Electronic structure of Co-doped ZnO nanorods Ahmed Neffati, Hajer Souissi, and Souha Kammoun Citation: J. Appl. Phys. 112, 083112 (2012); doi: 10.1063/1.4757634 View online: http://dx.doi.org/10.1063/1.4757634 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i8 Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 112, 083112 (2012)
Electronic structure of Co-doped ZnO nanorods Ahmed Neffati,a) Hajer Souissi, and Souha Kammoun Universit e de Sfax, Laboratoire de Physique Appliqu ee, Groupe de Physique des Mat eriaux Luminescents, D epartement de Physique, Facult e des Sciences de Sfax, B.P. 802, 3018 Sfax, Tunisia
(Received 15 June 2012; accepted 10 September 2012; published online 26 October 2012) The optical transmission spectra, the photoluminescence (PL), and the photoluminescence excitation (PLE) spectra of the cobalt doped zinc oxide nanorods Zn1xCoxO (x ¼ 0.01, 0.10) were measured by Loan et al. [J. Phys. D: Appl. Phys. 42, 065412 (2009)] in the region 1.5-4 eV. These spectra exhibit a group of ultraviolet narrow lines in the region of 3.0-3.4 eV related to the nearband-edge emission of the host ZnO materials and a group of emission lines in the red region of 1.8–1.9 eV assigned to the radiative transitions within the tetrahedral Co2þ ions in the ZnO host crystal. The group of lines in the visible region provides important information about the electronic structure of the cobalt doped zinc oxide nanorods. This work investigates a theoretical crystal-field analysis of the visible lines associated to the Co2þ ion transition occupying a Td site symmetry in ZnO host crystal. A satisfactory correlations were obtained between experimental and calculated energy levels. The electronic structure was compared with the reported for cobalt transition ion doped in ZnO nanoparticles and bulk crystals [Volbers et al., Appl. Phys. A 88, 153 (2007) and H. J. Schulz and M. Thiede, Phys. Rev. B 35, 18 (1987)]. In order to explain the existence of excitation peaks observed near the band edge of the ZnO host, an energy transfer mechanism is C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4757634] proposed. V
I. INTRODUCTION
One-dimensional (1D) nanostructures including nanowires, nanorods, nanofibres, nanobelts, and nanotubes have attracted a great deal of interest in both academic research and potential applications in electronic, optoelectronic, electrochemical, and electromechanical nanodevices.1–5 In the past few years, considerable effort has been devoted to developing various 1D semiconductor nanostructures. Many nanostructures based on various metal oxides, III–V and II–VI compound semiconductors were synthesized. Among the one-dimensional (1D) nanostructures, zinc oxide (ZnO) nanorods is one of the most important nanomaterials for nanotechnology in today’s research.6 ZnO, as a well-known semiconductor with direct wide band gap energy (3.37 eV) and a large exciton binding energy at room temperature (60 meV), has attracted considerable scientific attention due to its importance for applications in the fields of piezoelectronics, photonics, optoelectronics, and high power electronic devices. Furthermore, ZnO is bio-safe and biocompatible and may be used for biomedical applications without coating.5,6 Recently, 3d transition-metal elements (Co, Ni, V, Mn, and Cu) have been alloyed with ZnO and their properties have been investigated.1–3,7–18 Due to their spintransport properties, transition-metal doped ZnO is a diluted magnetic semiconductor and has attracted much attention because of the possibility of its application in spintronic devices.1 The electronic structure as well as the optical and magnetic properties of Co2þ doped ZnO nanoparticles have been characterized.2 The optical absorption spectra show two bands: an intense absorption peak in the ultraviolet region (3.80 eV) and a weak band in the visible region a)
