Quantum Confinement Model Research Articles

A Quantum Confinement Study of the Electronic Energy of some Nanocrystalline Silicon Quantum-Dots

We have employed a quantum confinement (QC) model to the study of different shapes of nanocrystalline silicon (nc-Si) quantum dot. Each dots (shapes), although within the limits of an effective diameter of 3nm, exhibits divergence leading to different electronic energy based on the transitions from the quantum selection rule. Also, the graphical representation of the energies from each shape as a function of the effective diameter gives a qualitatively similar spectrum of discrete energies. The results obtained in this work using QC model are in good agreement with experiment and other models in literature.

A Quantum Confinement Study of the Electronic Energy of some Nanocrystalline Silicon Quantum-Dots

We have employed a quantum confinement (QC) model to the study of different shapes of nanocrystalline silicon (nc-Si) quantum dot. Each dots (shapes), although within the limits of an effective diameter of 3nm, exhibits divergence leading to different electronic energy based on the transitions from the quantum selection rule. Also, the graphical representation of the energies from each shape as a function of the effective diameter gives a qualitatively similar spectrum of discrete energies. The results obtained in this work using QC model are in good agreement with experiment and other models in literature.

Study of oxidation effect on the photoluminescence in p‐type nanostructured silicon

AbstractPorous silicon samples containing crystallites in the nanometer scale are produced and treated with an oxidizing solution. Photoluminescence (PL) spectra, attenuated total reflectance (ATR) and contact angle (CA) measurements on these samples are recorded. ATR and CA results are compared to extract information concerning the oxidation degree of the surfaces. PL spectra are deconvolved into nanocrystal structures and surface states contributions by the use of the stochastic quantum confinement model (SQCM). A correlation between nanocrystals sizes and oxidation layer presence is obtained. This result could be important for eventual PL device development based on this semiconductor material. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical emission fromSiO2-embedded silicon nanocrystals: A high-pressure Raman and photoluminescence study

We investigate the optical properties of high-quality Si nanocrystals $(\mathrm{NCs})/\mathrm{Si}{\mathrm{O}}_{2}$ multilayers under high hydrostatic pressure with Raman scattering and photoluminescence (PL) measurements. The aim of our study is to shed light on the origin of the optical emission of the Si $\mathrm{NCs}/\mathrm{Si}{\mathrm{O}}_{2}$. The Si NCs were produced by chemical-vapor deposition of Si-rich oxynitride $(\mathrm{SRON})/\mathrm{Si}{\mathrm{O}}_{2}$ multilayers with 5- and 4-nm SRON layer thicknesses on fused silica substrates and subsequent annealing at 1150 \ifmmode^\circ\else\textdegree\fi{}C, which resulted in the precipitation of Si NCs with an average size of 4.1 and 3.3 nm, respectively. From the pressure dependence of the Raman spectra we extract a phonon pressure coefficient of $8.5\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/\mathrm{GPa}$ in both samples, notably higher than that of bulk $\mathrm{Si}\phantom{\rule{0.28em}{0ex}}(5.1\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/\mathrm{GPa})$. This result is ascribed to a strong pressure amplification effect due to the larger compressibility of the $\mathrm{Si}{\mathrm{O}}_{2}$ matrix. In turn, the PL spectra exhibit two markedly different contributions: a higher-energy band that redshifts with pressure, and a lower-energy band which barely depends on pressure and which can be attributed to defect-related emission. The pressure coefficients of the higher-energy contribution are $(\ensuremath{-}27\ifmmode\pm\else\textpm\fi{}6)$ and $(\ensuremath{-}35\ifmmode\pm\else\textpm\fi{}8)\phantom{\rule{0.16em}{0ex}}\mathrm{meV}/\mathrm{GPa}$ for the Si NCs with a size of 4.1 and 3.3 nm, respectively. These values are sizably higher than those of bulk $\mathrm{Si}\phantom{\rule{0.28em}{0ex}}(\ensuremath{-}14\phantom{\rule{0.16em}{0ex}}\mathrm{meV}/\mathrm{GPa})$. When the pressure amplification effect observed by Raman scattering is incorporated into the analysis of the PL spectra, it can be concluded that the pressure behavior of the high-energy PL band is consistent with that of the indirect transition of Si and, therefore, with the quantum-confined model for the emission of the Si NCs.

