Stark shift of an on-center donor binding energy

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Transcript Stark shift of an on-center donor binding energy

Παρουσίαση ερευνητικών αποτελεσμάτων
του προγράμματος
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Τίτλος Υποέργου: Μοντελοποίηση-σχεδιασμός και ανάπτυξη ημιαγώγιμων
νανοδομημένων υλικών
Κωδικός Υποέργου: D. 237 (ΠΕΡΙΦΕΡΕΙΑ D. 207)
Επιστημονικός Υπεύθυνος: Σωτήριος Μπασκούτας, Βαθμίδα: Αναπληρωτής Καθηγητής,
Tμήμα Επιστήμης των Υλικών
Χρόνος Υλοποίησης: 2011 – 2014 (36 μήνες)
Ι. Παρουσίαση ερευνητικών αποτελεσμάτων
Stark effect of donor binding energy in a self-assembled GaAs quantum dot
subjected to a tilted electric field
We theoretically investigated the donor binding energy distribution with respect to the dopant
positions in a self-assembled GaAs/AlGaAs quantum dot (QD) in the presence of a tilted electric
field. It is found that there is a critical line in a doping plane, corresponding to zero Stark shift of
the donor binding energy. At low electric fields, our work reveals that Stark shift of an on-center
donor binding energy is a “purely” quadratic function of the electric field strength, irrespective of
QD dimensions and field orientations. This scaling law permits us to indirectly estimate the
impurity polarizability in a self assembled QD.
Contour plots of the Stark shift of the donor binding energy (in unit of
meV) for various dopant positions distributed in xoz plane of a GaAs/Al0.3Ga0.7As
cylindrical QD with different dot configurations subjected to an uniform electric
field (F = 50 kV/cm, θ= 45).
Zaiping Zeng, Christos S. Garoufalis, Sotirios Baskoutas , Andreas F. Terzis, Physics Letters
A 376 (2012) 2712–2716
Tuning the binding energy of surface impurities in cylindrical GaAs/AlGaAs
quantum dots by a tilted magnetic field
Using the potential morphing method in the framework of the effective-mass approximation, we
have studied theoretically the effect of a tilted magnetic field on the binding energy of surface
impurities in GaAs/Al0:3Ga0:7As cylindrical quantum dots. It is found that contrary to what was
expected based on the existing literature for growth-direction magnetic fields, the presence of a
tilted field does not always contribute positively to the binding energy of surface impurities.
The shape (aspect ratio) and size of the cylindrical QD as well as the dopant positions at the QD
surface play an important role. Furthermore, we find that decrease of the QD size can reduce
the sensitivity of the variation of the donor binding energy with respect to the field strength
(orientation), but it cannot change its general behaviour.
The first column corresponds to the three dimensional electron probability density of an impurity free GaAs=Al0:3Ga0:7As cylindrical QD with
different aspect ratios in the absence of the magnetic field. The following three columns show suitably chosen isosurfaces of electron
probability density differences for non-vanishing field along various directions.
Zaiping Zeng, Christos S. Garoufalis, Sotirios Baskoutas, and Andreas F. Terzis, JOURNAL OF APPLIED PHYSICS 112, 064326
(2012)
Combination effects of tilted electric and magnetic fields on donor binding energy in
a GaAs/AlGaAs cylindrical quantum dot
We have performed a systematic study on the ground-state binding energy of an on-center donor
impurity confined in a GaAs/Al0.3Ga0.7As cylindrical quantum dot (QD), subjected to simultaneously
applied electric and magnetic fields. The two fields are tilted with respect to the QD growth direction
and they are either parallel or perpendicular to each other. All the calculations are based on the
potential morphing method which is employed within the framework of the effective-mass
approximation. Our results show that when the tilted electric and magnetic fields are parallel, the
magnetic shift of the donor binding energy is a monotonic function of the magnetic field strength. On
the other hand, when the two fields are perpendicular to each other, the magnetic shift of the donor
binding energy varies nonmonotonically with respect to the magnetic field strength, exhibiting a
minimum value at a critical magnetic field strength. The position of this minimum value and its
dependence on the QD size, its aspect ratio and the orientation of the tilted magnetic field is
systematically investigated. Moreover, we discuss in detail the competition effects which appear in the
presence of the two fields, showing that the critical line which corresponds to zero shift of the
donor binding energy can be manipulated by suitably adjusting the QD size, the aspect ratio
and the relative orientation of the two fields.
