Abstract
It is interesting in low-dimensional nanostructures of silicon that the two quantum effects play different roles in nanosilicon emission, in which the quantum confinement (QC) effect opens band gap and makes emission shift into shorter wavelengths (blue-shift) as the size of the nanocrystals is reduced; however the breaking symmetry originating from impurities on nanosilicon produces the localized electronic states in band gap and makes emission shift into longer wavelengths (red-shift). The results of experiment and calculation demonstrated that the energy levels of nanosilicon can be manipulated through these quantum effects, where the curved surface (CS) effect of impurity atoms bonding on nanosilicon is important in breaking symmetry of nanosilicon system. Here, the CS effect plays an important role on impuritied nanosilicon in smaller scale with larger surface curvature, in which a few characteristic parameters have been found to describe the breaking symmetry of nanosilicon system, such as bonding angle and projecting length of bonds on curved surface. More interesting, the coupling ways between the QC effect and the CS effect determinate the levels position of localized states in band gap and manipulate emission wavelength, where a few new phenomena were explored.
Highlights
Despite bulk silicon has an indirect band gap, rendering the emission in the material relatively inefficient because of accompany by phonons for conservation of momentum, the significant progress has been made in the development of silicon photonics[1,2,3,4,5,6,7,8,9,10,11,12]
The energy EL of localized levels on nanosilicon with coupling of the quantum confinement (QC) effect and the curved surface (CS) effect can be described by the formula: EL = C/rm + βB1/(d+1) /R, where the first term relates to the QC effect and the second term relates to the CS effect[31], in which R is the curvature radius of surface and B is the bonding cover factor on surface whose index d is cover dimension, such as d = 0 for Si = O bond, d = 1 for Si-O-Si bond, and d = 2 for Si-N bond
The CS effect is often submerged in the size effect, which makes some confusion when the QC effect fails for smaller nanosilicon
Summary
It is interesting in low-dimensional nanostructures of silicon that the two quantum effects play different roles in nanosilicon emission, in which the quantum confinement (QC) effect opens band gap and makes emission shift into shorter wavelengths (blue-shift) as the size of the nanocrystals is reduced; the breaking symmetry originating from impurities on nanosilicon produces the localized electronic states in band gap and makes emission shift into longer wavelengths (red-shift). The simulating calculation on nanosilicon with spherical shape in diameter scale from 0.6 ~ 1.8 nm exhibits the detail change of the localized states in band gap, in which the a few characteristic parameters have been selected to describe the breaking symmetry of nanosilicon system, such as bonding angle and projecting length of bonds on curved surface. The results of experiment and calculation demonstrated that the QC effect produces blue-shift of emission wavelength with decrease of Si QDs size, but the CS effect generates emission red-shift with surface curvature increase of impurity atoms bonding on Si QDs, in which the breaking symmetry of nanosystem owing to Si = O, Si-O-Si and Si-N bonding on QDs surface produces the localized states obviously for manipulating the PL emission wavelength. We use the density functional theory (DFT) to calculate the density of states (DOS), which is carried out with the local density approximation (LDA) and gradient-corrected exchange-correlation function (GGA) for the self-consistent total energy methods
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