Stochastic quantum confinement in nanocrystalline silicon layers: The role of quantum dots, quantum wires and localized states

Nanocrystallites of Silicon have been produced by electrochemical etching of crystal wafers. The obtained samples show photoluminescence in the red band of the visible spectrum when illuminated by ultraviolet light. The photoluminescence spectra can be deconvolved into three components according to a stochastic quantum confinement model: one band coming from Nanocrystalline dots, or quantum dots, one from Nanocrystalline wires, or quantum wires, and one from the presence of localized surface states related to silicon oxide. The results fit well within other published models.

Electroluminescence from GaN Nanostructures

Electroluminescence (EL) intensity from GaN nanostructures is reported as a function of different parameters, such excitation wavelength, number of the nanostructures, applied voltage, temperature and time. Quantum confinement model (QCM) is used to develop the model equation that describes the EL intensity as a function of size of the nanostructures. It is shown that as the number of nanostructures decreases EL intensity increases. The highly efficient EL intensity is obtained at low operating voltage (6V). It is observed that EL intensity decreases as the temperature increases and degrades with time.

Optical properties of germanium nanoparticles synthesized by pulsed laser ablation in acetone

Germanium (Ge) nanoparticles (NPs) are synthesized by means of pulsed laser ablation of bulk germanium target immersed in acetone with ns laser pulses at different pulse energies. The fabricated NPs are characterized by employing different techniques such as UV-visible absorption spectroscopy, photoluminescence, micro-Raman spectroscopy, transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The mean size of the Ge NPs is found to vary from few nm to 40 nm with the increase in laser pulse energy. Shift in the position of the absorption spectra is observed and also the photoluminescence peak shift is observed due to quantum confinement effects. High resolution TEM combined with micro-Raman spectroscopy confirms the crystalline nature of the generated germanium nanoparticles. The formation of various sizes of germanium NPs at different laser pulse energies is evident from the asymmetry in the Raman spectra and the shift in its peak position towards the lower wavenumber side. The FESEM micrographs confirm the formation of germanium micro/nanostructures at the laser ablated position of the bulk germanium. In particular, the measured NP sizes from the micro-Raman phonon quantum confinement model are found in good agreement with TEM measurements of Ge NPs.

Compositional and optical properties of SiO x films and (SiO x /SiO y ) junctions deposited by HFCVD.

In this work, non-stoichiometric silicon oxide (SiO x ) films and (SiO x /SiO y ) junctions, as-grown and after further annealing, are characterized by different techniques. The SiO x films and (SiO x /SiO y ) junctions are obtained by hot filament chemical vapor deposition technique in the range of temperatures from 900°C to 1,150°C. Transmittance spectra of the SiO x films showed a wavelength shift of the absorption edge thus indicating an increase in the optical energy band gap, when the growth temperature decreases; a similar behavior is observed in the (SiO x /SiO y ) structures, which in turn indicates a decrease in the Si excess, as Fourier transform infrared spectroscopy (FTIR) reveals, so that, the film and junction composition changes with the growth temperature. The analysis of the photoluminescence (PL) results using the quantum confinement model suggests the presence of silicon nanocrystal (Si-nc) embedded in a SiO x matrix. For the case of the as-grown SiO x films, the absorption and emission properties are correlated with quantum effects in Si-nc and defects. For the case of the as-grown (SiO x /SiO y ) junctions, only the emission mechanism related to some kinds of defects was considered, but silicon nanocrystal embedded in a SiO x matrix is present. After thermal annealing, a phase separation into Si and SiO2 occurs, as the FTIR spectra illustrates, which has repercussions in the absorption and emission properties of the films and junctions, as shown by the change in the A and B band positions on the PL spectra. These results lead to good possibilities for proposed novel applications in optoelectronic devices.PACS61.05.-a; 68.37.Og; 61.05.cp; 78.55.-m; 68.37.Ps; 81.15.Gh

