Papers by Keyword: Quantum Confinement

Abstract: Pure and copper doped tin oxide nanoparticles were synthesized by co-precipitation method and are characterized by XRD, SEM, EDAX, UV-Visible, photoluminescence, and FT-IR analysis techniques. Tetragonal rutile structure is confirmed from XRD and the crystallite size is found to be between 3.8nm and 4.8nm. The optical band gap is observed from UV-Vis spectrum and is found to be 3.99eV and 3.93eV for tin oxide and copper doped tin oxide respectively. The optical band gap of pure and Copper doped tin oxide were blue shifted due to quantum confinement. Photoluminescence spectrum shows UV, blue and green emission peaks.

Abstract: This paper described the synthesis of graphene quantum dot (GQDs) by hydrothermal method using graphene (average thickness of 7 nm) as a precursor material. At first, graphene was diluted in n-butyl acetate to obtain uniform mixture through sonication. Then, graphene was transferred into Teflon-lined autoclave and to be heated at different period of times to obtain resultant GQDs.The synthesised GQDs was characterized by using FT-IR Spectrometer (FTIR), UV-VIS Spectroscopy, photoluminescence (PL) and transmission electron microscopy (TEM). Typically, the absorption peak of GQDs were observed around 260 nm and 330 nm in UV-VIS spectra due to π →π * transition of aromatic sp² domain. This GQD has a broad peak and emit strong PL, light centred at 440 nm upon excitation at 260 nm. Thus, blue-luminescent GQDs are demonstrated, with a material performance that is competitive with GQDs produced by other methods.

Abstract: Chalcogenide semiconductor nanoparticles and their self-assembly structures have become the most explored group of semiconductor nanomaterials due to the interesting physics involved in quantum confinement, surface chemistry and variety of applications. In the last couple of decades, facile routes for their synthesis and strategies for controlling the size, shape and morphology have been reported. In the present review, synthesis strategies of size and shape controlled nanoparticles belonging to II-VI group of semiconductor chalcogenides are presented and each method for preparation of nanoparticles is critically analysed. Role of various factors that affect the nucleation and growth of nanoparticles is discussed at length. Nanoparticles and self-assemblies of CdSe, CdTe, HgTe and ZnSe are synthesized using new and facile single molecular precursor based noble route by our group that uses non-pyrophoric, low temperature and non-toxic chemicals, their properties and synthesis scheme are discussed as future development in this field. Some recent applications of chalcogenides QDs in the fields of solar cell, optical fibre amplifiers, biosensing and bo-imaging are discussed and reviewed.

Abstract: The energetic, structural and electronic properties of CdS with different nanometer sizes were prepared by the precipitation method by changing the refluxing time of the reactants. The grain sizes were determined by X-ray diffraction method. By impedance analyses method, the band gap value of nanoparticles were calculated and compared with the bulk material. The nanocrystalline CdS was characterized by Impedance spectroscopy and corresponding measurements are discussed briefly in this paper.

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: Amorphous Si/SiO₂ quantum wells have been obtained at room temperature with atomic precision using magnetron sputtering. The Si/SiO₂ layer structure induces the higher optical transmittance at the visible wavelength region with increasing layer numbers. The tentative absorption coefficients are evaluated for integrated Si thicknesses. The absorption edge energy dependency on Si layer thickness E₀ = 1.61 + 0.75d^-2 is in accordance with effective mass theory for thicknesses 0.5 < d < 6nm. Quantum confinement effects of the Si/SiO₂ nanostructure layer are confirmed from optical transmittance and reflectance spectra.

Abstract: Gallium Phosphide (GaP) nanostructures were grown on p-Si substrates by Metal Organic Chemical Vapor Deposition (MOCVD) to study the structure of low dimensional IIIV semiconductor on Si substrates. It is found that at a temperature of 540 °C, nanostructures with diameter 4080 nm and height 515 nm were obtained. The density of the nanostructures was found to be 10¹⁴ m^-2. The UV-Vis-NIR spectra showed a blue shift of band gap. Photoluminescence measurements also confirmed the band gap enhancement.

Abstract: A modified pulsed laser deposition (PLD) was employed to deposit GaAs nanocrystals on the surface of PMMA microspheres. This novel approach is distinguished by the fact that laser ablated materials are deposited uniformly onto the surface of spherical particles that are held constantly in a particle fludization unit. The XRD, SEM, EDX, TEM, EDP and PL results confirmed that cubic structured GaAs nanocrystals were deposited uniformly on the surface of PMMA microspheres with an average diameter of about 15 nm.

Abstract: We Perform Density Functional Theory Calculations of the Hydrogen-Passivated Topological Silicon Carbide Quantum Dots (QDs) and Investigate their Structural, Electronic and Optical Properties. We Study Clusters Constructed from 3C-Sic with up to 8 Topological Shells, Corresponding to Diameters up to 2.2 Nm, Terminated Homogeneously with either Si-H or C-H Bonds. All Qds Exhibit Tensile Strain (1-5 %) within the Cluster Core. the Larger the Cluster, the Smaller the Strain in the Interior, however. Tensile Strain Increases from the inside of the Cluster towards the outside, Reaches a Maximum at the Second Layer below the Surface, and Vanishes only for Bonds Involving Surface Si or C Atoms. Quantum-Confinement Effects Are Observed for the Energy Gaps and Optical Gaps of SiC QDs. Size Has a Major Impact on the Absorption Edge in Comparison to a Weak Effect on the Photon Energy of the Spectra Maxima. Our Calculations Show that Surface Termination Plays a Crucial Role and Strongly Affects Energy Gaps, Optical Gaps and Optical Spectra. Orbitals around the HOMO-LUMO Gap Predominantly Localize within the Core of the Cluster, with Significant Contributions by the Surface for Si-H Terminated Clusters only.

Abstract: Synthesis of Si quantum dots (QDs), useful for multi-junction crystalline Si solar cells, using porous Silicon (PS) is presented in this paper. Four types of freestanding PS structures are fabricated by anodization method with modulation of current density between two levels. The level-1 current density is kept constant at 20 mA/cm2 (for reference monolayer structure - sample A) and 10 mA/cm2 (for all multilayer structures samples B, C, D). The level-2 is varied between 0 to 50 mA/cm2 (0, 20, 30, 50 mA/cm2 as sample A, B, C and D respectively). In order to obtain Si QDs from PS films, the films are subjected to sonication (120 W, 42 kHz) for 6 hours. HRTEM images confirm presence of Si nanoparticles in the range of 2 to 8 nm. Various spectroscopic analyses of Si nanoparticles are performed in order to evaluate quantum confinement behavior and surface modification observed during sonication. Analysis of de-convoluted Raman peaks shows frequency downshift and increase in full width half maximum due to formation of QDs. After sonication, PL spectroscopy indicates blue shift from 2.54 eV (sample A) to 2.85 eV (sample D_6HR), similar to the observations made by UV-Vis spectroscopy. FTIR spectra show oxidation of Si QDs during sonication. Spectroscopic and microscopic results are explained using quantum confinement and surface modification phenomenon.