Structural and Optical Properties of Co2+-Doped PbSe Nanocrystals

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Structural and Optical Properties of Co -Doped PbSe Nanocrystals in Chalcogeneide Glass Matrix Sidney Alves Lourenço, Ricardo Souza da Silva, Anielle Christine Almeida Silva, and Noelio Oliveira Dantas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01920 • Publication Date (Web): 14 May 2015 Downloaded from http://pubs.acs.org on May 20, 2015

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The Journal of Physical Chemistry

Structural and Optical Properties of Co2+-doped PbSe Nanocrystals in Chalcogeneide Glass Matrix Sidney A. Lourenço*a, Ricardo S. Silvab, A. C. A. Silvac, Noelio O. Dantasc a

Engenharia de Materiais, Universidade Tecnológica Federal do Paraná – UTFPR, CEP

86.812-460, Londrina, PR, Brazil. b

Departamento de Física, Universidade Federal do Triângulo Mineiro, 38.025-440,

Uberaba, MG, Brazil. c

Laboratório de Novos Materiais Isolantes e Semicondutores (LNMIS), Instituto de Física,

Universidade Federal de Uberlândia, CP 593, 38.400-902, Uberlândia, MG, Brazil.

Abstract: Semimagnetic chalcogeneide Pb1-xCoxSe nanocrystals were successfully synthesized by fusion protocol in a glass matrix and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic force microscopy (MFM), and optical absorption (OA) techniques. XRD, TEM and MFM measurements show that the asproduced Pb1-xCoxSe magnetic nanocrystals are single phases, nanosized, and crystallized in rock-salt structures. The OA spectra and crystal field theory indicate that part of Co2+ is incorporated in the tetrahedral site (Td) into PbSe nanocrystals, presenting characteristic structures in the visible and near-IR electromagnetic spectral range. The crystal field strength and the Racah parameters are estimated for the tetrahedral coordinated Co2+ ions into PbSe nanocrystals.

Keywords: PbCoSe Nanocrystals, Glass Matrix, Crystal field theory, tetrahedral coordinated

*Prof. Dr. Sidney Alves Lourenço. Engenharia de Materiais, Universidade Tecnológica Federal do Paraná – UTFPR, CEP 86.812-460, Londrina, PR, Brazil. E-mail:[email protected]

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1. Introduction Transition metal (TM) ion-doped chalcogenide semiconductor nanocrystals (NCs) have attracted much attention due to their unique optical and electronic properties compared to their bulk counterparts. The localization of magnetic ions in the same places as free-like electron and hole carriers occurring in these nanomaterials lead to high luminescence quantum efficiency and high optical gain with lower threshold.1-7 Their optical and electronic properties, generally controlled as a function of NC size, shape, and TM concentration in the NC have applications in telecommunication,1 photovoltaic detectors,2 saturable absorbers for near-infrared passively mode-locked and Q-switched solid-state lasers,3 fluorescence biological labels,4 and cancer therapy.6 Among these, diluted magnetic semiconductors (DMS) are expected to be key materials for future spintronic devices, since they carry charge and spin degrees of freedom in a single media with interesting magnetic, magneto-optical, magneto-electronic properties, among others.8-12 Although quantum dots doped with impurities (metal or magnetic) are currently being synthesized by colloidal chemistry techniques12-13 or molecular beam epitaxy (MBE),14 some possible applications require nanoparticles to be embedded in robust and transparent host materials. In this context, the melting-nucleation approach appears as an appropriate synthesis technique since it allows the growth of DMS NCs embedded in different glass matrices, which can avoid undesirable effects on the nanostructures, such as corrosion and humidity.15-17 An interesting case is the Co2+ ion. As this ion is introduced into the tetraedral site, it shows a characteristic d-d absorption/emission in the visible and near-infrared spectral range that can be used in a broad range of applications, as blue pigments (Al2O4:Co2+),18 optoelectronics and spintronics (ZnO:Co2+),19-20 phosphors (ZnAl2O4:Co2+),21-22

Q-switch

and

nonlinear

optics,23-27

and

infrared

lasers

(MgAl2O4:Co2+),26, 28-30 materials. In this work, semimagnetic Pb1-xCoxSe nanocrystals were synthesized, for the first time, by fusion protocol in a glass matrix and characterized by by X-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic force microscopy (MFM), and optical absorption (OA) techniques. XRD, TEM, MFM and OA strongly indicated the formation of Pb1-xCoxSe NCs in the magnetic phases embedded into the glass matrix, and optical


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measurements, together with crystal field theory, indicate that part of Co2+ is incorporated in the tetrahedral site (Td).

