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Nanoscale Res Lett. 2012; 7(1): 374.
Published online Jul 7, 2012. doi:  10.1186/1556-276X-7-374
PMCID: PMC3511169
Magneto-optical properties in IV-VI lead-salt semimagnetic nanocrystals
Silvio J Prado,1 Leonardo Villegas-Lelovsky,corresponding author2 Augusto M Alcalde,2 Victor Lopez-Richard,3 and Gilmar E Marques3
1Faculdade de Ciencias Integradas do Pontal, Universidade Federal de Uberlandia, Ituiutaba Minas Gerais 38302-000, Brasil
2Instituto de Fisica, Universidade Federal de Uberlandia
3Departamento de Fisica, Universidade Federal de Sao Carlos
corresponding authorCorresponding author.
Silvio J Prado: psprado/at/df.ufscar.br; Leonardo Villegas-Lelovsky: lvl/at/df.ufscar.br; Augusto M Alcalde: alcalde/at/fafis.ufu.br; Victor Lopez-Richard: vlopez/at/df.ufscar.br; Gilmar E Marques: gemarques/at/df.ufscar.br
Received February 2, 2012; Accepted July 7, 2012.
Abstract
We present a systematic study of lead-salt nanocrystals (NCs) doped with Mn. We have developed a theoretical simulation of electronic and magneto-optical properties by using a multi-band calculation including intrinsic anisotropies and magnetic field effects in the diluted magnetic semiconductor regime. Theoretical findings regarding both broken symmetry and critical phenomena were studied by contrasting two different host materials (PbSe and PbTe) and changing the confinement geometry, dot size, and magnetic doping concentration. We also pointed out the relevance of optical absorption spectra modulated by the magnetic field that characterizes these NCs.
Keywords: Nanocrystals, Quantum dots, DMS, II-VI semiconductors, Lead salts, Magneto-optical properties
Recently, the successful fabrication of IV-VI nanocrystals doped with Mn has shown possible effective tuning of the emission energy from infrared (dot radius [similar, equals] 200 Å) up to near-ultraviolet (dot radius [similar, equals] 20 Å) regions [1]. The IV-VI semiconductors, such as PbSe nanocrystals (NCs), provide access to the limit of strong quantum confinement where, besides the changes induced by very small dot size, the direct narrow band-gap that can also be engineered by the gradual addition of dilute amounts of magnetic Mn ions to the dot structure. The members of the lead-salt family, such as PbSe and PbTe, have rock-salt crystalline structure with a direct bandgap in the L-point and the energy branches are four-fold degenerate. The bottom of the conduction band has equation M1 symmetry with the top of the valence band displaying equation M2 symmetry of the double group D3. This corresponds to the opposite situation observed in III-V or II-VI zinc blend materials, since here the valence band-edge Bloch function displays s-like symmetry whereas the conduction band-edge Bloch function has pz-like symmetries, where z denotes the left angle bracket111right angle bracket direction of the cubic lattice [2].
In this letter, we contrast quantum dot electronic properties of two IV-VI semiconductor materials by modifying the quantum confinement from spherical to semispherical and varying the diluted concentration of incorporated Mn2 + ions. The electronic, magnetic, and optical properties are studied as a function of Mn content for varying temperature. The total Hamiltonian of the system is H = Hkp + V + Hx where Hkp is the hyperbolic or Kane-Dimmock [3]k · p Hamiltonian model for IV-VI semiconductors, V is a hard wall confinement potential and Hx is the exchange interaction between Mn2 + ions and conduction band (valence band) spins. Here, Hkp was slightly modified to explore spherical symmetries of the confinements
equation M3
(1)
where equation M4, with [nabla]2 as the 3D Laplacian operator, and equation M5equation M6 are electron and hole effective mass terms while Ptand Pl are the anisotropic conduction-valence Kane-Dimmock coupling parameters for longitudinal and transverse directions; Pz and P± = Px ± iPy are the momentum operators, whereas Eg is the bandgap and m0 is the free electron mass. The relevant Kane-Dimmock parameters for the materials analyzed in this work can be found in [4,5].
Also, Hx = − x/2left angle bracketSz(BT)right angle bracketN0 · αβ), where left angle bracketSz(BTx)right angle bracket is the mean field magnetization at temperature T, represented as a Brillouin function in dilute doped sample containing N0 unit cells and Mn content, x[6]. Finally, α and β are the exchange constants for the semimagnetic materials, N0 · α = −0.08 eV and N0·β = 0.02 eV for PbMnSe, while N0·α = −0.45 eV and N0·β = 0.29 eV for PbMnTe [5].
