LDC) and Ce 0.95 La 0.05 O 2−δ (0.05 LDC) showed a maximum conductivity of 8.89 × 10 −3 and 8.32 × 10 −3 S cm −1 at 700 °C , respectively. The σ t of 0.1 LDC = 1.01 × 10 −2 S cm −1 at 750 °C. The activation energy of 0.1 LDC and 0.05 LDC was found to be 0.70 eV and 0.87 eV, respectively. These values are higher than those reported for Ladoped CeO 2 in literature. The SOFC performance with 0.05 LDC as electrolyte showed open circuit voltage of 0.81 V and maximum power density of 41 mW cm −2 at 650 °C using hydrogen as fuel.

The Cu-doped titania (Ti 0.95 Cu 0.05 O 2-δ) is studied here as a solid-state ionic conductor for its possible application in high temperature energy devices such as an electrolyte for SOFC. The sample in the powder form was obtained by solid state method using TiO 2 and copper acetate by heating up to 1200 • C for 10 h. It was characterized by XRD, FT-IR, Raman, SEM/EDS, DRS-UV-Visible, photoluminescence, BET and ac-impedance techniques. The oxide ion conductivity (σ t) values obtained from ac-impedance measurements showed a linear increase with temperature from 300 − 700 • C. The σ t values are similar to that of Ln-doped ceria, and the highest conductivity of 1.41 × 10 − 4 Scm − 1 was recorded at 700 • C. The activation energy for total conductivity was found to be 0.82 eV. The ionic and electronic transport numbers are 0.79 and 0.21, respectively. This study suggests the plausible use of rutile TiO 2 based (low-cost and structurally stable) materials as electrolytes in SOFC.

Many thanks to Songyan Hou and Aurélien Olivier for all the insightful discussions and experiments we shared. I am thankful to all the staff in NTU for their help maintaining and fixing the equipments so I can work efficiently. A great thank to the Nano-fabrication fabrication center team -Mohammad Shamsul, Mak Foo Wah, Zulkiflee Bin Abdullah, Irene, Chong Gang Yih, Yang Xiaohong, Ngo Ling Ling and Chung Kwok Fai -the builders of Nanoelectronics lab -Hasman Bon Hassan and Debbie -the Characterisation lab's team -Seet Lye Ping, Muhd Fauzi Bin Abdullah and Guo Xin Tina at SiCOE -and Leng-Low Poh Chee at Photonics lab. To my colleagues from CINTRA -Xuan Quyen, Christophe Couteau -and of course, my fellow PhD students and friends, from discussions during lunch and coffee breaks to experiments and travels -thanks to

The ideal methods for the preparation of semiconductors should be reproducible and possess the ability to control the morphology of the particles with monodispersity yields. Apart from that, it is also crucial to synthesize a large quantity of desired materials with good control of size, shape, morphology, crystallinity, composition, and surface chemistry at a reasonably low production cost. Metal oxides and chalcogenides with various morphologies and crystal structures have been obtained using different anion metal precursors (and/or different sulfur sources for chalcogenides in particular) through typical synthesis methods. Generally, spherical particles are obtained as it is thermodynamically favorable. However, by changing the anion precursor salts, the morphology of a semiconductor is influenced. Therefore, precursors having different anions show some effects on the final forms of a semiconductor. This review compiled and discussed the effects of anions (NO3−, Cl−, SO42-, CH3CO...

WNx/GaAs Schottky contacts formed by reactive sputtering were found to be thermally stable up to an annealing temprature of . . . . . . . . 900 DC. The interface morphology and structure of this contact under high temperature annealing conditions ( > 700 DC ) have been investigated by transmission electron microscopy (TEM) and x-ray diffractometry techniques. For the as-deposited samples, the thin film had an amorphous structure. Mter annealing at high temperatures, the amorphous phase transformed to (Y-W and WzN phases. However, the contact interface remained thermally stable up to 850 DC. Cross-sectional TEM micrographs revealed that annealing at temperatures above 850 DC resulted in the formation of 'pockets' beneath the interface. This phenomenon has been correlated with the electrical properties of the contacts, e. g., an enhancement of the barrier height of the contact. Comparisons between the interface morphology of this system and other refractory metal nitride contacts (e. g., TiN/GaAs) are also presented.