2. Samples and Experimental procedures Nonmagnetic PbSe and semimagnetic Pb1-xCoxSe NCs were produced by the fusion method in

the

glass

matrix

with

the

following

nominal

composition:

40SiO2•30Na2CO3•1Al2O3•25B2O3•4PbO (%mol), herein quoted as SNABP glass matrix. This glass matrix has shown good properties to grow chalcogenide nanocrystals in a controllable way.17,31 The nominal composition of the nanocomposite was achieved by adding 2Se (%wt) plus xCo with respect the (1-x)Pb, with x = 0.0, 0.01, 0.05, 0.10 and 0.20. The chemical reagents used in the synthesis are from the Sigma-Aldrich Chemical Company, with purity precursors of approximately 99.9%. Samples were produced according to two major preparation steps. In the first step, the powder mixture was melted in an alumina crucible, at 1200 oC, for 30 minutes, followed by a quick cooling of the crucible containing a melted mixture of 1200 oC down to room-temperature. In the second step, a thermal annealing of the previously-melted glass matrix was carried out at 500 oC for different times, to either enhance the diffusion of Pb2+, Co2+, and Se2- species within the hosting matrix or to rearrange the ions entering the formation of the NCs. Due to the thermal annealing procedure, Pb1-xCoxSe NCs were formed within the glass template. Energy-dispersive X-ray spectroscopy (EDS) have shown that Pb peak is 9% higher than the Co peak, for sample with nominal concentration of 10% Co, and 18% for sample with nominal concentration of 20% Co. These results show that the fraction of cobalt ions in the samples are similar to that used in the nominal composition. Room temperature optical absorption (OA) spectra for the Pb1-xCoxSe NCs were obtained using a double beam UV–Vis–NIR spectrophotometer Varian (Cary 500). To identify the structural phases of the as-grown Pb1-xCoxSe NCs

embedded within the

SNABP glass matrix, X-ray diffraction (XRD) spectra were recorded using an XRD-6000 Shimadzu diffractometer operating with monochromatic Cu-Kα1 radiation (λ = 15.4056 nm). Scanning transmission electron microscopy (TEM, JEOL, JEM-2100, 200 kV) was used to investigate the formation and size of the Pb1-xCoxSe NCs. MFM images of the growing Pb1-xCoxSe NCs with x = 0.0 and 0.10 were recorded at room temperature with a


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Shimadzu Scanning Probe Microscope (SPM-9600). The tapping-mode was used to obtain the sample surfaces topography and the lift-mode was used for the magnetic phase. In the lift-mode, the tip-sample distance varied by tens to hundreds of nanometers. These images were recorded to confirm the formation of magnetic nanoparticles in the SNABP glass matrix.