A complete set of eigenfunctions for the total Hamiltonian H can be spanned in terms of products of periodic Bloch functions |J,Jzright angle bracket near the L-point and envelope functions. For spherical confinement, we expand the four-component spinor wave functions in two Hilbert subspaces with the general form [7,8].
equation M7
(2)
For the spherical model, these states fulfill the boundary condition equation M8 at the dot radius; thus, the function components have the form equation M9 where An,L is a normalization constant, jL(x) is the spherical Bessel function, and equation M10 are the spherical harmonics. The subspaces must be constructed with special combinations of even ( equation M11) or odd ( equation M12) with wave number equation M13, where equation M14 is the nth zero of jL(x) = 0. For the semispherical structures, the states must also fulfill the boundary condition equation M15 at the equator plane which restricts the set of quantum numbers L and M to the condition |LM| = odd number. Hence, the parities of the spinor components differ from the full spherical case and the states equation M16 for a semispherical confinement require the replacement 2L (2L + 1) in the second (third) line of Equation 2 by 2L + 1 (2L).
Figure Figure1a,b1a,b shows the changes in the magnetic energy dispersions for the first few levels in Pb1−xMnxSe dots with R = 300 A when the confinement is changed from spherical to semispherical. The broken symmetry induces stronger changes on the electron than on the hole energy dispersions by inducing anti-crossing regions. The exchange coupling affects mainly the conduction carrier dispersion. However, for Pb1−xMnxTe dots with the same size R, shown in Figure Figure2a,b2a,b with both broken symmetry and exchange interaction, induce strong changes on both carrier magnetic dispersions but with the valence-band being more sensitive. The interplay between the usual Zeeman effect and the exchange interaction gives place to the crossing between spin-split levels at certain critical field, Bc, as displayed in Figure Figure22 for both spherical and semispherical dot spatial confinements.
Figure 1
Figure 1
Conduction and valence band energy levels as function of magnetic field in Pb1−xMnxSe NCs with spherical (a) and semispherical (b) confinements of radius R = 300 A andT = 1.8 K. The subbands structure with (solid line) and without Mn-doping (dashed (more ...)
Figure 2
Figure 2
Conduction and valence band energy levels as function of the magnetic field in Pb1−xMnxTe NCs with spherical (a) and semispherical (b) confinements with radius R = 300 A and T = 4.8 K. The other subband structure details are given in Figure (more ...)
Figure Figure3a,b3a,b shows that the critical field strength for Pb1−xMnxTe dots, at a fixed temperature, increases with increasing Mn content for different dot sizes. Note that the smaller the dot size R, the larger the critical concentration xc where equation M17. For the limit equation M18, we have calculated the Landè g-factor of the conduction band ground state of Pb1−xMnx Te dots as geμBB = E(e↑,1/2,N)−E(e↓,−1/2,N), where equation M19 is the Bohr magneton, E(e↑(),Fz N) is the energy of the corresponding spin state, and Fz = Lz + Jz is the z-component of total angular momentum F = L + S. The ge-values for Pb1−xMnxTe dots as shown in Figure Figure3c,d3c,d displays similar behavior as reported in [9,10]ge(BRx)to approximately1/R.
Figure 3
Figure 3
Critical magnetic field as function of the Mn concentration for different Pb1−xMnx Te NC radii Critical magnetic field as function of the Mn concentration for different Pb1−xMnx Te NC radii (a,b); Landè g factor in the limitequation M20as (more ...)
As noted in Figure Figure3c,d,3c,d, there are Mn concentration regions where the g factor becomes strictly positive or negative, independent of the confinement shape. For fixed dot radius, it is possible to predict the existence of a zero critical field value for a certain value xc for different dot and confinement geometries. For large dot sizes, a nonlinear increasing of Bc is observed for low values of x and a quasi-linear behavior otherwise.
In order to discuss the optical absorption spectrum, the probability for dipole-allowed optical transitions between single electron and hole states has to be evaluated in detail. Within the electrical dipole approximation, the oscillator strength is a linear combination of the matrix elements of the optical transitions, equation M21. Here, equation M22 is the light polarization vector, equation M23 is the momentum operator, fjand ujare the envelope and periodic Bloch functions at the L point for each involved carrier j, respectively. The second term on the right-hand side is responsible for intraband optical transitions, since left angle bracketuj|ujright angle bracket = δjj. In this case the incident light couples, in the same band, state with different symmetries whenever the term equation M24 for a given polarization. In our case the complete set of selection rules are obtained from the nonvanishing products of the matrix elements Ie,hδLe,Lhπα,α, where πα,α is the matrix of the parity operator, and Ie,h = left angle bracketfe,α|fh,αright angle bracket is the overlap integral of the electron-hole envelope functions allowed by the interband transition equation M25. The allowed transitions between states belonging to the Hilbert subspaces described by spinors (2) are determined from the angular dependence of the wave functions equation M26.