Experiments and theory have shown that the energy spectrum of nanostructures is extremely sensitive to the built-in strain. Knowledge of the strain distribution is therefore of utmost importance for the design of optical devices with prescribed light emission spectrum. The widely used Valence-Force-Field (VFF) model developed by Keating and later Martin relies on a parabolic expansion of the inter-atomic potential. However, for the lattice mismatches present in Ge/Si and InAs/GaAs heterostructures (4% of the equilibrium bond length in Ge/Si and 7% in InAdGaAs), which are materials widely used in the design of optical and electronic devices, anharmonic terms are expected to play an important role.

The development of new generations of laser source for fiber optic’s applications require more fast new photosensors with spectral characteristics in visible range. The idea of this work is development of technology and device – fast photosensor with spectral range adapted to modern lasers for the quickly increasing needs of optoelectronics and fiber optics communications. By pulsed laser deposition (PLD) usage UV N2 Laser for ablation λ=337.1 nm, energy per pulse 8 mJ and CW 60 W CO2 Laser for heating are produced thin CdSxSe1-x films on quartz substrate. Polycrystalline thin films with thickness from 0.5 to 2.0 μm and dominant orientation – (002) are formed by energy density 4.5 J/cm and repetition rate 20 Hz .The thin films are investigated by EDAX and SEM. The films are additional thermo treatment for increasing ratio photocurrent – dark current. This ratio reaches 10. By means of TEA UV N2 Laser with energy per pulse-0.3 mJ, pulse duration 2 ns are formed planar Ohm contacts fr...

in the PL lifetime ("lifetime blinking") occur without appreciable changes in the PL intensity. Single-dot measurements under controlled electrochemical charge injection [1] yield the PL lifetimes of neutral and charged excitons. We show that the observed "lifetime blinking" are due to random charging/discharging of the nanocrystal [2]. Indeed, the injection of electrons does not appreciably modify the PL quantum yield, which explains the coexistence of a nonblinking intensity with a "blinking" lifetime. At higher excitation power, charged excitons dominate the PL emission. We build a quantitative model showing that nanocrystal charging is caused by Auger-assisted ejection of a hole, producing negatively charged species. Importantly, Auger recombination that involves excitation of an electron is suppressed while hole-based processes remain efficient.

We study coherent charge tunneling through a one-dimensional interacting ring with a one-dimensional quantum dot embedded in one of its arms through bosonisation. The symmetries of the effective action explain many of the features such as phase change between resonances, in-phase successive resonances and phase-locking, which have been observed in experiments of coherent transport in mesoscopic rings, with a quantum dot. We also predict changes in the behaviour of the tunneling conductance in the presence of an Aharanov-Bohm flux through the ring. We argue that these results hold true in general for any dot.

Semiconductor-based metal oxide gas detector of five mixed from zinc chloride Z and tin chloride S salts Z:S ratio 0, 25, 50, 75 and 100% were fabricated on glass substrate by a spray pyrolysis technique. With thickness were about 0.2 ±0.05 µm using water soluble as precursors at a glass substrate temperature 500 ºC±5, 0.05 M, and their gas sensing properties toward CH 4 , LPG and H 2 S gas at different concentration (10, 100, 1000 ppm) in air were investigated at room temperature which related with the petroleum refining industry. Furthermore structural and morphology properties were scrutinize. Results shows that the mixing ratio affect the composition of formative oxides were (ZnO, Zn 2 SnO 4 , Zn 2 SnO 4 +ZnSnO 3 , ZnSnO 3 , SnO 2) ratios mentioned in the above respectively, and related with the sensitivity of the reduction tested gases, best sensitivity was for H 2 S gas, have sensitivity about 80.61% and a response time of 10 seconds for the binary oxides and 89.57% and a response time of (5-2) seconds for the mixed ternary oxides.

