We apply perturbative effective mass theory as a broadly applicable theoretical model for quantum confinement (QC) in all Si and Ge nanostructures including quantum wells (QWs), wires (Q-wires) and dots (QDs). Within the limits of strong, medium, and weak QC, valence and conduction band edge energy levels (VBM and CBM) were calculated as a function of QD diameters, QW thicknesses and Q-wire diameters. Crystalline and amorphous quantum systems were considered separately. Calculated band edge levels with strong, medium and weak QC models were compared with experimental VBM and CBM reported from X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS) or photoluminescence (PL). Experimentally, the dimensions of the nanostructures were determined directly, by transmission electron microscopy (TEM), or indirectly, by x-ray diffraction (XRD) or by XPS. We found that crystalline materials are best described by a medium confinement model, while amorphous materials exhibit strong confinement regardless of the dimensionality of the system. Our results indicate that spatial delocalization of the hole in amorphous versus crystalline nanostructures is the important parameter determining the magnitude of the band gap expansion, or the strength of the quantum confinement. In addition, the effective masses of the electron and hole are discussed as a function of crystallinity and spatial confinement.

The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO 2 , and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si 3 N 4 and Al 2 O 3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters. V

Visible luminescence from ZnO nanorods (NRs) is attracting large scientific interest for light emission and sensing applications. We study visible luminescent defects in ZnO NRs as a function of post growth thermal treatments, and find four distinct visible deep level defect states (VDLSs): blue (2.52 eV), green (2.23 eV), orange (2.03 eV), and red (1.92 eV). Photoluminescence (PL) studies reveal a distinct modification in the UV (3.25 eV) emission intensity and a shift in the visible spectra after annealing. Annealing at 600 °C in Ar (Ar600) and O2 (O600) causes a blue and red-shift in the visible emission band, respectively. All samples demonstrate orange emission from the core of the NR, with an additional surface related green, blue, and red emission in the As-Prep, Ar600, and O600 samples, respectively. From PL excitation (PLE) measurements we determine the onset energy for population of the various VDLSs, and relate it to the presence of an Urbach tail below the conduction band due to a presence of ionized Zni or Zni complexes. We measured an onset energy of 3.25 eV for the as prepared sample. The onset energy red-shifts in the annealed samples by about 0.05 to 0.1 eV indicating a change in the defect structure, which we relate to the shift in the visible emission. We then used X-ray photoemission spectroscopy (XPS), and elastic recoil detection analysis (ERDA) to understand changes in the surface structure, and H content, respectively. The results of the XPS and ERDA analysis explain how the chemical states are modified due to annealing. We summarize our results by correlating our VDLSs with specific intrinsic defect states to build a model for PL emission in ZnO NRs. These results are important for understanding how to control defect related visible emission for sensing and electroluminescence applications.

The electronic states and optical properties of Si quantum dots (QDs) with variable size prepared by ion implantation in a SiO 2 matrix are studied by x-ray photoemission spectroscopy (XPS), photoluminescence (PL), and Raman spectroscopy. The results are compared with several theories of quantum confinement. Our Si 2p binding energies and the valence band energies do not change as a function of QD diameter nor compared to the bulk Si values. Raman spectra show no signs of stress on the Si-QDs. XPS data indicates the presence of a Si 2 O 3 interfacial layer between the Si-QDs and the surrounding SiO 2 matrix, which is understood to relieve stress in the QDs and to cause pinning of the valence level. Our XPS results for ion-beam implanted QDs are compared with other group's studies for Si-QDs prepared by alternative methods, and discrepancies in the interfacial compositions are discussed. These results call into question the fundamental predictions and assumptions of many quantum confinement models. It is concluded that the lack of a shift in the valence band is due to a symmetry-breaking process in the hole states, which is not currently accounted for by theory, demonstrating the importance of the hole states during radiative events. This work is intended as a first step in highlighting the features that should be present in a theoretical formalism for embedded Si-QDs, and cause is given to abandon particular formalisms.