The corresponding selection rules for each optical transition in any polarization can be precisely obtained according to Kang et al. [2]. Due to the differences in the angular momenta L (symmetry and parity) of electron and hole spinor components, the allowed transitions occur only between initial (hole) and final (electron) states belonging to different Hilbert subspaces equation M27 or equation M28 for linear light polarization Πz and for circular light polarization σ±. Moreover, the preservation of the total angular momentum Fz, between initial and final states requires that ΔM = 0 for Voigt- Πz, and ΔM = ±1 for Faraday- σ± geometry. For the circular polarization, the optical matrix element takes the form
equation M29
(3)
where
equation M30
(4)
with β = 2L + 1/2[minus-or-plus sign]1/2. In the same way, the equation M31 transitions can be obtained by interchanging 2L + 1/2 [minus-or-plus sign] 1/2 by 2L + 1/2 ± 1/2. The absorption coefficient can then be written as follows [7]:
equation M32
(5)
where α0 is a magnitude which includes the bulk P parameter and the dielectric constant. The material parameters can be found in [4,5]. For the linear light polarization Πz, the optical matrix element becomes
equation M33
(6)
where
equation M34
(7)
and the related absorption coefficient turns
equation M35
(8)
In the case of semispherical geometry, the selection rules for the circular light polarization are the same as for the spherical case; meanwhile, for the linear light polarization, these allow transitions within the same subspace due to the parities of the components of the wave functions in the subspaces.
The excitonic resonances for Πz and σ+ , calculated as a function of the magnetic field for each Mn-doped lead-salt dot and confinements, are shown in Figure Figure4a,b,c,d,e,f,g,h.4a,b,c,d,e,f,g,h. In Figure Figure5,5, we displayed the corresponding excitonic resonances for σ+ of the reference samples (without Mn doping) for spherical confinement. Comparing Figures Figures4e4e and and5a5a and Figures Figures4g4g and and5b,5b, we confirm that the effect of Mn doping on the absorption spectra is stronger on the bandgap renormalization than on the subband levels in the doped salt-selenide unlike the salt-telluride, where the Mn presence strongly modifies all the band structure [11,12]. The resonant transitions shown in Figure Figure5a,b5a,b involve just the conduction band ground state of spherical and semispherical PbMnSe dots. The corresponding spectra for PbMnTe, shown in Figure Figure4c,d,4c,d, correspond to the transitions to the first crossing conduction band levels. Figure Figure4d4d displays an absorption bottleneck due to the level crossing (see Figure Figure2a,b)2a,b) for PbMnSe spherical dots. Another absorption quenching appears at B = 1.2T in Figure Figure4e4e caused by the character admixture close to a level crossing. In turn, Figure Figure4f4f displays a single transition to the conduction band ground state. In Figures 4g,h two transitions appear that fade-off for lower and higher fields, respectively. This effect is produced by the modulation of the oscillator strength. For small nanocrystal size, the spectra will show quantitative variation due to the effective gap modulation and the subsequent weakening of the intersubband coupling.
Figure 4
Figure 4
Interband absorption spectra as function of magnetic field for polarization Πz (a-d) and σ+ (e-h) Pb0.99Mn0.01Se NCs with spherical (a,e) and semispherical (b,f) confinements and Pb0.99Mn0.01Te NC with spherical (c,g) and semispherical (more ...)
Figure 5
Figure 5
Interband absorption spectra as function of magnetic field for polarization σ+ of PbSe (a) and PbTe NCs (b) with spherical confinement of R = 300 A and temperatures 1.8 and 4.8 K, respectively.
Summarizing, we have investigated the electronic and magneto-optical properties of Pb1−xMnx Se and Pb1−xMnx Te semimagnetic dots by taking advantage of their strong sensitivity to spatial confinement asymmetry and properties induced by the Mn doping. We have shown the appearance of the critical phenomena as the spin level crossing for certain concentration of Mn on the Pb1−xMnx Te and the modulation of the optical absorption controlled by field B and confinement anisotropy. Subtle effects of Mn content variation were predicted for the energy spectra of the Pb1−xMnx Se dots, whereas important consequences are expected for Pb1−xMnx Te dots. We believe that these results may stimulate research groups working on these important materials to explore device applications working on the wide spectral range.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
SJP carried out the calculation of the band structure and absorption spectra and participated in the study of the electronic and magneto-optical properties. LVL, VLR and GEM participated in the design of the problem, and its study and coordination. AMA conceived of the study and participated in the design of the problem and first stages of calculation. All authors read and approved the final manuscript.
Acknowledgements
The authors acknowledge the financial support from the Brazilian agencies, FAPEMIG (SJP, LV-L), INCT-IQ (AMA) and FAPESP and CNPq (VL-R, GEM).
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