Zaiping Zeng, Christos S Garoufalis and Sotirios Baskoutas, J. Phys. D: Appl. Phys. 45 (2012) 235102
Competition Effects of Electric and Magnetic Fields on Impurity Binding Energy in a
Disc-Shaped Quantum Dot in the Presence of Pressure and Temperature
We have theoretically studied the combination effects of the electric and magnetic fields on the binding energy of an on-center
donor impurity in disc-shaped GaAs/Al0.3Ga0.7As quantum dots (QDs) with emphasis on the competition effects between the
two fields under externally applied pressure and temperature. The electric field is applied along the radial direction of the QD,
while the magnetic field is applied along the growth direction. Our results show that the two fields exhibit a competition effect
on the donor binding energy, leading to an invariant binding energy as in the zero field case at a critical line. This line separates
the region corresponding to the “red shift” of the donor binding energy from the one referring to the “blue shift” of the binding
energy. Comparing to the magnetic field effect, increasing in the QD sizes or applied pressure is found to favor more its electric
counterpart, enlarging the region satisfied by the “red shift” of the donor binding energy. The temperature effect appears to
decrease the donor binding energy. However, the competition effects between the two fields retain the same when the
temperature varies.
Zaiping Zeng, George Gorgolis, Christos S. Garoufalis,
and Sotirios Baskoutas, Science of Advanced Materials
Vol. 6, pp. 586–591, 2014
Contour plot of the binding energy of an on-center donor impurity Eb (upper panel, in
unit of meV) and the energetic shift of the donor binding energy (δEb) (lower panel,
in unit of meV) for various electric (E) and magnetic (B) fields in GaAs/Al0:3Ga0:7As
disc-shaped QDs with radius R = 20 nm at zero pressure (P = 0) . Two temperature
values (T = 10 K (left panel)and 150 K (right panel)) are considered, respectively.
Strong quantum confinement effects in SnS nanocrystals
produced by ultrasound-assisted method
Nanocrystalline SnS powder has been prepared using tin chloride (SnCl2) as a tin ion source and sodium
sulfide (Na2S) as a sulfur ion source with the help of ultrasound irradiation at room temperature. The assynthesized SnS nanoparticles were quantitatively analyzed and characterized in terms of their morphological,
structural, and optical properties. The detailed structural and optical properties confirmed the orthorhombic
SnS structure and a strongly blue shifted direct band gap (1.74 eV), for synthesized nanoparticles. The
measured band gap energy of SnS nanoparticles is in a fairly good agreement with the results of theoretical
calculations of exciton energy based on the potential morphing method in the Hartree–Fock approximation.
Yashar Azizian-Kalandaragh • Ali Khodayari , Zaiping Zeng
• Christos S. Garoufalis, Sotirios Baskoutas, Lionel Cervera
Gontard, J Nanopart Res (2013) 15:1388
Tuning of the Optical Emission Polarization of ZnO Nanorods by an
Applied Hydrostatic Pressure
In the present work, we have performed a systematic theoretical study of the electronic and optical properties of ZnO nanorods
as a function of an externally applied hydrostatic pressure. Our calculations are based on the empirical pseudopotential method
and configuration interaction to consider the excitonic effects. The optical emission polarization is found to exhibit a transition
between in plane and out-of-plane polarization with increasing pressure. The critical value of the hydrostatic pressure at which
the crossover takes place appears to depend strongly on the nanorod’s radius. In particular, when the nanorod’s radius is smaller
than the ZnO exciton Bohr radius, the necessary pressure to reach the crossover point is significantly smaller than the pressure
required for a nanorod with radius larger than the exciton Bohr radius. For large pressures, the lowest exciton state is a nearly
pure state (0,0), where electron and hole have dominant S orbital character and the hole has a nearly pure parentage from the
bulk Bloch C-band.
Oscillator strengths for the emission |X⟩ to |0⟩ at room temperature in L = 5 nm, D = 2.2 nm (left panel) and L = 5 nm, D = 4 nm (right
panel) calculated at the CI level. Emission polarized along the out-of-plane direction (c-axis) is shown in red, while that polarized in-plane is
shown in black. The numbers in brackets refer to the dominant single-particle levels involved in the transitions (e,h). The blue curve is the total
emission spectrum where a Lorentz line shape with a broadening of 1.5 meV has been used.