3. Results and Discussion XRD patterns of the Pb1-xCoxSe NCs growth in SNABP glass matrix samples, for x = 0.0, x = 0.01, x = 0.05, x = 0.10 and x = 0.20 are shown in Fig. 1. Evidently, the amorphous character of the host glass matrix where the NCs are embedded hampers the observation of the diffraction peaks related to the NCs. The obtained XRD patterns were compared with the patern bulk-PbSe rock-salt crystal structure, herein represented at the bottom of Fig. 1(a). It shows that the rock-salt structure is the most common phase for PbSe NCs32 and that the high intense (2 0 0) XRD peaks cannot be identified for the NC samples due to the presence of the host glass matrix amorphous band. However, a less intense (1 1 1) XRD peak is clearly observed in all NC samples, confirming their successful growth and the expected rock-salt crystal structure. Figure 1(b), where the browned amorphous band was subtracted, shows a slight shift of the (1 1 1) peak toward higher diffraction angles values an increase of the xCo-concentration in the Pb1-xCoxSe structure takes place, indicating a change of the lattice parameter with the incorporation of Co2+ ions in PbSe NCs. Similar shift has been observed for Cd1-xCoxSe,33 and Cd1-xCoxS,34 NCs quantum dots in the wurtzite structure. By using the Bragg's law for the cubic crystalline structure, we made an estimate of the d-spacing value of Pb1-xCoxSe NCs in function of the xCo-concentration, as shown in Fig. 1(c). The change observed in the d-value is due to the substitution of the Pb2+-ion, with larger ionic radius (1.19 Å) by the Co2+-ion, with lower ionic radius (0.72 Å), a reduction by 40% in the ionic radius. This is a strong evidence of Co2+ incorporation in PbSe NC. Recently, Erwin35 showed, using a simple diffusion model of impurities in PbSe NCs, which substitutional activation energy for diffusion of Co into PbSe NCs is ~ 2.0 eV. Thus, temperatures higher than ~430 oC are needed for the diffusion to happen. The melting-nucleation protocol, used to prepare our samples, works with annealing temperatures of 500 oC, which is sufficient to promote diffusion in the PbSe NCs. In


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Fig. 1(a). XRD diffraction patterns of Pb1-xCoxSe NCs (x = 0.0, 0.01, 0.05, 0.10 and 0.20) embedded in the host glass matrix, with annealing of 500 oC by 500 min are compared to the values in the powder diffraction standard of PbSe cubic (rock-salt) phase (JCPDS no. 78.1902). The browned curve refers to the glass. (b) The effects associated with the Co2+ incorporation into the PbSe NCs are seen as the shift to a higher 2θ diffraction angle of the (1 1 1) peak with an increase in xCo-concentration. (c) The intensity of the shift is observed with the application of Bragg’s law. The doted curve in (c) is a guide to the eye.

semiconductors, the diffusion of substitutional impurities can be mediated by vacancies or interstitials holes.15 The melting-nucleation procedure, working with relatively high temperatures, can generate vacancies in the PbSe NC cubic structure, which can favor incorporations of cobalt ions into the NCs. Possibly, part of Co2+ ions can also be incorporated into tetrahedral interstitial sites contributing to the relaxation of the d-spacing value. The experimental dependence of the d value for (1 1 1) planes with cobalt


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concentration, showed in Fig. 1(c),

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shows this relaxation clearly, where the d value

decreases nonlinearly as the cobalt concentration increases, confirming that this process does not follow Vergard’s law.36 Thus, the doping protocol may cause lattice distortion in the NC, which can be intensified due to the compressive strain induced by the host glass matrix on the surface of NC, since NCs, in rigid hosts, experience stress due to a mismatch between the thermal expansion coefficients of the dot and host, as well as the pressure arising from the surface tension.34,37-38

Fig. 2. TEM images of (a) PbSe and (b) Pb0.90Co0.10Se nanocrystals, grown in SNABP glass matrix, at 500 oC, by 200 min, with size histogram information on the average diameter (D) and size-dispersion (σ). For the time annealing of 500 min, at 500 oC, TEM images of (c) PbSe and (d) Pb0.90Co0.10Se nanocrystals are observed, with a diameter of around 8.5 nm.


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Figure 2 shows the TEM images of (a) PbSe and (b) Pb0.90Co0.10Se nanocrystals, grown at 500 oC, for 200 minutes. Insets show size histogram information on the average diameter (D) and size-dispersion (σ). The Gaussian adjust were carried out to estimate sizedispersion. The estimated average diameter (size-dispersion) for PbSe and Pb0.90Co0.10Se NCs was 3.5 nm (0.8 nm) and 3.0 nm (0.7 nm), respectively. TEM images of (c) PbSe and (d) Pb0.90Co0.10Se nanocrystals with annealing at 500 oC for 500 minutes are observed. The average diameter for the PbSe and Pb0.90Co0.10Se NCs annealing at 500 oC for 500 minutes, is around 8.5 nm (not shown here). In Fig. 2(c) and (d), it is possible to identify the (200) crystalline plane of PbSe NC. MFM images, represented in Fig, 3, show the topography ((a) and (c)) and the