In this study, the effects of GaN capping layer on the behaviour of AlGaN/GaN nanostructure based laser is considered. We have employed the self-consistent solution of Poisson and Schrodinger equations for calculation of the energy levels, wave functions and conduction and valance bands profile. The impact of different thicknesses of the capping layer has been studied for sheet carrier density, then on optical gain. The results indicate that, by increasing the thickness of the cap layer, the optical gain decreases.

In this work, silicon samples ligated with gallium and antimony impurity atoms were investigated by the 4-probe method. The diffusion process was carried out at temperatures of 1000, 1100, 1175 and 1250 °C. The results of the experiment revealed that 1100 °C is the most favorable temperature for the formation binary compound GaSb of Ga and Sb impurity atoms. These obtained results require a deeper study of the bonding conditions of Ga and Sb impurity atoms.

The growth selectivity on patterned GaAs (311)A substrates differs qualitatively from that on low-index (100) and (111) substrates. During molecular beam epitaxy of (Al,Ga)As, [01-1] oriented mesa stripes develop a fast growing convex sidewall. A continuous transition occurs towards the slow growing concave sidewall upon turning the mesa along the perpendicular [-233] direction without breaking up the growth front into microfacets. This allows their systematic combination at the corner or edge of intersecting mesa stripes appropriately inclined from [01-1] among which we highlight those involving fast growing sidewalls. The scenario, which is unique for patterned GaAs (311)A substrates, offers a novel degree of flexibility for the design of lateral functional semiconductor nanostructures.

Density functional theory (DFT) using MPW1PW91 and B3LYP hybrid functionals was utilized for quantum-based investigations of three major sulfur compounds (H 2 S, SO 2 , and SO 3 ) adsorption onto fullerene-like Ga 12 N 12 nanocluster. All chemicals showed high chemisorption with the order of SO 3 ˃SO 2 ˃˃H 2 S. Results of charge analysis showed that during adsorption, transfer of charge is from H 2 S to nanocluster while reverse direction of charge transfer is found for SO 2 and SO 3 molecules. Partial dissociation is found for adsorbates especially for SO 2 and SO 3 molecules. Results of thermochemistry analysis show negative values for enthalpy and Gibbs free energy of adsorption, confirming exothermic spontaneous process. Analysis of frontier molecular orbital (FMO) showed important role of orbital hybridizing towards formation of new bonds upon adsorption. As a result, we introduce Ga 12 N 12 nanocluster as a strong adsorbent for sulfur compounds.

Density functional theory (DFT) using MPW1PW91 and B3LYP hybrid functionals was utilized for quantum-based investigations of three major sulfur compounds (H 2 S, SO 2 , and SO 3 ) adsorption onto fullerene-like Ga 12 N 12 nanocluster. All chemicals showed high chemisorption with the order of SO 3 ˃SO 2 ˃˃H 2 S. Results of charge analysis showed that during adsorption, transfer of charge is from H 2 S to nanocluster while reverse direction of charge transfer is found for SO 2 and SO 3 molecules. Partial dissociation is found for adsorbates especially for SO 2 and SO 3 molecules. Results of thermochemistry analysis show negative values for enthalpy and Gibbs free energy of adsorption, confirming exothermic spontaneous process. Analysis of frontier molecular orbital (FMO) showed important role of orbital hybridizing towards formation of new bonds upon adsorption. As a result, we introduce Ga 12 N 12 nanocluster as a strong adsorbent for sulfur compounds.

