Optical properties of acceptor–exciton complexes in ZnO/SiO2 quantum dots (original) (raw)
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Solid State Communications, 2011
Assuming finite depth and within the effective mass approximation, the energies of exciton states and of the acceptor-exciton complexes confined in spherical ZnO quantum dots (QDs) embedded in a SiO 2 matrix are calculated using a matrix procedure, including a three-dimensional confinement of carrier in the QDs. This theoretical model has been designed to illustrate the two emission bands in the UV region observed in our experimental Photoluminescence spectrum (PL), with the first emission band observed at 3.04 eV and attributed to the bound ionized acceptor-exciton complexes, and the second one located at 3.5 and assigned to the free exciton. Our calculations have revealed a good agreement between the matrix element calculation method and the experimental results.
Optical properties of exciton confinement in spherical ZnO quantum dots embedded in matrix
Superlattices and Microstructures, 2009
The optical characteristics of ZnO nanoparticles in SiO 2 matrix have been determined by UV-visible and photoluminescence (PL) studies. The PL spectrum of the ZnO quantum dots shows two different bands: the first one is a broad green emission band related to deep level emission in the visible range and the second one is situated at 3.48 eV in the ultraviolet region, and is attributed to the recombination of electrons in the conduction band and holes in the valence band. The experimental result that we found was simulated numerically using different methods. Our calculations revealed a good agreement between the matrix element calculation method and the experimental result. We have also calculated the ground state binding energy, the oscillator strength and the radiative lifetime of the exciton in a ZnO spherical quantum dot. It was clearly shown from our calculation that these physical parameters are very sensitive to the quantum dot size. As a consequence, the optical and electronic properties of the dot can be controlled and also tuned through the nanoparticle size variation. These results could be particularly helpful, since they are closely related to experiments performed on such nanoparticles; this may allow us to improve the stability and efficiency of semiconductor quantum dot luminescence, which is considered critical.
Linear and nonlinear intraband optical properties of ZnO quantum dots embedded in SiO2 matrix
AIP Advances, 2012
In this work we investigate some optical properties of semiconductor ZnO spherical quantum dot embedded in an amorphous SiO 2 dielectric matrix. Using the framework of effective mass approximation, we have studied intraband S-P, and P-D transitions in a singly charged spherical ZnO quantum dot. The optical properties are investigated in terms of the linear and nonlinear photoabsorption coefficient, the change in refractive index, and the third order nonlinear susceptibility and oscillator strengths. Using the parabolic confinement potential of electron in the dot these parameters are studied with the variation of the dot size, and the energy and intensity of incident radiation. The photoionization cross sections are also obtained for the different dot radii from the initial ground state of the dot. It is found that dot size, confinement potential, and incident radiation intensity affects intraband optical properties of the dot significantly.
ZnO Quantum Dots: Physical Properties and Optoelectronic Applications
Journal of Nanoelectronics and Optoelectronics, 2006
We present a review of the recent theoretical and experimental investigation of excitonic and phonon states in ZnO quantum dots. A small dielectric constant in ZnO leads to very large exciton binding energies, while wurtzite crystal structure results in unique phonon spectra different from those in cubic crystals. The exciton energies and radiative lifetimes are determined in the intermediate quantum confinement regime, which is pertinent to a variety of realistic ZnO quantum dots produced by wet chemistry methods. An analytical model for the interface and confined polar optical phonons is presented for spheroidal quantum dots of different size and barrier materials. The experimental part of the review covers results of the nonresonant and resonant Raman spectroscopy and photoluminescence study of ZnO quantum dots with sizes comparable to or larger than the exciton diameter in ZnO. The origins of the Raman phonon shifts and the mechanisms of the carrier recombination in ZnO quantum dots are discussed in detail. The reviewed properties of ZnO quantum dots are important for the proposed optoelectronic applications of these nanostructures.
Quantum Confinement Effects on Electronic Properties of ZnO Quantum Dots
Advanced Science, Engineering and Medicine, 2014
We present a theoretical investigation on the quantum confinement effects (QC) on the electronic properties of ZnO quantum dots (QDs) embedded in MgO matrix. The latter material acts as a huge barrier for both electrons and holes so that the ZnO QD behaves as a three-dimensional quantum well. As a computational method, the tight-binding with sp 3 minimal basis set is employed to probe the electronic band structure and inspect the number and the confinement energies of the bound states versus QD size (up to 20 Å) and the valence band offset (VBO). Excellent agreement is achieved between the theoretically obtained band-gap energy (E g ) and the available experimental photoluminescence (PL) data, especially when VBO = 1 eV, which correspond to the maximum compromised confinements between holes and electrons. Furthermore, theoretical results show that the QC energy follows a power-law rule, indicating strong confinement, and is the main reason behind the UV emissions in ZnO QDs. The strong QC of excitons would further explain the enhancement of the oscillator strength and recombination rate. The excellent agreements between our results and the available experimental data do corroborate our claims.