Blue photoluminescence from nanocrystalline porous silicon structure fabricated by high-current pulsed electron beam irradiation

N-type Si (111) wafers have been processed by high-current pulsed electron beam (HCPEB) treatment with an increasing number of irradiation (1, 10 and 20 pulses). The results of this work show that a highly porous nanostructure was formed after irradiation. Moreover, the high-density Si nanocrystals (Si-ncs) about 3 nm were distributed on the surface of Si wafers and exhibited 3.02 eV Photoluminescence (PL) emission in blue band. The PL intensity increases with the increase in the Si-ncs’ density in accordance with the quantum confinement model, which can be ascribed to the different pulse time of HCPEB treatment. The possible formation mechanisms of micropores and Si-ncs are discussed.

Phonon confinement effects in Raman spectra of porous silicon at non‐resonant excitation condition

Light emitting porous silicon samples with different porosities, i.e. crystalline sizes, were produced from the low level doped p‐type silicon wafers by the anodization process. The effects of strong phonon confinement, redshift and broadening, were found on the O(Γ) phonon mode of the Raman spectra recorded at non‐resonant excitation condition using a near infrared 1064 nm laser excitation wavelength. Similarly, the blueshift of the photoluminescence peak was observed by reducing the crystalline sizes. Vibrational and optical findings were analysed within the existing models of confinement on the vibrational and electronic states of silicon nanocrystals. Since the energy of the photoluminescence peak of small nanocrystals also depends on the oxygen content on the surface of nanocrystals, the surface oxidation states were examined using infrared and energy dispersive spectroscopy. The partial coverage of the surface of nanocrystals was found due to the sample exposure to air. As a consequence, the photoluminescence energy did not increase as would be expected from the quantum confinement model. These results further indicate that the oxygen passivation along with the quantum confinement determines the electronic states of the silicon nanocrystals in porous silicon. Copyright © 2014 John Wiley & Sons, Ltd.

Simulation of the luminescence spectrum of CdSe quantum dots

CdSe quantum dots were prepared via a colloidal route using an organic capping agent consist thiol group (SH). We explicitly calculated the exciton binding energy and we incorporate the exciton contribution then combined it with quantum confinement model and surface states to see its effect on photoluminescence. The results obtained by using MATLAB programming. Calculations took place at room temperature on small quantum dots (1 - 5 nm). In conclusion, our results are quite new and it effect some of the feature that has quite close correspondence with the experimental. The results are also in conformity with other theoretical and experimental investigations. This study shows the applications of photoluminescence spectrum in determination the size of nano particle, distribution of size and surface energy level of CdSe quantum dots.

A Simple ZnO Nanocrystal Synthesis Illustrating Three-Dimensional Quantum Confinement

Semiconductor nanocrystals or “quantum dots” are attractive materials for exploring quantum confinement effects. For example, band-gap energies of these materials depend on particle size, and this dependence can be explained using simple, quantum mechanical models. We outline here an undergraduate physical chemistry laboratory involving the room-temperature synthesis and absorption spectroscopy of ZnO nanocrystals. The experimental results are compared with the predictions of a three-dimensional quantum confinement (or “particle-in-a-sphere”) model. The ease of synthesis and data collection, the common precursors employed, and the simple spectroscopic analysis allow facile incorporation of this experiment into essentially any undergraduate laboratory program.