The semiconductor industry is being radically impacted by the development of hetero-devices and systems for a wide spectrum of technological applications that are drivers for new materials and decreasing form factors. Material selection has been constrained by existing fabrication process technology, limiting chemical sources or precursors to those that match existing CVD and ALD associated processes. Processing and integration of material structures will require the introduction of novel chemistry and deposition processes, and manufacturing worthy protocols that provide new methods for atomic level control of near-zero-thickness thin films. Potential processes and precursors that embody this strategy include: • P-CVD SiNx: This process combines precursor thermal adsorption and the reaction with remote plasma ammonia. A demonstration is presented in which thermal adsorption of tri(isopropyl)cyclotrisilazane (TICZ) ensures a substrate driven nucleation and growth process that leads ultimately to conformal SiNx in extremely high aspect ratio structures. Concurrently, the application of a remote NH3 plasma eliminates any undesirable plasma-induced effects. • ALD and CVD Through Vapor Phase Transient Species Generation: The discrete generation of disilylsilylene as a transient species in physical and energetic environments distinct from the substrate deposition and the parent precursor supply will be discussed. This process enables the deposition of epitaxial silicon films in lower thermal or energetic substrate environments. It also enables the deposition of films with greater conformality. • Integrated Synthesis and Deposition (ISD): A manufacturing paradigm comprising a method and system for real-time, closed-loop synthesis, supply, and consumption of precursors in IC manufacturing processes. In its simplest form, the system consists of a precursor synthesis chamber being physically interfaced with a thin film processing chamber, with precursor being synthesized on demand and supplied into the thin film processing chamber where it is consumed. This allows highly reactive chemistry that would be considered otherwise unacceptable due to explosive or toxicity concerns. • Area Specific Deposition by Topmost Atomic Layer Modification and Restructuring (ASD-TLM): The selective deposition of dielectric or conducting films enabled by molecular layer deposition (MLD) or self-assembled monolayers (SAMs) will be discussed. Trihydridosilanes can be grafted without catalyst to amorphous hydrogenated silicon, gold and titanium substrates. The mechanism involves dissociative adsorption and dehydrocoupling.

We study the effect of hydrostatic pressure on resonant frequency (ν 1) and its associated lifetime (τ 1), and energy (E 1) for electrons tunneling through GaAs-AlGaAs two-barrier nanostructure (TBNS). The effective mass mismatch for well and barrier materials is considered using the effective mass theory. Pressure and the Al content, which mainly affect the barrier height and consequently the TBNS's, are found to have a significant impact on resonant lifetime, resonant frequency, and resonant energy. The current study shows that the resonance lifetime, resonant frequency, and energy are strongly influenced by the barrier thickness and well width. When comparing the results of this study to the data from the experiment, good agreements are found. The GaAs-AlGaAs TBNS's electronic devices are controlled mainly by the hydrostatic pressure.

Maintaining stable fluxes for multiple source elements is a challenging task when the source materials have significantly different oxygen affinities in a complex-oxide MBE environment. Although this problem has been known to the complex oxide MBE community since the late 1980s, a detailed study and solution is still lacking. Here, using Sr as a test source, because of its easy

All praises are due to Almighty Allah, the Creator and Sustainer of the universe. He is the origin of all the knowledge and wisdom. Without His will nothing could be happened. All the regards to Holy Prophet Hazrat Muhammad (S. A. W.), Who paved our way towards success with the essence of faith in Allah and Who is forever an ivory tower for all of us. I wish to express my deep gratitude and appreciation to my respected supervisor Prof. Dr. Arshad Saleem Bhatti for his inspiring guidance, affectionate teaching and fruitful discussions. I would always look up to him as my guiding star and mentor. I am greatly indebted to my guidance and supervisory committee members Dr. Fakhar ul Inam and Dr. Umair Hassan for their motivating suggestions and guidance during the research. I greatly acknowledge Prof. Dr. Sajid Qamar (HoD) and Dr. Sadia Manzoor (chairperson) for providing a platform to utilize my skills in the research work.