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(1.8-2.4 eV). The absorption at high energies is due to the bandgap absorption of ZnO, and the visible band is due to the Co2þ d–d internal transitions. Low temperature photoluminescence (PL) spectrum is also investigated. The optical properties of cobalt doped zinc oxide nanorods Zn1xCoxO (x ¼ 0.01, 0.10) were investigated by Loan et al.1 The PL spectra at low temperatures exhibit a group of ultraviolet (UV) narrow lines in the near-band-edge region of 3.0– 3.4 eV and a very broad band peaked at 3.20 eV. Another group of emission lines in the red region of 1.8–1.9 eV have been revealed. These emission lines were assigned to the radiative transitions in the high-spin state of the tetrahedrally coordinated Co2þ (3d7) ions substituting Zn2þ ions.1 In the present work, we present a detailed crystal-field analysis of the electronic energy levels of cobalt doped zinc oxide nanorods Zn1xCoxO (x ¼ 0.01, 0.10). This analysis is based on the interpretation of the PL and the photoluminescence excitation (PLE) spectra at low temperature 15 K in the red region (1.8-1.9 eV) related to the emission transitions within Co2þ ions. The theoretical study reposes on the Racah tensor algebraic methods and was carried out for the Co2þ(3d7) centre with a Td site symmetry. This study permits us to determine the electronic structure of Co-doped ZnO nanorods and then to deduce the energy levels, which are not observed experimentally. A comparison of the obtained results for cobalt ion doped in ZnO nanorods with those reported for cobalt ion doped in ZnO nanoparticles and bulk crystals is realized in order to determine the effect of reduced dimensionality. II. EXPERIMENTAL AND THEORETICAL DATA
The electronic structure of cobalt transition ion doped zinc oxide nanorods Zn1xCoxO (x ¼ 0.01, 0.1) are obtained 112, 083112-1
C 2012 American Institute of Physics V
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FIG. 3. Experimental1 and calculated (this work) PL spectra of Zn1xCoxO (x ¼ 0.10) at low temperature 15 K, excited with the wavelength 17 695 cm1 (2.194 eV). FIG. 1. Room temperature optical transmittance spectrum of Zn1xCoxO with x ¼ 0.01.1
from the interpretation of the optical spectra measured by Loan et al.1 For this reason, we present briefly in this section the results of their spectroscopic studies.1 The optical transmission spectrum of Zn1xCoxO with x ¼ 0.01 measured at room temperature is shown in Fig. 1. In this spectra, the absorption edge at about 25 971 cm1 (3.22 eV) corresponds to the band gap Eg for the host ZnO material. The absorption bands at about 15 381 cm1 (1.907 eV), 16 413 cm1 (2.035 eV), and 17 656 cm1 (2.189 eV) are assigned to 4A2(4F) ! 2E(2G), 4A2(4F) ! 4T1(4P), and 4 A2(4F) ! 2A1(2G) transitions, respectively. The PL spectra of the Zn1xCoxO nanorods with x ¼ 0.10 are represented in Figs. 2 and 3.1 The UV emission can be assigned to the free excitons (denoted by XA) at 27 230 cm1 (3.376 eV), the excitons bound to neutral donor (D X) at 27 157 cm1 (3.367 eV), the recombination of electrons bound on a donor with free holes in the valence band (BF) at 26 730 cm1 (3.314 eV), its longitudinal optical (LO) phonon replica (BF1LO) at 26 157 cm1 (3.243 eV), and donor-acceptor pair (DAP) emission at 25 318 cm1 (3.139 eV) (DAP1) and at