Size-dependent photoluminescence in silicon nanostructures: quantum confinement effect

Visible photoluminescence (PL) from laser-etched silicon nanostructures has been analysed. A systematic size dependence study of PL from silicon nanostructures has been performed. The PL from these structures is attributed to the quantum confinement effect. Different quantum confinement models have been used for PL and Raman lineshape fitting to calculate the mean size and size distribution of silicon nanostructures and the results are comparatively studied. Calculated values of oscillator strength and radiative lifetime show that PL is due to radiative recombination of confined excitons.

Effect of post-annealing treatment on the microstructure and optical properties of ZnO/PS nanocomposite films

In this study, porous silicon (PS) templates were formed by electrochemical anodization on p-type (100) silicon wafer and ZnO films were deposited on PS substrates using radio frequency reactive magnetron sputtering technique. The effects of annealing temperature on the microstructure and optical properties of the ZnO/PS nanocomposite films were systematically investigated by X-ray diffraction (XRD), UV–Visible spectroscopy and fluorescence spectrophotometry. XRD results indicated that all ZnO films were polycrystalline in nature with a hexagonal wurtzite structure and the (0002) oriented ZnO films deposited on PS substrates had the best crystal quality under annealing at 700°C. It was demonstrated that the optical band edge shifted to longer wavelength as the annealing temperature shifted from room temperature (RT) to 700°C due to the quantum confinement effect. Furthermore, the optical band gaps calculated based on the quantum confinement model were in good agreement with the experimental values. Photoluminescence (PL) measurements at RT revealed that ZnO/PS films formed a broad PL band including the violet, blue and green emissions from ZnO and red–orange emission from the PS. The mechanism and interpretation of broadband PL of the films were discussed in detail.

The capacitance–voltage measurement of the screen-printed silicon solar cells with electrochemically etched nanostructured porous silicon antireflection coating

In the present study, capacitance–voltage measurement of screen-printed silicon solar cells with electrochemically etched nanostructured porous silicon structure has been investigated. The structural and optical properties of the freshly etched as well as thermally oxidized porous silicon surfaces was characterized by FTIR, Raman spectroscopy, photoluminescence and atomic force spectroscopy measurements. The values of the doping level, NA ≅ 8.4 × 1016 atoms cm−3 in the bulk Si substrate and the built-in potential, Vbi = 0.91 V was estimated by extrapolating the plot of the inverse-square capacitance–reverse bias voltage (I/C2–VR plot). The impedance spectra indicate a significant change in the frequency response of thermally oxidized porous silicon surface which is more distinct in the lower frequency range. The Raman spectra of the etched porous silicon has a sharp peak with a broadening and downshift of phonon energy which indicates that porous silicon is characterized by a nanoscale distribution of the pores and crystallites network and, thereby giving a detailed description on the ‘quantum confinement model’ for the light emission in the visible range from electrochemically etched porous silicon structure.

Surface Reconstruction of Electrochemically Etched Nano-structured Solar Cell

Inexpensive silicon solar cell utilizes nanotechnology offers great possibility to enhance the conversion efficiency and reduce the maintenance requirements as well. Moreover, the large surface area of the nanostructured solar cell enables this cell to reduce the number of the required cells. The surface of ordinary solar cell has been reconstructed by electrochemical etching in suitable cell. It is found that the etching time and current density have optimum values to obtain higher conversion efficiency. Effects of these two parameters on the optoelectronic properties of the solar cell have been studied. Spectroscopic methods involving Raman and photoluminescence emission were carried out on the reconstructed surface of the solar cell.Fitting of Raman line shape with theoretical quantum confinement model implies that nanostructure of mean value of 6 nm was established. While the surface morphology investigation reveal synthesize of a pore-like structure separated by thin walls consisting silicon nanostructures.

