Thesis (M. Phil.)--University of Hong Kong, 2010. / Includes bibliographical references. Also available in print.

Recent scientific progress has resulted in the development of sophisticated hybrid nanostructures composed of semiconductor and metal nanoparticles. These hybrid structures promise to produce a new generation of nanoscale optoelectronic devices that combine the best attributes of each component material. The optical response of metal nanostructures is dominated by surface plasmon resonances which create large local electromagnetic field enhancements. When coupled to surrounding semiconductor components, the enhanced local fields result in strong absorption/emission, optical gain, and nonlinear effects. Although hybrid nanostructures are poised to be utilized in a variety of applications, serious hurdles for the design of new devices remain. These difficulties largely result from a poor understanding of how the structural components interact at the nanoscale. The interactions strongly depend on the exact composition and geometry of the structure, and therefore, a quantitative comparison between theory and experiment is often difficult to achieve. Colloidal semiconductor quantum dots are strong candidates for integration with metal nanostructures because they have a variety of desirable optical properties, such as tunable emission and long term photostability. However, one potential drawback of colloidal quantum dots is the intermittency in their fluorescence (commonly referred to as “blinking”). Blinking was first observed over a decade ago, yet there is still no complete theory to explain why it occurs. In spite of the lack of a full theoretical explanation, multiple methods have been used to reduce blinking behavior, including modifying quantum dot interfaces and coupling quantum dots with metal nanostructures. This thesis focuses on studying the coupling between colloidal quantum dots and metal nanoparticles in simple model systems. Atomic force microscopy nanomanipulation is used to assemble the hybrid structures with a controlled geometry. The experimental studies report for the first time the modified fluorescence decay, emission intensity, and blinking of a single quantum dot coupled to a single Au nanoparticle. Since the geometry of the structure is known, these studies provide reliable information on the interparticle coupling, and quantitative experimental results are shown to be consistent with classical electrodynamic theories. / text

　Zinc oxide (ZnO) is a wide band gap (3.4eV at 300K) II-VI semiconductor with an exciton binding energy up to 60meV and is promising in the realization of excitonic or polaritonic lasing effect. Photoluminescence is widely used in studying the band gap and defect levels of ZnO. However, understanding in defects of ZnO is still far from satisfaction and remains controversial. Different authors suggest different explanations and mechanisms. 　In the present study we investigate in the photoluminescence spectra of four kinds of ZnO single crystal, namely as-grown (not implanted) Zn-face polished, Zn-implanted, O-implanted and He-implanted. The samples are annealed both in air and argon gas at a temperature of 350, 650, 750, 900 and 1200oC. The results show that O-implanted sample is weaker in excitonic emission and has an annealing effect tendency not consistent with that of Zn-implanted and He-implanted. Ion implantation would introduce defects in favor of yellow luminescence and the defects would anneal out gradually as the annealing temperature is rising. / published_or_final_version / Physics / Master / Master of Philosophy

Indium nitride (InN) is a group III-V semiconductor that is part of the Al,Ga:N family. It is an infrared bandgap semiconductor with great potential for use in photovoltaic applications. Being an intrinsically n-type material, p-type doping is naturally one of the ongoing hot topics in InN research, which is of interest in the fabrication of pn junctions. Plasma-assisted molecular beam epitaxy (PAMBE) grown Mg doped InN thin film was investigated via systematic optical characterizations. Photoluminescence (PL) measurement has been a key part of the research, exhibiting a wide range of spectral lines between 0.54 and 0.67 eV. In a critical Mg concentration range of 2.6×10¹⁷ and 1.0×10¹⁸ cm⁻³, a strong luminescence line at 0.6 eV has been associated with a Mg-related deep acceptor. Correspondingly, a variable magnetic field Hall (VFH) effect measurement has successfully probed a buried hole-mediated conductivity path underneath a surface electron accumulation layer. This specific doping range also led to a manifestation of a “true” band-to-band transition at 0.67 eV. Such an observation has not previously been reported for InN and in our case this assignment is convincingly supported by the quadratic characteristic of the excitation power law. This established that a rigorous control of Mg flux can sufficiently compensate the background electron concentration of InN via the substitutional incorporation on In sites (Mg_In). However, introduction of donor-like complexes somewhat suppressed this process if too much Mg or even alternative dopants such as Zn and Mn were used. Also distinctively observed was a strongly quenched PL quantum efficiency from heavily doped films, where time-resolved differential transmission (TRDT) measurement showed a biexponential carrier lifetime decay curve owing to the onset of Auger recombination processes. These observations certainly have profound implications for devices and beyond.