24 278 cm1 (3.010 eV) (DAP2). Figure 3 shows a broad emission band in the visible region with a width at halfheight of 600 cm1. The PLE spectra for the sample of Zn1xCoxO with x ¼ 0.10 were measured at a temperature of 15 K (Figs. 4 and 5).1 The UV group consists of a peak at 27 076 cm1 (3.357 eV) (Fig. 4), which is related to the nearband-edge absorption of the ZnO host materials. The visible group consists of four peaks at 15 438 cm1 (1.914 eV), 16 317 cm1 (2.023 eV), 17 704 cm1 (2.195 eV), and 20 212 cm1 (2.506 eV) (Fig. 5), and the first three peaks among them are very close to the absorption peaks in the above-mentioned transmission spectrum. These four peaks are attributed to the transitions from the basic state 4A2(4F) to the 2E(2G), 4T1(4P), 2A1(2G), and 2T1(2P) excited states of tetrahedrally coordinated Co2þ ions, respectively. For comparison, we give also some experimental data for Co2þ in ZnO nanoparticles.2 Three main peaks are observed in the visible absorption spectrum at positions 15 244 cm1 (1.89 eV), 16 292 cm1 (2.02 eV), and 17 906 cm1 (2.22 eV).2 They have been identified as the internal transitions 4A2(4F) ! 2T1(2G), 4T1(4P), 2A1(2G) of substitutional Co2þ on Zn sites in ZnO, respectively.2 The PL spectra of Co2þ in ZnO
FIG. 2. PL spectrum of the Zn1xCoxO nanorods (x ¼ 0.10) at a temperature of 50 K, excited with the wavelength of 30 771 cm1 (3.815 eV).1
FIG. 4. PLE spectrum monitored at the emission line 14 679 cm1 (1.820 eV) in the UV region of Zn1xCoxO (x ¼ 0.10) measured at a temperature of 15 K.1
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based on MAPLE software to calculate the energy levels and state vectors for any transition metal ions with the 3dN configuration (N ¼ 2, 3, 7, 8) located at sites with symmetry given by any of the 32 crystallographic point groups. IV. RESULTS AND DISCUSSION
FIG. 5. PLE spectrum monitored at the emission line 14 679 cm1 (1.820 eV) in the visible region of Zn1xCoxO (x ¼ 0.10) measured at a temperature of 15 K.1
nanocrystals recorded at a temperature of 4 K show a band at 15 131 cm1 (1.876 eV), which is attributed to the 4T1(P) ! 4A2(4F) transition. III. THEORETICAL CRYSTAL-FIELD STUDY OF Co21 DOPED ZnO NANORODS
The total Hamiltonian used to study the electronic energy level structure of cobalt transition ion Co2þ doped into ZnO is given by:19,20 H ¼ H0 þ HTrees þ HCF þ HSO;
(1)
H0 is the Coulomb interaction including electron-electron repulsions. This Hamiltonian gives the 2Sþ1L terms: two quartet terms (4F ground state and 4P excited state) and six doublet excited terms (2H, 2G, 2F, 2D, 2D0 , and 2P) for the cobalt ion Co2þ with a 3d7configuration. The eigenvalues of the Hamiltonian H0 are expressed as a function of the Racah parameters B and C.21,22 HTrees is the Trees correction describing the two-body orbit-orbit polarization interaction (a is the Trees parameter).19,20 HSO is the spin-orbit coupling (n is the SO parameter). HCF is the crystal field Hamiltonian, which is represented for tetrahedral symmetry Td in the Wybourne’s notation by the following equation:23,24 " # rffiffiffiffiffi 5 ð4Þ ð4Þ ð4Þ C þ C4 ; HCF ðTd Þ ¼ Btet (2) 4 C0 þ 14 4 B4tet is the tetrahedral CF parameter and Cq(k) are the Racah tensor operators. The matrix elements of Cq(k) operators are calculated numerically by using the Racah tensor algebraic methods,25 whereas the Racah parameters B and C, the Trees parameter a, the crystal field parameter B4tet, and the SO parameter n are to be determined from the optical spectra. Since for 3dN transition ions of the first series in crystals, the crystal-field is of the intermediate strength,21–23 the basis functions {jdNLSMLMSi} in the LS-coupling scheme have been adopted in our computer package. As a part of a larger project, we have set to develop a computer package