Sotirios Baskoutas, Zaiping Zeng, Christos S. Garoufalis, and Gabriel Bester, J. Phys. Chem. C 2012, 116, 26592−26597
Band-gap tuning at the strong quantum confinement regime in magnetic
semiconductor EuS thin films
Ultraviolet-visible absorption spectra of nanoscaled EuS thin films reveal a blue shift of the energy between the
top-valence and bottom-conduction bands. This band-gap tuning changes smoothly with decreasing film
thickness and becomes significant below the exciton Bohr diameter 3.5 nm indicating strong quantum
confinement effects. The results are reproduced in the framework of the potential morphing method in
Hartree Fock approximation. The large values of the effective mass of the holes, due to localization of the EuS fstates, limit the blue shift to about 0.35 eV. This controllable band-gap tuning of magnetic semiconductor EuS
renders it useful for merging spintronics and optoelectronics.
Panagiotis Poulopoulos, Björn Lewitz, Andreas Straub, Spiridon D. Pappas, Sotirios A. Droulias, Sotirios
Baskoutas, and Paul Fumagalli, Applied Physics Letters 100, 211910 (2012)
Synthesis and Characterizations of Cd-Doped ZnO
Multipods for Environmental Remediation Application
Well-crystalline Cd-doped ZnO multipods were synthesized by simple and facile hydrothermal process by using zinc chloride,
cadmium chloride, hexamethylenetetramine and ammonium hydroxide at low-temperature. The synthesized materials were
characterized in terms of their morphological, structural, compositional and optical properties. The morphological investigations
done by field emission scanning electron microscopy (FESEM) reveal that the synthesized products are multipods shaped and
grown in high density. The structural and compositional properties, observed by X-ray diffraction (XRD), energy dispersive
spectroscopy (EDS) attached with FESEM and Fourier transform infrared (FTIR) spectroscopy exhibit that the synthesized
multipods are well-crystalline and possessing wurtzite hexagonal phase pure Cd-doped ZnO. The as-synthesized Cd-doped ZnO
multipods exhibited good optical properties as was confirmed by UV-vis. spectroscopy. Finally, the as-synthesized Cd-doped ZnO
multipods were used environmental remediation application. For this, the synthesized multipods were used as an effective
photocatalyst for the photocatalytic degradation of acridine orange (AO) which exhibit ∼92.4% degradation within 90 min. This
work demonstrates that doped ZnO materials could be used as efficient photocatalyst for the photocatalytic degradation of
various organic dyes and chemicals.
Suzan A. Khayyat, M. Abaker, Ahmad Umar, Manal O. Alkattan, Najlaa D. Alharbi, and S. Baskoutas,
Journal of Nanoscience and Nanotechnology Vol. 12, 8453–8458, 2012
II. Επαγγελματική πορεία υπότροφου
Ο κ. Zaiping Zeng συνεχίζει την έρευνά του σαν
μεταδιδάκτορας
στο
Laboratoire
de
Simulation
Atomistique, CEA, Genoble, FRANCE, όπου έγινες
δεκτός μετά απο διεθνή πρόσκληση εκδήλωσης
ενδιαφέροντος. Αξίζει να σημειωθεί ότι επιλέχθηκε μεταξύ
80 συνυποψηφίων.
III. Επιτεύγματα του έργου που πρέπει να αναφερθούν
Collaborations:
• Max Planck Institute for Solid State Research, Stuttgart, Germany
• Department of Chemistry, University of Hamburg, Germany (MOU under preparation)
• Advanced Materials and Nano Research Center, Najran University, Saudi Arabia
(MOU under preparation)
• Free University of Berlin, Institute of Experimental Physics, Berlin, Germany
• Department of Physics, University of Mohaghegh Ardabili, Ardebil, Iran
• University of Kashan, Institute of Nanoscience & NanoTechnol, Kashan, Iran
IV. Χρησιμότητα του έργου στην μετέπειτα πορεία των
εμπλεκομένων
• CODES FOR PARALLEL COMPUTING
• CODES FOR THE ACCURATE DETERMINATION OF OPTICAL PROPERTIES OF SEVERAL
SEMICONDUCTOR NANOSTRUCTURES
• CODES FOR NANOSTRUCTURES UNDER APPLIED PRESSURE WITH RELAXATION PROCCESS
• DEVELOPMENT OF SEVERAL CHARACTERIZATION TECHNIQUES
• DEVELOPMENT OF THERMAL EVAPORATION METHOD
• DEVELOPMENT OF HYDROTHERMAL METHOD