Photoluminescence spectroscopy of bulk zinc oxide under different annealing conditions was examined. The effect of the annealing atmosphere, temperature and time on the optical properties of zinc oxide were studied to investigate the influence on the intrinsic defects present. The wafers used were bulk +c ZnO grown by Tokyo Denpa using the hydrothermal technique. The annealing effect on both zinc and oxygen faces was investigated. The dominant donor bound exciton related to aluminum, labelled in the literature as I₆ demonstrated a splitting of 0.3 meV. The origin of this splitting has been linked to an interaction between aluminum and hydrogen, through its reaction to atmospheric dependent annealing. The removal of the hydrothermal hydrogen peak at 3.3624 eV has uncovered some fine structure. After Arrhenius analysis of this fine structure it was shown it is excited states of bound excitons. This fine structure has been loosely associated with vibrational and rotational excited states. The behaviour of all the optical features present in the photoluminescent spectra under annealing has a relation with the carrier concentration of the samples.

Organic light emitting diodes (OLEDs) are an emerging display technology with the advantages of being efficient, bright, portable and flexible. In this work, a number of novel compounds have been developed for incorporation into OLEDs as emitting dopants. A series of ligands containing dipyrido[3,2-a:2�,3�-c]phenazine substituted at the 11-position with ethyl ester, bromo-, nitrile and 5-phenyl-1,3,4-oxadiazole moieties have been synthesised. Each of the ligands were coordinated to Re(I), Cu(I), Ru(II) and Ir(III) metal centres. Ligands and complexes were characterised by �H NMR and IR spectroscopy, mass spectrometry and microanalysis. Single crystal X-ray analyses were performed on fac-chlorotricarbonyl(dipyrido[3,2-a:2�,3�-c]phenazine-11-carboxylic ethyl ester)rhenium (triclinic, P-1, a = 6.403(5) Å, b = 10.388(5) Å, c = 16.976(5) Å, α = 84.087(5)�, β = 84.161(5)�, γ = 79.369(5)�, Z = 2, R1 = 0.0536, wR2 = 0.0978), fac-chlorotricarbonyl(11-bromodipyrido[3,2-a:2�,3�-c]phenazine)rhenium.CH₃OH (monoclinic, C2/c, a = 19.506(5) Å, b = 18.043(5) Å, c = 13.320(5) Å, α = γ = 90�, β = 114.936(5)�, Z = 4, R1 = 0.0345, wR2 = 0.0827), fac-chlorotricarbonyl(11-cyanodipyrido[3,2-a:2�,3�-c]phenazine)rhenium (triclinic, P-1, a = 6.509(5) Å, b = 12.403(5) Å, c = 13.907(5) Å, α = 96.88(5)�, β = 92.41(5)�, γ = 92.13(5)�, Z = 2, R1 = 0.0329, wR2 = 0.0701), bis-2,2�-bipyridyl(2-(11-dipyrido[3,2-a:2�,3�-c]phenazine)-5-phenyl-1,3,4-oxadiazole)ruthenium triflate.2CH₃CN (triclinic, P-1, a = 10.601(5) Å, b = 12.420(5) Å, c = 20.066(5) Å, α = 92.846(5)�, β = 96.493(5)�, γ = 103.720(5)�, Z = 2, R1 = 0.0650, wR2 = 0.1458) and bis-(2-phenylpyridine-C�,N�)(dipyrido[3,2-a:2�,3�-c]phenazine)iridium(III) hexafluorophosphate.(CH₃)₂CO (triclinic, P-1, a = 13.505(5) Å, b = 16.193(5) Å, c = 19.788(5) Å, α = 92.857(5)�, β = 98.710(5)�, γ = 93.432(5)�, Z = 2, R1 = 0.0494, wR2 = 0.1097). The ground and excited state properties of the ligands and complexes were investigated by a range of techniques, including electrochemistry, absorption and emission spectroscopy, spectroelectrochemistry and excited state lifetime studies. Complexes of dppz-based ligands typically show MOs which are segregated over either the bpy or phz region of the dppz backbone. The properties of the Ru(II) and Ir(III) complexes of the ligand series investigated in this work were consistent with this model, and the LUMOs of these complexes were assigned as the b₁(phz) phz-localised MO. The Re(I) and Cu(I) complexes of the ligand series appeared to show MOs which were delocalised over the entire dppz ligand. A modular complex containing an electron transport group, hole transport group and emitting centre was synthesised. The complex fac-tricarbonyl(trans-(E)-1-((2,2�:5�,2��-terthiophen)-3�-yl)-2-(4�-pyridyl)-ethane)(2-(11-dipyrido[3,2-a:2�,3�-c]phenazine)-5-phenyl-1,3,4-oxadiazole)rhenium(I) hexafluorophosphate was oxidised and reduced readily, encouraging efficient transport of both holes and electrons. However, this resulted in the complex having a small band gap and hence a low quantum yield of emission. Emission from this complex appeared to be from more than one state. The complexes containing the dppz-based ligand series show complicated excited state behaviour. Emission behaviour is consistent with input from more than one state for many of the Re(I), Cu(I) and Ir(III) complexes. The Ru(II) complexes of the ligand series emit from a �MLCT state between metal-based and bpy-based MOs located on the dppz ligands, as is usual for complexes of this type. All complexes containing 11-cyanodipyrido[3,2-a:2�,3�-c]phenazine showed extremely short excited state lifetimes consistent with extremely efficient non-radiative deactivation of the excited state. Ligands and complexes were incorporated into OLEDs with the structure [ITO/PEDOT:PSS/PVK:BuPBD:dopant/BCP/Alq₃/LiF/Al] to test their ability to behave as emissive dyes. Many of the compounds behaved poorly as dopants due to their low emission quantum yields, and poor alignment of HOMO and LUMO energy levels with those of the other compounds within the device. �MLCT-based emission was achieved through energy transfer from the PVK host for the devices containing chlorotricarbonylrhenium(I) complexes of the ligand series. The OLEDs containing Ru(II) and Ir(III) complexes also emitted from dopant-centred �MLCT states. In these devices, dopant excitation appeared to occur through direct charge trapping from the adjacent hole transport and electron transport layers.