For the cobalt Co2þ doped in ZnO, the theoretical energies are obtained by diagonalizing the full 120 120 matrix associated to the Hamiltonian of Eq. (1). These energies are function of the B, C, a, B4tet, and n parameters. These parameters, determined from the PLE spectra, permit as to deduce the calculated energy levels listed in the Table I. The set of calculated parameters B, C, B4tet, Dq, a, and n presented in Table II reproduce well the energies of the observed bands of Fig. 5. Moreover, we have deduced the energy levels, which are not observed experimentally. The parameter Dq shows the crystal field strength and is characteristic of transition metals. From this crystal field analysis, we remark that the peak at 15 781 cm1 observed in Fig. 5 is assigned to the 4A2(4F) ! 2T1(2G) transition. This peak is not mentioned in Ref. 1. Moreover, our theoretical calculation do not confirm the observed PLE peak at 20 213 cm1, which is declared in Ref. 10 and assigned to the 4A2(4F) ! 2T1(2P) transition. The theoretical results give values of B ¼ 760 cm1 and C ¼ 3519 cm1 (assuming C/B ¼ 4.63 of the free ion), B4tet ¼ 8316 cm1 (Dq ¼ 396 cm1 and Dq/B ¼ 0.52), a ¼ 10 cm1, and n ¼ 390 cm1. The Racah parameters B and C are reduced compared to the free ion values (Bfree ion ¼ 971 cm1 and Cfree ion ¼ 4497 cm1)21–23 due to covalency effects. With bB ¼ Bcomplex ion/Bfree ion and bC ¼ Ccomplex ion/Cfree ion, it follows: bB ¼ bC ¼ 0.78. The Racah B and C and the crystal field B4tet parameters presented in Table II are in good agreement for Co2þ ions in ZnO nanorods, nanoparticules, and bulk crystals. This result is predictable and explained by the fact that the spectrums of these semiconductors are almost identical in the visible region associated to Co2þ d-d internal transitions.1–3 The Tanabe-Sugano diagram for Co2þ ion in tetrahedral symmetry site is drawn in Fig. 6. It indicates the general behavior of Co2þ ion energy levels as a function of the local field strength measured in terms of Dq/B. The vertical line shown in Fig. 6 represents the appropriate value for Dq/B found for Co2þ in ZnO nanorods. From Tanabe-Sugano diagram (Fig. 6), we remark that the 4T1(4P) quartet state and the 2E(2G), 2T1(2G), and 2A1(2G) doublet states are nearly for the calculated parameters Dq, B, and C. This shows a strong coupling between the electronic states of different multiplicities. All these excited states are subdivided into doublet Kramer’s states E1/2, E5/2, and G3/2 by spin-orbit coupling. The interaction between the same symmetry levels (E1/2, E5/2, or G3/2) deriving from electronic states of different multiplicities leads to the observed spin-forbidden transition from 4A2(4F) ground state to 2E(2G), 2T1(2G), and 2 A1(2G) excited states (Td symmetry) with lower intensity compared to a spin-allowed transition. Figure 7 shows the interaction between the 4T1(4P) quartet state and the 2E(2G) doublet state by spin-orbit coupling.
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TABLE I. Experimental and calculated energies (cm1) of Co2þ occupying a Td symmetry in ZnO nanorods (The number in parenthesis indicates the degenerate of the energy level; the slash (“/”) signifies that the states are not visible). Eobs (absorption) x ¼ 0.011
Eobs (PLE) x ¼ 0.11
0 /
0 /
0 3960
4
/
/
6855
2
15 381 /
15 438 15 781
15 357 15 890
4
16 413
16 317
16 325
2
17 656 /
17 704 /
17 637 18 407
Td 4
A2(4F) T2(4F)
4
T1(4F)
E(2G) T1(2G)
2
T1(4P)
A1(2G) T2(2G)
2
T1(2P)
ECal [this work]
ECala[this work] 0(4) 3852(2) 3945(4) 4037(4) 4114(2) 6656(2) 6734(4) 7145(4) 7381(2) 15340(4) 15 840(4) 1591(2) 16 179(2) 16 280(4) 16 570(4) 16 573(2) 17 823(2) 18 288(2) 18 535(4) 20 374(2) 20 551(4) 20 853(2) 20 952(4) 22 452(4) 23 177(2) 23 352(4) 24 587(2) 24 641(4)
2
/
2
/
/
20 617
2
/ /
/ /
22 336 23 216
2
/
/
24 501
2
/
/
25 401
25 518(4) 25 672(2)
2
E(2a Da ) A2(2F) 2 T2(2F)
/ / /
/ / /
27 784 32 757 33 203
2
/
/
34 460
27 931(4) 32 825(2) 33 176(2) 33 336(4) 34 464(2) 34 597(4)
2
/
/
51 046
51 136(4)
2
/
/
51 697
51 807(4) 51 829(2)
T1(2H) E(2H) T1(2H)
2
T2(2H) T2(2a Da )
2
T1(2F) E(2b Db ) T2(2b Db )
20 572
a
with spin orbit coupling.
