Papers by Keyword: Exciton

Abstract: The unbound exciton properties in multilayered cylindrical quantum dot (CQD) (core/shell/shell) have been studied theoretically, within the effective mass approximation and two-band model. The uncorrelated energy of an exciton confined in GaAs/Ga_1-x1Al_x1As/Ga_1-x2Al_x2As CQD as a function of the core and first shell radius is presented. The numerical results show that the quantum dot size and the confinement potentials depth significantly adjust the ground state uncorrelated energy of exciton. However, the exciton wave function parameters are dependent on the core and first shell radius (R₁ and R₂), as well as the concentration of the barrier’s materials.

Abstract: A p-type thin film of hydrogenated amporphous silicon (a-Si:H) has successfully been fabricated by radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD) technique. Substrate used in the deposition process is indium tin oxide (ITO) layer coated having size of 10 x10 cm² and being cleaned with 97% alcohol using ultrasonic bath. According to Atomic Force Microscope (AFM) observation, the layer thickness of p-type a-Si: H film was 150 nm. The Transmission spectrum at room temperature obtained from UV-Vis measurement demonstrates a large period modulation, which is due to the interference within the film. At wavelength longer than 1000 nm (or energy <1 eV), the interference modulation in the transmission spectrum of the film is seen to broaden. It is shown in a zoomed - scale around the related band gap area that one may find an exciton structure.

Abstract: There are presented optical properties of strongly in-plane elongated nanostructures the so called quantum dashes made in InAs/InP material system by molecular beam epitaxy. They have been investigated systematically by a spectroscopic manner on both the entire ensemble and on the single dash level. Their properties are discussed with respect to the fundamental electronic and optical properties as the polarization of emission and the corresponding driving factors, exciton fine structure splitting, biexciton binding energy, the characteristic exciton to biexciton lifetimes ratio and exciton decoherence via interaction with acoustic phonons. The experimental results are analyzed supported by previous energy level calculations within the eight-band kp theory and the rate equation modeling of the exciton kinetics.

Abstract: The room temperature luminescence intensity as a function of the size and the voltage of silicon (Si) and germanium (Ge) nanowires (NWs) having 5 to 30 atoms per wire with diameter ranging from 1.2 nm to 3.5 nm are investigated. The effects of exciton energy states, localized surface states and the quantum confinement are integrated in our phenomenological model to derive an analytical expression for the photoluminescence (PL) and electroluminescence (EL) intensity. By controlling a set of fitting parameters in the model, one can tune the EL and PL peak and intensity. Our results show that both quantum confinement and surface passivation in addition to exciton effects determine the optical and electronic properties of Si and Ge NWs. We observed that the EL and PL intensities occurs at the same energy, however the EL intensity has sharp Gaussian sub peaks and red shifted compared to the PL intensity.

Abstract: One of the key technologies in future optoelectronics is control of excitons in oxide materials by the coupling with plasmons on noble metal surfaces. Optical properties of ZnO nanowires decorated with Au nanoparticles were studied to understand fundamental mechanism of the coupling and to develop optoelectronic devices with new functionalities. Light intensity at the main peak position in the photoluminescence (PL) spectra of ZnO nanowires was enhanced with the coverage of Au nanoparticles. Lifetime of excitons excited optically decreased by the decoration of Au nanoparticles. Understanding of the coupling between excitons and plasmons leads to optical control of excitons and will pave the way for new type of optoelectronic devices.

Abstract: High yield ZnO nanorods are synthesized by a simple wet chemical method. The crystal morphology and structure of the ZnO nanorods are examined by transmission electron microscopy (TEM) and X-Ray Diffraction (XRD), respectively. The properties of the excitonic luminescence are investigated by temperature dependent photoluminescence (PL) spectra. Barely observed visible emission band indicates the good optical quality of the ZnO nanorods and the abnormal position and intensity changes of the emission peaks indicates the localization property of exciton.