Fig. 3. Topographic (left panel) and magnetic phase (rigth panel) MFM image showing a high amount of (a, b) PbSe and (c, d) Pb0.90Co0.10Se NCs at the sample’s surface, respectively, with annealing of 500 oC for 500 min. The contrast between the South (S) and North (N) magnetic poles identifies the orientation of the total magnetic moment of semimagnetic NCs. The contrast observed of some NCs selected in red, reinforces the formation of semimagnetic nanostructures.


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magnetic phase ((b) and (d)) measurements for the Pb1-xCoxSe NCs samples, for x = 0.0 and 0.10, grown in SNABP glass matrix, with annealing at 500 oC, for 500 min. The contrast observed in the magnetic phase is due the magnetic response of the semimagnetic NCs embedded in the host glass matrix with a magnetic tip, considering that the dark area (represented by S in figure 3(d)) is a result of the attraction and bright area (represented by N in figure 3(c)) is a result of the repulsion. The phase magnetic contrast observed is more evident for

samples with Pb0.90Co0.10Se NCs in relation to the PbSe NCs, strongly

indicating incorporations of Co2+ ions in the PbSe nanocrystalline structure. Hence, we may conclude that the total magnetic moment of each NC (observed in Fig. 3) has a contribution of the sp-d exchange interactions. The crystal field theory (CFT) together with optical absorption spectroscopy are effective tools to analize the configuration around Co2+ impurits.24,39 Figure 4 shows the room temperature optical absorption for PbSe:Co2+ NCs embedded in the glass matrix for five different cobalt concentrations (Pb1-xCoxSe; x = 0.0, 0.01, 0.05, 0.10, and 0.20) and the Tanabe-Sugano diagram for Co2+ in tetrahedral site.40 Using the CFT and the experimental optical data, the crystal field strength ∆ and the Racah parameters B can be calculated for the electronic configuration of Co2+ (3d7) in tetrahedral sites by fitting the spin-allowed transitions in the d3 Tanabe-Sugano diagram40 (Fig. 4(A)). In the tetrahedral sites, the ground level 4F of a free ion Co2+ is split into three levels, 4T1, 4T2 and 4A2, by the crystal field (Fig. 4(A)). The absorption spectra in the visible light region (Fig. 4(B)) present a component close to 587 nm assigned to the spin and electric-dipole allowed 4A2(F)→4T1(P) Co2+ ligand-transition.24,41-42 Bands close to 530, 639 and 650 nm are due to three spinforbidden transitions. These bands are assigned to 4A2(F)→2A1(G), 4A2(F)→2T1(G), and 4

A2(F)→2E(G) transitions, respectively, according to the Tanabe–Sugano diagram of d7(Td)

for C/B = 4.5.24,43-44 Their transition energies can be well described by a Racah parameter, B = 788.2 cm−1 and by crystal-field splitting, ∆= 3882.9 cm−1, and they are shown by spots at the ∆/B = 4.93 value in Fig. 4(B). Our experimental results are in agreement with CFT for Co2+ in tetrahedral sites. All samples have extremely intense absorption in the visible and NIR regions, and the intensity of the absorption band increases with the increasing of the Co2+ concentration. This increase is associated with the increasing of Co2+ ions located in the tetrahedral sites of PbSe:Co2+ NCs.