TABLE II. Crystal field parameters (in cm1) of Co2þ (3d7) in ZnO nanorods and comparison with parameters of Co2þ in ZnO nanoparticles and bulk crystals. (The slash (“/”) signifies that the authors could not determine the values of the parameters.) Materials
B
C
B4tet
Dq
a
n
2þ
760
3519
8316
396
10
390
770 760
3560 3500
8337 8400
397 400
/ /
/ 429
775
3490
8190
390
/
630
Co in ZnO nanorods [this work] Co2þ in ZnO nanoparticles2 Co2þ in ZnO Bulk crystals3 Absorption at T ¼ 4.2 K Co2þ in ZnO Bulk crystals3 Absorption at T ¼ 77
The highest-energy narrow peak in the luminescence spectrum of Fig. 3 is located at 15 107 cm1. According to Schulz and Thiede, the homolog emission peak at 15 161 cm1 corresponds to transition from the mixed 2E(2G), 2 T1(2G), 4T1(4P), and 2A1(2G) levels to the 4A2(4F) ground state for ZnO:Co bulk crystal.3 This is explained by the fact that 2E(2G), 2T1(2G), 4T1(4P), and 2A1(2G) levels are almost nearly as its is seen on the Tanabe-Sugano diagram. Following this reasoning, the initial state of the emission should be the 2E(2G) level, which is reached after relaxation in the excited state. Figure 8 schematically represents the lowest energy levels split of Co2þ ions in the crystal field. The excitation and the emission transitions within the tetrahedral Co2þ ions are shown in this figure. Indeed, based on the PL and the PLE spectral analysis, we reveal that the visible light can
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FIG. 8. The energy levels split in the crystal field of Co2þ ions (Td symmetry) and the excitation and emission transitions within the tetrahedral Co2þ ions. FIG. 6. Tanabe-Sugano diagram for tetrahedrally coordinated Co2þ ion with C/B ¼ 4.63. The vertical line represents the case of ZnO:Co nanorods.
immediately excite electrons from the basic state 4A2(4F) to the 2E(2G), 2T1(2G), 4T1(4P), and 2A1(2G) excited states of tetrahedrally coordinated Co2þ ions. The electrons relax from the higher excited state 2T1(2G), 4T1(4P), and 2A1(2G) to the 2 2 E( G) doublet excited state and then return to the 4A2(4F) basic state, emitting the photon of 15 107 cm1. The luminescence spectrum of Co2þ doped in ZnO nanorods shows a few narrow, well-resolved peaks located on the high-energy side of the band, as illustrated by the arrows in Fig. 3. These peaks form two vibronic progressions, one with an interval of 108 cm1 and the other with an interval of 432 cm1. According to a published analysis of the Raman spectrum,10,26 there are two modes of vibration E2 (high) and E2 (low) observed, respectively, at a frequency of 434 cm1 and 105 cm1 for a polycrystalline ZnO sample, which are in agreement with the observed replicas phonons in Fig. 3. Table III indicates the energies of the 2E(2G) ! 4A2(4F) emission and the phonons replicas with the coupling mode of vibration. The calculated luminescence spectrum is also represented in Fig. 3. This spectrum is obtained by the Fourier transformation of the autocorrelation hujuðtÞi, i.e., the overlap of u(t) with u(t ¼ 0),27 ð þ1 n o E00 2 2 Ilum ðxÞ ¼ Cx3 eixt hujuðtÞieC t þi h t dt; (3) 1
with FIG. 7. Electronic states of 4T1(4P) and 2E(2G) of Co2þ in ZnO nanorods. Solid arrows denote spin-allowed transition. Dotted arrows connect pairs of interacting levels under spin-orbit coupling.
" #! X DQ2j ik t j ð1 eikj t Þ hujuðtÞi ¼ exp : 2 2 j
(4)
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TABLE III. Experimental energies of the PL spectrum and there attribution. Peaks A Phonon replicas A1 A2 A3 A4 A5 A6 A7 A8
Energies (cm1) 15 107 14 999 14 891 14 783 14 675 14 567 14 459 14 243 13 811
Attribution 2
2
4
Vibration mode 4
E( G) ! A2( F) E2 (low) E2 (low) E2 (low) E2 (high), E2 (low) E2 (low) E2 (low) E2 (high) E2 (high)
In Eq. (3): E00 is the energy of the origin, x is the frequency of the luminescence spectrum, and C is a phenomenological damping factor determining the width of each line in the spectrum. All quantities are in wavenumber units. In Eq. (4), DQj denotes the displacement along the normal coordination Qj associated to the E2(low) and E2(high) modes, kj are the vibrational frequencies of each mode. Frequencies of 108 and 432 cm1 are considered, respectively, for k1 and k2. The parameters E00, C, DQ1 , and DQ2 are adjusted to fit the experimental spectrum. The obtained values are listed in Table IV. The optical transmission spectrum of Fig. 1 shows a broad absorption band at 2.5-3.2 eV, which is not mentioned in Ref. 1. This band was assigned as the charge transfer process