Synthesis of Silicon Nanostructures: Comparative Study

Silicon nanoparticles/nanostructures have been prepared by different methods. Lasers of different operational modes have been employed to prepare silicon nanoparticles by laser-induced etching, laser ablation and laser annealing. Moreover, electrochemical and photoelectrochemical etching were performed to synthesize silicon nanostructures. Optimum etching rate of 2.4 µm/min is obtained for the porous layer prepared by electrochemical etching under optimum conditions of 15 mA/cm2 and 10 minutes etching current density and etching time, respectively. Characterization of the prepared silicon nanostructures / nanoparticles was carried out using various methods. The experimentally observed Raman spectra of nanostructured layers prepared by three etching techniques reveal a red shift to 518 cm-1 and line broadening of 12 cm-1. While fitting of these spectra with the quantum confinement model provide an average size for nanostructured layers 6, 5.5 and 2 nm for photochemical, electrochemical and phooelectrochemical etching, respectively. The surface morphology investigation and their analysis provide valuable details on silicon nanostructure/nanoparticle size and size distribution.

Quantum confinement of volume plasmons and interband transitions in germanium nanocrystals

The plasmonic properties of individual quantum-sized Ge nanocrystals (NCs) were observed and systematically analyzed by aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). For this purpose, Ge NCs embedded in an SiO${}_{2}$ matrix with controllable size, density, and structure were fabricated using magnetron sputtering. The size dependence of the Ge plasmon energies in the size range of 5--9 nm is shown to be well depicted by the so-called medium quantum confinement (QC) model, with an effective mass of 0.57${m}_{0}$ (contrary to expectations of a stronger quantum effect). In the very low-loss region of the EEL spectra, an apparent blue shift of the ${E}_{2}$ interband transition peak up to 2 eV and a strong reduction in the oscillator strength were measured for the NCs in the size range of 4--6 nm. It indicates for this smaller size range a transition to a QC regime where the band structure and the density of states are modified dramatically. These trends are explained by a combination of low-loss and core-loss EELS results, which show that the Ge NCs are surrounded uniformly by nearly stoichiometric SiO${}_{2}$. This local chemistry is shown to provide an infinite potential barrier and to confine electrons and holes in the spherically shaped Ge NCs. In addition to pure QC effects in the Ge NCs, the SiO${}_{2}$ matrix thus plays an important role in the strength of the observed QC and interband transitions.

Photoluminescence spectra of InAs quantum dots embedded in GaAs heterostructure

Self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE) were subjected to growth ripening pause of comparatively short durations (0–50s) at growth temperature (520°C). Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) studies indicate that the QD islands increase in size with the growth pause and correspondingly their density decreases. Photoluminescence (PL) studies were carried out on these QDs. Both ground and excited state PL spectra are reported. We present a formalism based on the quantum confinement model and a distribution of dot sizes to explain our results. A minimal set of parameters is employed whose numerical values are obtained from independent experiments and/or microscopic theories. Calculations based on our formalism satisfactorily account for observed PL spectra. The temperature dependence of the PL is also studied and models to explain it are explored.

Electronic Properties of Pristine and Se Doped [001] Silicon Nanowires: An Ab Initio Study

We report the results of first-principles density functional theory calculations of the properties of pristine and Se doped hydrogen passivated silicon nanowires oriented along the [001] direction. We show that the effect of confinement in the orthogonal directions is to increase the bandgap as the diameter of the nanowire is progressively reduced, as expected from the simple quantum confinement model. Moreover, we observe that the introduction of surface reconstruction before H passivation can lead to an indirect bandgap due to a shift of the valence band maximum away from the convergence point. We also study the formation energy and hyperfine parameters of Se located in different positions in a nanowire with diameter of 0.8 nm. Our results confirms a tendency to surface segregation for Se atoms, probably driven by deformations close to the nanowire surface. These deformations cause also a steep decrease in the hyperfine contact term of Se in surface positions compared to the situation when Se is placed near the axis of the nanowire. The latter values of the hyperfine contact term are instead much greater than the corresponding bulk values.