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Fig. 4. (A) Tanabe-Sugano diagram of d7(Td) for C/B = 4.5. It shows spin allowed and spin forbidden transitions. (B) The room-temperature absorption spectra of Pb1-xCoxSe NCs (x = 0.0, 0.01, 0.05, 0.10 and 0.20) embedded in the SNABP glass matrix annealed for 200 min, at 500 oC. For comparison purposes, the absorption spectra of the SNABP glass matrix is shown in the Figure (blue line). Fig. (B) shows the simple energy level diagram of Co2+ (3d7) doped in a tetrahedral host. The experimental crystal-field energies obtained from the optical spectra are represented by spots at the ∆/B = 4.93 value. Fitting parameters are B = 788.2 cm−1 and ∆= 3882.9 cm−1. The spin-orbit coupling, splits the 4T1(4F) excited estate into three sub-states: Γ6, Γ8 and Γ8 + Γ7. Figure (B) shows the optical absorption energy involving this splitting states with energies E21, E22 and E23, according to Ref.39, 42, 45 The observed structures in the optical absorption spectra of PbSe:Co2+ system, analyzed in this study, are similar to those observed in other materials, both in bulk as well as in the nanocrystalline form,24,26-30,39,41,45-46 when Co2+ ion is in tetrahedral site, and are not consistent with the optical properties of Co2+ ion when it is in the octahedral sites.39,42 The crystal field strength ∆ for the tetrahedral site is lower than the charged ion in the octrahedral site (∆ 0.5 ∆ .18,24 This leads to a big change in the optical transitions’ energy observed in the optical absorption spectra.42 The near-infrared broad absorption band centered at 1483 nm is attributed to the spin and electric-dipole allowed 4A2→4T1 (4F) transition. This spin-allowed transition can be explained by the spin-orbit coupling interaction that splits the 4T1(4F) excited estate into three sub-states: Γ6, Γ8 and Γ8 + Γ7. Figure 4(B) shows the optical absorption energy


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involving this splitting states, with energies: E21, E22 and E23.39,42,45 This result is in agreement with those observed in other matterials.39,42 The optical absorption spectra of the SNABP matrix (represented by a blue line) and the cobalt-free PbSe NCs (represented by a green line) are also shown in Figure (4B) for comparison purposes. The shoulder at 1100 nm, in the cobalt-free PbSe NCs, is associated with the excitonic transition of PbSe NC.17,47-48 The excitonic absorption energy of this transition, 1100 nm (1.12 eV), is above the three excited estate energies Γ6, Γ8 and Γ8 + Γ7. These excited states of ions Co2+ are located within the PbSe NC band gap energy. Thus, we show that Co2+-dopped PbSe NCs were grown in the SNABP glass matrix and Co2+ ion is in tetrahedral sites. However, we should note that PbSe NCs crystallizes in the rock-salt crystal structure, and in this structures Co2+ may be substitutionally incorporated in the octahedral symmetry. Thus, our optical absorption data indicates that part of Co2+ into PbSe NCs is incorporated interstitially in the tetrahedral sites. The low ionic radius of Co2+ -ion (0.72 Å) in relation to Pb2+-ion radius (1.19 Å) may facilite interstitial doping in tetrahedral holes of the PbSe NCs with fcc lattice.

4. Conclusions This study reports on the successful synthesis of semimagnetic Pb1-xCoxSe nanocrystals (NCs), with an average diameter of ~3.0 nm and crystallized in rock-salt structures, embedded in a SNABP glass matrix by the fusion method. Our results confirm that the magnetic doping Co2+ ions were incorporated in the NCs. The investigation of semimagnetic PbSe:Co2+ NCs using the experimental techniques (X-ray diffraction ˗ XRD, transmission electron microscopy ˗ TEM, magnetic force microscopy ˗ MFM, and optical absorption - OA) showed that it was possible to control the structural, magnetic and optical properties of these DMS NCs, confirming the high quality of the synthesized samples. The OA spectra and the crystal field theory indicate that part of Co2+ dopping is incorporated in the tetrahedral site (Td) into PbSe:Co2+ nanocrystals, leading to strong OA structures in the visible and near-IR electromagnetic spectral range. We believe that these results can motivate further investigations of these systems in search for possible device applications.


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5. Acknowledgement The authors gratefully acknowledge the financial support from the following Brazilian agencies: MCT/CNPq, Capes, Rede Mineira de Química (RQ), Fundação Araucária and Fapemig. We are also thankful to Instituto de Física (INFIS), Universidade Federal de Uberlândia (UFU), for letting us use their facilities for the MFM measurements, supported by a grant (Pró-Equipamentos) from CAPES (Brazilian Federal Agency for Support and Evaluation of Graduate Education).