of Co2þ ! Co1þ attributed to electronic transfer between the O 2p and Co 3d orbitals.15,16,18 Many recent studies deals with transition metal ions doping in semiconductors usually results in the quenching of the host’s band edge emission and the activation of the dopant’s luminescent transitions. These processes are typically associated with an energy or charge transfer process from the host to the dopant as well as the opening up of additional nonradiative defect related relaxation channels.17,28 These recent studies suggest that intrinsic defects can function as energy storage centres that mediate the energy transfer from the ZnO host to the transition metal ions. The defects has often been ascribed to the radiative recombinaison of photogenerated holes with electrons induced by the oxygen vacancies VO.17,29–32 Other sources such as VZn and DAP recombination have also been proposed.28–32 In Fig. 4, the PLE spectrum shows a UV peak, related to the near-band-edge absorption of the ZnO host materials, at 3.357 eV (27 076 cm1) for the sample with x ¼ 0.1. This peak is shifted to 3.341 eV for the sample with x ¼ 0.01.1 TABLE IV. Parameters used to calculate the luminescence spectrum shown in Fig. 3. Parameter E00 (cm1) C (cm1) k1 (cm1), DQ1 (dim, less) k2 (cm1), DQ2 (dim, less)
Value 15 362 28 108, 1.49 432, 1.20
Otherwise, the substitution of Co2þ ions into the ZnO matrix reveals the increase of the band-gap energy. This fact may be owing to the size effect: the ionic radius of Co2þ ˚ ) is smaller compared to ionic radius of Zn (rCo2þ ¼ 0.58 A 2þ ˚ (rZn ¼ 0.60 A). In addition, the band-gap energy of the (undoped and doped) ZnO nanocrystals is demonstrated to be located at higher energies compared to that of bulk ZnO.2 This may be due to quantum confinement effect. V. CONCLUSIONS
The PL and PLE spectra of the Zn1xCoxO nanorods were composed of two groups of emission lines, one in the UV region (2.8–3.4 eV) related to the near-band-edge emission of the host ZnO materials and another in the red region assigned to the radiative transitions within the tetrahedral Co2þ ions in the ZnO host crystal. The visible peaks in the PLE spectra are attributed to the transitions from the basic state 4A2(4F) to the 2E(2G), 2T1(2G), 4T1(4P), and 2A1(2G) excited states of the tetrahedrally coordinated Co2þ ion. From these transitions, the electronic structure of the cobalt ion Co2þ doped ZnO nanorods was determined using the crystal field theory. This theoretical study provides an exact assignment of the observed transitions. The Tanabe-Sugano diagram plotted with the values of Dq, B, and C obtained from this theoretical study shows that the excited states 2 2 E( G), 2T1(2G), 4T1(4P), and 2A1(2G) are almost nearly. So, the interaction by spin-orbit coupling between these excited states leads to the observed spin-forbidden transitions from the 4A2(4F) ground state to the 2E(2G), 2T1(2G), and 2A1(2G) excited states. This theoretical study confirms that the red emitting state at 15 107 cm1 in the visible PL spectrum is 2 2 E( G). The PL spectrum associated to the 2E(2G) ! 4A2(4F) transition and its phonon replicas is reproduced theoretically using the Fourier transformation of the autocorrelation function. The band-edge of the nanocrystals is located at higher energies to that of bulk ZnO due to the quantum confinement effect. Whereas, the electronic structure of the Co2þ ion in ZnO nanocrystals is almost identical with that of Co2þ ion in ZnO bulk crystals. In order to explain the existence of excitation peaks observed near the band edge of the ZnO host, an energy transfer mechanism is proposed as follows: when the Zn1xCoxO nanocrystals are excited by UV light, electrons in the valence band of the ZnO host absorb the photon energy and transfer to the conduction band, generating free electrons in the conduction band and free holes in the valence band. These photogenerated electrons and holes then relax to the band edge of the conduction band and valence band, where they are rapidly trapped at the defects or undergo subsequent band edge radiative emission. By means of a resonant energy transfer process, the trapped carriers at the defects vacancies could transfer their energy to the Co2þ subsystem. The mechanism of charge transfer Co2þ ! Co1þ is also probably present. As a final step of the energetic process, Co2þ ions go through the radiative transition from 2 2 E( G) ! 4A2(4F), giving out the red emission at at 15 107 cm1.
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