Because of quantum confinement, semiconductor quantum dots have size dependent electronic states, and the corresponding optic properties. Besides, ultrasmall quantum dots have additional red-side broadband emission, which can cover visible light. Suitable controlling the size thus can be a good candidate for the white light emitting material. In the dissertation, we study broadband emission mechanism of ultrasmall CdSe/ZnS quantum dots by single molecule detection system. Photoluminescence excitation spectrum in the solution indicates that the broadband emission and band transition fluorescence come from the same excitation. In addition, we measure the emission spectrum of single quantum dot, which also shows the similar broadband red-emission. The emission is separated into red emission and blue emission by a dichroic mirror. Blue emission channel is for the band transition fluorescence and red emission channel is for broadband emission. Fluorescence from two channels have different decay dynamics, which indicate the different emission mechanism. According to Hanbury Brown and Twiss experiments, we obtain the photon anti-bunching behavior at zero delay time. The result is consisted with ensemble average results that the exciton from quantum dot would choose either blue emission or red emission to release its excitation, but only single photon were emitted at each excitation. When increasing the laser power, we observe cascade emissions from multi-exciton. Antisymmetric bunching behavior at zero delay time indicates strong correlation between the two channel¡¦s arriving photons. Summing over the results, we conclude that the red-emission is from an electron transfer state that either electron or hole trapped in the surface states, but the counter carrier delocalized in the QD, which is very similar to the emission from the type II semiconductor structure.

No description available.

Recently, ZnO is more and more popular in years because ZnO which is the same with GaN is a wide bandgap semiconductor. Moreover, ZnO can be made as UV region photonic devices and it is transparent. In this article, the main discussion is the Photoreflectance spectroscopy and contactless electroreflectance spectroscopy of ZnO. The next step is that measures contactless electroreflectance spectroscopy at different temperatures. After experiments, using Lorentzian profile fits the experiment data. And observing the transition type of ZnO.

InN is a semiconductor material of vary high electron mobility, so InN have potential for high speed electronic device. But the bandgap is not sure. We use photoreflectance spectroscopy to investigate bandgap of InN at different temperature. We use third derivative reflectance formula of low field to fit experimental data and appraisal the type of electron transition.

The excitonic transitions of a c-oriented wurtzite InN thin film, grown on sapphire substrate by plasma-assisted molecular beam epitaxy, were studied by photoreflectance (PR) measurement from 10 to 120 K. The energies of the observed features have a tendency to decrease with increasing temperature. They are assigned to excitonic transition rather than band-to-band transitions because the features can be observed only below 110 K. The justifications of such assignments are discussed in the context of binding energies of the excitons. The PR spectra of various power of pumping beam (Ppu) were also measured. The energies of the observed features become red-shifted with decreasing Ppu. This is consistent with assignment of the excitonic transition.

Thesis (M.Sc.(Natural and Agricultural Sciences)) -- University of Pretoria, 2009. / Summary in English. Includes bibliographical references (leaves 55-56).

The Best M.Phil Thesis in the Faculties of Dentistry, Engineering, Medicine and Science (University of Hong Kong), Li Ka Shing Prize,1991-1993 / published_or_final_version / Physics / Master / Master of Philosophy

No description available.

Semiconducting single-walled carbon nanotubes (S-SWCNTs) have direct band gaps with a range of 0.5 to 2 eV depending on the SWCNT chirality. The photoluminescence (PL) quantum efficiency and the carriers’ radiative lifetime have been previously studied but neither of them have been confirmed due to the large variation resulting from ensemble averaging, environmental effects, SWCNT defects, and SWCNT bundles. For example, quantum efficiency was estimated to be 0.01% to 7% and radiative lifetime was estimated or calculated to be 10 to 100 ns. In this thesis, we study absorption cross section, PL quantum efficiency and exciton relaxation dynamics from single air-suspended S-SWCNTs and extract “intrinsic” S-SWCNT properties. The photo-excited carriers are electron-hole pairs (called excitons) in a SWCNT due to the strong Coulomb interactions in the nm-scale system. We selected relatively bright and less defected S-SWCNTs on our samples for investigation. For each SSWCNT, the tube length, orientation, absorption and emission spectra were recorded. Experimentally, we observed that PL from a single S-SWCNT increases linearly at low excitation intensity (linear regime) and saturates at higher intensity (saturation regime). We also studied the exciton relaxation dynamics on each S-SWCNT by femtosecond excitation correlation (FEC) spectroscopy and resolved two relaxation time constants which were independent of the excitation intensity. We compare the simulation results based on a stochastic model to the experimental data and extract essential parameters including S-SWCNT unitless absorption coefficient (typically 0.02 to 0.06), PL quantum efficiency (typically 7 to 20 %) and exciton relaxation time constants. We observed very fast nonlinear exciton-exciton annihilation rate (>(2 ps)^−1) in a typical 5 μm-long S-SWCNTs. The exciton dynamics were consistent from 4 different S-SWCNTs in the saturation regime and the average total exciton number per pulse per tube in this saturation regime ranges from 2 to 12. Compared to past work, the results (PL saturation curves and FEC data) between S-SWCNTs are very consistent which supports our belief that we are studying “intrinsic properties”. We found a higher absorption coefficient, and higher PL quantum efficiency of S-SWCNTs compared to previous work. We also observe very fast nonlinear exciton-exciton annihilation in a relatively longer S-SWCNT and at lower exciton numbers. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-01-08 20:38:56.433

211 leaves : ill., tables, graphs ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.1978) from the Deaprtment of Physical and Inorganic Chemistry, University of Adelaide