The optical properties of Mg-doped InN thin films grown on YSZ substrates have been investigated by photoluminescence (PL). A series of InN:Mg samples with various Mg cell temperatures (TMg) were produced by molecular beam epitaxy. The effect of Mg concentration on PL emission properties have been explored by various excitation power and temperature dependent measurements. The PL spectra as a function of excitation power exhibited a pronounce blueshift, indicating prominent band filling caused by the Burstein-Moss effect. Meanwhile, a typical redshift was observed as temperature increased due to bandgap shrinkage. The samples with TMg below 210 ˚C have a dominant peak at energy of 0.68 eV. In contrast, the PL peak emissions for films with a high TMg between 210~230 ˚C were centred near 0.6 eV. No PL emission was observed from the films with TMg above 230 ˚C. By fitting with an empirical Arrhenius equation, the activation energies yield approximately 20 meV and 15 meV for the lower and higher energy transitions, respectively. The fundamental optical properties of Zn doped InN were also examined. InN:Zn films were grown under In-rich conditions. The samples showed well defined PL emission spectra implying that the quality of the film has been improved over the Mg-doped series. The PL spectra of InN:Zn exhibited prominent features containing various emission peaks. The combination of excitation power and temperature dependent measurements supports a precise determination for the origins of the observed transitions. The comparison between the optical properties of Mg and Zn doped InN provide the motivation for more precise quantitative interpretation of p-type InN.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (Ph. D.)--University of Hong Kong, 1990.

Includes bibliographical references (p. 104-110).