Potential well

About: Potential well is a research topic. Over the lifetime, 1430 publications have been published within this topic receiving 30812 citations.

Quasiparticle energies, exciton level structures and optical absorption spectra of ultra-narrow ZSiCNRs

Abstract: The hydrogen-passivated N chain zigzag silicon carbide nanoribbons (N-ZSiCNRs) are indexed by their width N (the number of zigzag Si–C chains across the nanoribbon). Based on first-principles GW and Bethe–Salpeter equation (BSE) approaches, we investigated the quasiparticle band structures, exciton level structures and optical absorption spectra of the ultra-narrow N-ZSiCNRs with N = 2–3. It is found that the GW band gap of 3-ZSiCNR is 0.804 eV, which is more than two times larger than the HSE06 band gap (0.401 eV). The GW band gap of 2-ZSiCNR is 2.911 eV, which is also almost more than two times larger than the HSE06 band gap (1.621 eV). These results indicate that for 1-dimensional structure materials, HSE06 approaches underestimate the band gap of the system. The GW + BSE calculations demonstrate that the optical absorption spectra of the N-ZSiCNRs are dominated by edge-state-derived excitons with large binding energy, composed of a characteristic series of exciton states. It is found that the edge-state excitons of N-ZSiCNR belong to charge-transfer excitons, where the excited electron is confined to a Si edge while the hole is located on a C edge. The exciton binding energy increases with decreasing width N, which indicates that the quantum confinement effect enhances with decreasing width N. The excitons in 2-ZSiCNR can have a binding energy up to 1.78 eV. In addition, the exciton level structure and wave function are classified. It is very interesting to find a relationship between the node of the exciton wave functions and the incoming polarization light exciton excitation. For example, in the longitudinal optical absorption spectra, if the exciton whose wave function possesses an odd number of nodes is optically active, then the exciton whose wave function possesses an even number of nodes is optically inactive. In contrast, in the transverse optical absorption spectra, the exciton whose wave function possesses an odd number of nodes is optically inactive, while the exciton whose wave function possesses an even number of nodes is optically active.

Tuning of band-edges in type-I core–shell nanocrystals through band-offset engineering: selective quantum confinement effect

Abstract: The addition of a shell layer onto semiconducting quantum dots is an approach to control the optical bandgap of core–shell systems. In this direction, we have grown several core–shell nanostructures having a type-I heterostructure configuration. The nature of the energy-offset has been varied by a suitable shell-material with the aim to control the conduction and the valence band-offsets separately. Confined holes or electrons could hence be relaxed selectively leading to an increase in the valence band-edge or a decrease in the conduction band-edge. In this work, after forming such core–shell systems with a control over the shell thickness, we characterized the nanostructures with scanning tunneling spectroscopy in order to determine the density of states and finally the conduction and the valence band-edges of the core–shell systems. We found that while a large band-offset strictly localizes the carriers in the core, a small perturbation indeed delocalizes the carriers up to the shell-layer shifting the relevant band-edge towards the Fermi energy and thereby decreasing the transport gap. The decrease in the transport gap was in agreement with the optical absorption spectra. The results provide a novel route to delocalize a selective type of carrier up to the shell layer of core–shell nanostructured systems.

Band-edge photoluminescence of SiGe/strained-Si/SiGe type-II quantum wells on Si(100)

Abstract: High-quality completely lattice-relaxed SiGe buffer layer has been grown on Si(100) by using gas source molecular beam epitaxy. Pseudomorphic Si layer has been grown on this lattice-relaxed SiGe buffer layer to form SiGe/strained-Si/SiGe quantum wells. Intense band-edge photoluminescence has been observed from these quantum wells for the first time. Quantum confinement effect in SiGe/strained-Si/SiGe quantum well has been demonstrated from the systematic shift of photoluminescence energy peaks with the width of the quantum well.

Effect of Sn Doping on the Properties of Nano-Structured ZnO Thin Films Deposited by Co-Sputtering Technique.

Abstract: In this study, tin doped zinc oxide (ZnO:Sn) nano-structured thin films were successfully deposited by co-sputtering of ZnO and Sn on top of glass substrate. The effect of Sn doping on the microstructure, phase, morphology, optical and electrical properties of the films were extensively investigated by means of XRD, EDX, SEM, AFM, Hall Effect measurement, and UV-Vis spectrometry. The results showed that the undoped ZnO film exhibited preferred orientation along the c-axis of the hexagonal wurtzite structure. With increase of Sn doping, the peak position of the (002) plane was shifted to the higher 20 values, and ultimately changed to amorphous structure. The absorption edge was shifted to blue region which confirmed the excitonic quantum confinement effect in the films. Consequently, improved surface morphology with optical bandgap, reduced average particle size, reduced resistivity, enhanced Hall mobility and carrier concentration were observed in the doped films after vacuum annealing. Among all of the as-deposited and annealed ZnO:Sn films investigated in this study, annealed film doped with 8 at.% of Sn concentration exhibited the best properties with a bandgap of 3.84 eV, RMS roughness of 2.51 nm, resistivity of 2.36 ohm-cm, and Hall mobility of 83 cm2 V(-1) s(-1).

Atomic and electronic structure of silicon nanocrystals embedded in a silica matrix

Abstract: The atomic structures and the optical and electronic properties of silicon nanocrystals (nc-Si) in a β cristobalite matrix are studied using DFT calculations provided by the AIMPRO code. Five atomic models are considered (two nanocrystal diameters of 5.6 and 11 A with and without interface defects). After total relaxation, the mean Si–Si distances in nc-Si are found to be 6% higher than those in perfect bulk silicon. The optical and electronic properties are influenced by many parameters, among which are the nanograin density and size. The quantum confinement effect is demonstrated by the increase of energy gap when decreasing nanograin size. The energy gap of nc-Si is adjusted by using B3LYP functional calculations; the energy gap of 5.6 A nc-Si is found to be equal to 3.4 eV while that of 11 A nc-Si is equal to 3.1 eV. In the band structure, the levels due to nc-Si appear in the forbidden band of SiO2. The electronic density of these levels is presented in 3D. A redshift is observed in the optical absorption spectrum as the nc-Si size increases, and the absorbance of nc-Si/SiO2 is proportional to the nanograin density. The system is more stable as the distance between nanograins increases. We have also studied two kinds of nc-Si/SiO2 interface defects (Si–O–Si and Si = O bonds). It is found that the Si–O–Si bridge bond leads to the most stable configuration. The presence of Si = O double bonds reduces the nc-Si energy gap and leads to a redshift in the absorption spectrum. The Si–O–Si bonds produce the inverse effect, i.e. an energy gap increase associated with a blueshift in the absorption spectrum.