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30. Liu, L.; Yang, L.; Pu, Y.; Xiao, D.; Zhu, J., Optical Properties of Water-Soluble Co2+:ZnS Semiconductor Nanocrystals Synthesized by a Hydrothermal Process. Mater. Lett. 2012, 66, 121-124. 31. Dantas, N. O.; Monte, A. F. G.; Qu, F. Y.; Silva, R. S.; Morais, P. C., Energy Transfer in PbS Quantum Dots Assemblies Measured by Means of Spatially Resolved Photoluminescence. Appl. Surf. Sci. 2004, 238, 209-212. 32. Yu, W. W.; Falkner, J. C.; Shih, B. S.; Colvin, V. L., Preparation and Characterization of Monodisperse PbSe Semiconductor Nanocrystals in a Noncoordinating Solvent. Chem. Mater. 2004, 16, 3318-3322. 33. Hanif, K. M.; Meulenberg, R. W.; Strouse, G. F., Magnetic Ordering in Doped Cd1Co Se Diluted Magnetic Quantum Dots. J. Am. Chem. Soc. 2002, 124, 11495-11502. x x 34. Freitas Neto, E. S.; Silva, A. C. A.; Silva, S. W.; Morais, P. C.; Gomez, J. A.; Baffa, O.; Dantas, N. O., Raman Spectroscopy of Very Small Cd1-xCoxS Quantum Dots Grown by a Novel Protocol: Direct Observation of Acoustic-Optical Phonon Coupling. J. Raman Spectrosc. 2013, 44, 1022-1032. 35. Erwin, S. C., Doping PbSe Nanocrystals: Predictions Based on a Trapped-Dopant Model. Phys. Rev. B 2010, 81, 235433. 36. Denton, A. R.; Ashcroft, N. W., Vegard Law. Phys. Rev. A 1991, 43, 3161-3164. 37. Freitas Neto, E. S.; Dantas, N. O.; Silva, S. W.; Morais, P. C.; Silva, M. A. P., Confirming the Lattice Contraction in CdSe Nanocrystals Grown in a Glass Matrix by Raman Scattering. J. Raman Spectrosc. 2010, 41, 1302-1305. 38. Olkhovets, A.; Hsu, R. C.; Lipovskii, A.; Wise, F. W., Size-Dependent Temperature Variation of the Energy Gap in Lead-Salt Quantum Dots. Phys. Rev. Lett. 1998, 81, 35393542. 39. Dondi, M.; Ardit, M.; Cruciani, G.; Zanelli, C., Tetrahedrally Coordinated Co2+ in Oxides and Silicates: Effect of Local Environment on Optical Properties. Am. Mineral. 2014, 99, 1736-1745. 40. S. Sugano; Y. Tanabe; H. Kamimura, Multiplets of Transition Metal Ions in Crystals: New York, 1970. 41. Feng, S. Y.; Yu, C. L.; Chen, L.; Li, S. G.; Chen, W.; Hu, L. L., A Cobalt-Doped Transparent Glass Ceramic Saturable Absorber Q-Switch for a Ld Pumped Yb3+/Er3+ Glass Microchip Laser. Laser Phys. 2010, 20, 1687-1691. 42. Torres, F. J.; Rodriguez-Mendoza, U. R.; Lavin, V.; de Sola, E. R.; Alarcon, J., Evolution of the Structural and Optical Properties from Cobalt Cordierite Glass to GlassCeramic Based on Spinel Crystalline Phase Materials. J. Non-Cryst. Solids 2007, 353, 4093-4101. 43. Keppler, H., Crystal-Field Spectra and Geochemistry of Transition-Metal Ions in Silicate Melts and Glasses. Am. Mineral. 1992, 77, 62-75. 44. Bohle, D. S.; Spina, C. J., Controlled Co(ii) Doping of Zinc Oxide Nanocrystals. J. Phys. Chem. C 2010, 114, 18139-18145. 45. Orera, V. M.; Merino, R. I.; Cases, R.; Alcala, R., Luminescence of Tetrahedrally Coordinated Co2+ in Zirconia. J. Phys.-Condens. Mat. 1993, 5, 3717-3726. 46. Duan, X. L.; Yuan, D. R.; Cheng, X. F.; Wang, Z. M.; Sun, Z. H.; Luan, C. N.; Xu, D.; Lv, M. K., Absorption and Photoluminescence Characteristics of Co2+:MgAl2O4 Nanocrystals Embedded in Sol-Gel Derived SiO2-Based Glass. Opt. Mat. 2004, 25, 65-69.