Excitons in ZnO Quantum Dots: The Role of Dielectric Confinement
Journal of Physical Chemistry C, 2022
Quantum dots (QDs) grown by chemical synthesis methods are relatively unstable and present difficulties in dispersion and preservation. The most common way of surpassing the stability issues is to embed (and passivate) the QDs in a matrix material. However, some fundamental questions concerning the influence of the dielectric confinement caused by the matrix environment on the exciton energy and exciton binding energy of the QDs have not yet been properly addressed. Here we explore the exciton fine structure of wurtzite ZnO QDs by means of planewave million-atom atomistic pseudopotential calculations and a configuration interaction approach taking into account the dielectric confinement from the surrounding material. Our results indicate that the exciton energy increases and the exciton binding energy decreases as the dielectric constant of the surrounding material increases. The behavior of the exciton binding energy is caused by the presence of self-polarization potential induced by the dielectric mismatch in the surface of the QD. This mainly alters the localization of the hole charge density and thus reduces the electron−hole overlap and inevitably the exciton binding energy.
Excitons States in Semiconductor Quantum Dots
2016
It was found that within the band gap of the quantum dot of zinc selenide appears a zone of exciton states, located at the bottom of the conduction band. It has been shown that a decrease in the band gap width in this nanosystems conditioned by transition of the electron from quantum-dimensional level within the valence band of the quantum dot to the levels of the zone of exciton states. The dependence of the energy of a base state of an exciton from the radius of QD was obtained using a modified method of the effective mass.
Investigation of confinement effects in ZnO quantum dots
Nanotechnology, 2009
We report a simple method for the synthesis of Na + doped and stable zinc oxide quantum dots, using the quantum confinement atom method. An intense broad green photoluminescence (PL) was observed with a maximum located at ∼535 nm when excited by UV radiation of 332 nm. The PL peak intensity is found to be highly dependent on the size of the quantum dots (QDs). Electron microscopy observation revealed that the radius of the QD was ∼1 nm, which clearly indicated that the QDs are in the strong quantum confinement region (exciton Bohr radius, r B , for bulk ZnO is 1.8 nm). Phase purity of ZnO and the presence of Na + was confirmed by x-ray diffraction (XRD) and atomic absorption spectroscopy (AAS), respectively. The results are well incremented by x-ray photoelectron spectroscopy (XPS) studies. Intentional ageing of QDs for several days under controlled experimental conditions such as temperature, relative humidity and pH etc, facilitated the formation of various nanostructures with a slight red shift in the PL peak position. Time resolved emission spectroscopy measurements indicated that PL decay time changes from 35 ns for QDs to 1660 μs for nanocrystals. The observed high-intensity and stable green PL emissions have been analyzed and thoroughly discussed.
IEEE Photonics Journal, 2014
In this paper, we investigate the linear and nonlinear photoabsorption processes in the conduction-band-confined levels of a singly charged ZnO quantum dot (QD) surrounded by HfO 2 and AlN matrices. We also investigate the photoelectric process in which the conduction band electron ejects from the dot to the vacuum. We use the effective mass approximation with a finite barrier height at the dot-matrix interface. We consider the self-energy of the electron in the dot and the local field effect. The electromagnetic interaction of the incident radiation with the electron in the dot is considered in the electric dipole plus quadrupole approximation. Results for the photoabsorption coefficient and the photoelectric process are presented for different dot sizes and different intensities of incident radiation. It is found that the inclusion of the quadrupole effect reveals new photoabsorption peaks in the absorption spectra. Both the photoabsorption and photoelectric processes significantly depend on the dot size and the surrounding matrix. The change in the intensity of the incident radiation significantly influences the nonlinear photoabsorption. The photoabsorption coefficient and the photoelectric cross sections are found to be relatively higher for the ZnO QD embedded in the high-dielectric constant matrix HfO 2 as compared with the lower-dielectric constant AlN matrix.
Giant exciton-light coupling in ZnO quantum dots
Applied Physics Letters, 2002
We investigate the strength of the coupling of the electronic states with the electromagnetic field in ZnO nanospheres, taking into account the retardation effect. We show that the coupling strength is particularly strong: the bulk properties are so enhanced that the radiative decay time can reach some 200 ps for quantum dot sizes of some 30 nm.