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Quantum confinement (QC) typically assumes a sharp interface between a nanostructure and its environment, leading to an abrupt change in the potential for confined electrons and holes. When the interface is not ideally sharp and clean, significant deviations from the QC rule appear and other parameters beyond the nanostructure size play a considerable role. In this work we elucidate the role of the interface on QC in Ge quantum dots (QDs) synthesized by rf-magnetron sputtering or plasma enhanced chemical vapor deposition (PECVD). Through a detailed electron energy loss spectroscopy (EELS) analysis we investigated the structural and chemical properties of QD interfaces. PECVD QDs exhibit a sharper interface compared to sputter ones, which also evidences a larger contribution of mixed Ge-oxide states. Such a difference strongly modifies the QC strength, as experimentally verified by light absorption spectroscopy. A large size-tuning of the optical bandgap and an increase in the oscillator strength occur when the interface is sharp. A spatially dependent effective mass (SPDEM) model is employed to account for the interface difference between Ge QDs, pointing out a larger reduction in the exciton effective mass in the sharper interface case. These results add new insights into the role of interfaces on confined systems, and open the route for reliable exploitation of QC effects.

Two deep level defects (2.25 and 2.03 eV) associated with oxygen vacancies (V o ) were identified in ZnO nanorods (NRs) grown by low cost chemical bath deposition. A transient behaviour in the photoluminescence (PL) intensity of the two V o states was found to be sensitive to the ambient environment and to NR postgrowth treatment. The largest transient was found in samples dried on a hot plate with a PL intensity decay time, in air only, of 23 and 80 s for the 2.25 and 2.03 eV peaks, respectively. Resistance measurements under UV exposure exhibited a transient behaviour in full agreement with the PL transient indicating a clear role of atmospheric O 2 on the surface defect states. A model for surface defect transient behaviour due to band bending with respect to the Fermi level is proposed. The results have implications for a variety of sensing and photovoltaic applications of ZnO NRs.PACS numbers: 68.43. Fg, 68.43.Tj, 73.20.Hb, 73.20.At Zinc oxide is a wide band gap (∼ 3.2→3.4 eV) ntype semiconductor with a large exciton binding energy (60 meV) and is a promising material for a range of applications 1 . However, studies on low cost ZnO nanostructures (NSs) and thin films are unclear as to the source of n-type conductivity and persistent photoconductivity (PPC), the UV sensing mechanism, and the defect landscape 2,3 . In particular, the correlation between the defect landscape and sensing (whether gas, pH, or UV) responsivity in low cost ZnO NRs is debated. In one model, the neutral oxygen vacancy (V o ) is an n-type donor state 3 , atmospheric O 2 absorbs at this site 2 , and a depletion region forms beneath the surface 4 . UV excitation creates electron-hole pairs, holes migrate to the depletion region and O 2 desorption occurs, thus reducing the depletion region and increasing the conductivity 4,5 . Therefore, the kinetics of O 2 desorption determine the response time of a UV sensor. In a second model, V o is reported to be a deep level state (DLS) The quality, diameter, and length of the NRs was measured using a Gemini field emission scanning electron microscopy (SEM) Carl Zeiss SUPRATM 25. CBD introduces water based absorbates 13-15 , we controlled this parameter by comparing samples as-prepared not dried (ND), dried at 100 o C for 20 min on a hot plate, and left to dry over two weeks. Additionally, we annealed a sample at 600 o C for 30 min in O 2 to understand the role of V o .Resistance measurements were performed under exposure to 364 nm UV light. The samples were biased to force a current of 1 nA between two probes 1 mm apart to extract the resistance. This value was used to avoid compliance in the samples and gave a good overall response under UV exposure. PL measurements were performed by pumping at 1.5 mW the 325 nm line of a He-Cd laser chopped through an acousto-optic modulator at a frequency of 55 Hz. The PL signal was analyzed by a single grating monochromator, detected with a Hamamatsu visible photomultiplier, and recorded with a lock-in am-

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