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47. Silva, R. S.; Baffa, O.; Chen, F.; Lourenco, S. A.; Dantas, N. O., Luminescence in Semimagnetic Pb1-xMnxSe Quantum Dots Grown in a Glass Host: Radiative and Nonradiative Emission Processes. Chem. Phys. Lett. 2013, 567, 23-26. 48. Silva, R. S.; Morais, P. C.; Alcalde, A. M.; Qu, F.; Monte, A. F. G.; Dantas, N. O., Optical Properties of PbSe Quantum Dots Embedded in Oxide Glass. J. Non-Cryst. Solids 2006, 352, 3522-3524.


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TOC Graph


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Fig. 1(a). XRD diffraction patterns of Pb1-xCoxSe NCs (x = 0.0, 0.01, 0.05, 0.10 and 0.20) embedded in the host glass matrix, with annealing of 500 oC by 500 min are compared to the values in the powder diffraction standard of PbSe cubic (rock-salt) phase (JCPDS no. 78.1902). The browned curve refers to the glass. (b) The effects associated with the Co2+ incorporation into the PbSe NCs are seen as the shift to a higher 2θ diffraction angle of the (1 1 1) peak with an increase in xCo-concentration. (c) The intensity of the shift is observed with the application of Bragg’s law. The doted curve in (c) is a guide to the eye. 258x216mm (300 x 300 DPI)


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Fig. 2. TEM images of (a) PbSe and (b) Pb0.90Co0.10Se nanocrystals, growth in glass matrix at 500 oC by 200 min, with the size histogram information of the average diameter (D) and size-dispersion (σ). To the time annealing of 500 min at 500 oC is observed TEM images of (c) PbSe and (d) Pb0.90Co0.10Se nanocrystals with diameter around 8.5 nm. 193x210mm (96 x 96 DPI)



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Fig. 3. (Color on line) Topographic (left panel) and magnetic phase (rigth panel) MFM image showing a high amount of (a, b) PbSe and (c, d) Pb0.90Co0.10Se NCs at sample’s surface, respectively, with annealing of 500 oC for 500 min. The contrast between the South (S) and North (N) magnetic poles identifies the orientation of total magnetic moment of semimagnetic NCs. The contrast observed of some NCs selected in red, reinforce the formation of semimagnetic nanostructures. 75x85mm (96 x 96 DPI)


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Fig. 4. (Color on line) (A) Tanabe-Sugano diagram of d7(Td) for C/B = 4.5. It shows spin allowed and spin forbidden transitions. (B) The room-temperature absorption spectra of Pb1-xCoxSe NCs (x = 0.0, 0.01, 0.05, 0.10 and 0.20) embedded in the SNABP glass matrix annealed for 200 min, at 500 oC. For comparison purposes, the absorption spectra of the SNABP glass matrix is shown in the Figure (blue line). Fig. (B) shows the simple energy level diagram of Co2+ (3d7) doped in a tetrahedral host. The experimental crystalfield energies obtained from the optical spectra are represented by spots at the ∆/B = 4.93 value. Fitting parameters are B = 788.2 cm-1 and ∆= 3882.9 cm-1. The spin-orbit coupling, splits the 4T1(4F) excited estate into three sub-states: Γ6, Γ8 and Γ8 + Γ7. Figure (B) shows the optical absorption energy involving this splitting states with energies E21, E22 and E23, according to Ref. [39, 42, 45] 216x261mm (300 x 300 DPI)


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Structural and Optical Properties of Co2+-Doped PbSe Nanocrystals

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