Abstract: InGaN quantum dots (QDs) formed on top of GaN pyramids have been fabricated by means of selective area growth employing hot wall MOCVD. Upon regrowth of a patterned substrate, the growth will solely occur in the holes, which evolve into epitaxially grown wurtzite based pyramids. These pyramids are subsequently overgrown by a thin optically active InGaN well. The QDs are preferably nucleating at the apices of the pyramids as evidenced by the transmission electron microscopy (TEM). The emission from these QDs have been monitored by means of microphotoluminescence (μPL), in which single emission lines have been detected with a sub-meV line width. The μPL measurements undoubtedly reveal that the QDs are located in the apexes of the pyramids, since the sharp emission peaks can only be monitored as the excitation laser is focused on the apices in the µPL. It is also demonstrated that the emission energy can be changed in a controlled way by altering the growth conditions, like the growth temperature and/or composition, for the InGaN layers. The tip of the GaN pyramid is on the nm scale and can be made sharp or slightly truncated. TEM analysis combined with µPL results strongly indicate that the Stranski-Krastanow growth modepreferably is taking place at the microscopic c-plane truncation of the GaN pyramid. Single emission lines with a high degree of polarization is a common feature observed for individual QDs. This emission remains unchanged with increasing the excitation power and sample temperature. An in-plane elongated QD forming a shallow potential with an equal number of electrons and holes is proposed to explain the observed characteristics of merely a single exciton emission with a high degree of polarization.

Abstract: In bulk heterojunction solar cells, the donor and acceptor materials are intimately blended throughout the bulk, so that the excitons generated will reach the interface within their lifetime. In this work, Rosebengal (RB) is used as the donor material and nanocrystalline Titanium dioxide (nc TiO₂) as the acceptor material. Devices with device structure ITO/RB:TiO₂/Ag are prepared and their optical and electrical properties are compared at different temperatures. Optical absorption spectroscopic analysis shows that the absorption of Rose bengal ranges from 650-800 nm corresponding to a band gap of 1.98 eV. Cyclic voltametric analysis, and photo voltaic properties are analysed. Using simulation, the dark current parameters such as ideality factor (n), mobility (µ) potential barrier (φ_b) and carrier concentration are extracted and tabulated.

Abstract: A phenomenological model is developed by integrating the effect of excitonic energy states, localized surface states and quantum confinement (QC) to obtain an analytical expression for the room temperature photoluminescence (PL) intensity. We calculate the binding energy of strongly confined excitons in silicon (Si) quantum dots (QD) having sizes 1 to 7.75 nm to examine its contribution on optical band gap and electronic properties. The band gap with excitonic contribution is found to decrease as much as 0.23 eV for the smallest dot. The effect of exciton states explains almost accurately the experimental PL data. Our model provides the mechanism for controlling the PL intensity through fitting parameters. Huge excitonic effects, which depend strongly on QD size and shape, characterize the optical spectra. The results for the size dependence of the optical band gap, the PL intensity, and oscillator strength are presented the role excitonic effects on optical and electronic properties are discussed.

Abstract: To obtain the maximum luminous efficiency from an organic material, it is necessary to harness both the spin-symmetric and anti-symmetric molecular excitations (bound electron–hole pairs, or excitons) that result from electrical pumping. Here, we demonstrate that this deficiency can be overcome by using a phosphorescent sensitizer to excite a fluorescent dye. The photoluminescence and sensitization effect between tris (2-phenylpyridine) iridium (Ir (ppy) 3) and 5,6,11,12-tetraphenylnaphthacene (rubrene) in the host of 4,4'-N,N-dicarbazole-biphenyl (CBP) were investigated. The energy transfer characteristics in the electroluminescent process of the system of CBP, Ir (ppy) 3 and Rubrene has been discuss in this article. The Ir (ppy) 3 sensitizer affords an effective way to improve the device performance. In the organic light-emitting diodes based on the Ir (ppy) 3, rubrene and CBP system, both the singlet and triplet excitons can be used.