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Huge enhancement of photoluminescence emission from porous silicon film doped with Cr(III) ions
Journal of Luminescence 199 (2018) 109–111
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Huge enhancement of photoluminescence emission from porous silicon film doped with Cr(III) ions
T
⁎
Walter J. Salcedoa, , Mauro S. Bragab, Ruth F.V.V. Jaimesc a
Laboratório de Microeletrônica, Escola Politécnica, Universidade de São Paulo, C.P.61548, 05424-970 São Paulo, SP, Brazil Instituto Federal de Educação, Ciência e Tecnologia de São Paulo – IFSP, CEP:11533-160 Cubatão, SP, Brazil c Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, CEP 09210-580 Santo Andre, SP, Brazil b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous silicon Photoluminescence Cr(III) ion emission
In the present work, it is reported the high photoluminescence (PL) enhancement of porous silicon (PS) films doped with Cr+3 ions by electrochemical process. The experimental results showed that PL emission on these samples have been achieved enhancement of 6 orders of magnitude relative to emission intensity from the normal PS sample. The PL peak position of these samples takes place below 710 nm suggesting that Cr3+ ions in PS structure have been under strong crystal-field action. The Raman spectra results suggest that Cr3+ ions have been incorporated at the surface of porous structure forming the oxide complex.
1. Introduction Porous Silicon (PS) films have been used as an excellent photonic material for optical devices fabrication, e.g. wave guides, photonic band gap crystal, optical filter and electro luminescence devices [1]. The tunable optical properties of porous silicon have been successfully used for sensor and biosensor device fabrication, for example the mesoporous silicon was used as reusable bio catalytic device after functionalization by enzyme molecules [2]. The corrugated porous silicon filter with its surface hidrocarbonized was tested as a biocompatible optical sensor applied for microenvironmental control in cells cultures assay [3]. The porous silicon photoluminescence change was used as sensor response for glucose and urea detection reverting to initial intensity after interference of some ions metals [4]. In order to improve the sensors sensitivity, the photoluminescence (PL) emission from porous silicon have been used as principal response parameter in sensor and bio-sensor application. In this sense many effort of PL enhancement has been reported, for example the PL emission features have been tuned by changing the forming electrochemical parameters such as current density, etching time and electrolyte concentration. These experimental procedure tuned the peaks position and the high porosity zones showed highest PL intensity [3,12]. The supercritical drying process at the final stage of porous silicon formation increases the PL intensity in two times [5]. The silicon wafer excitation with He-Ne laser, during of PS electrochemical formation process, also improve the PL intensity [10]. Otherwise, PS fabrication and post-fabrication process have been suggested such as PS based nanocomposite
⁎
formation or metal ions doped PS.For example,the PS/Ce-oxide nanocomposite enhanced the PL emission in 30 times; this enhancement was assigned to efficient energy transfer from Ce-oxide to Si nano-crystal into PS layer [4]. The PS/ZnS nano composite,obtainedby electrochemical process, showed high PL, the authors suggest that the PL emission come out predominantly from ZnS emission [5]. The Co deposition on the PS surface by simple immersion in the aqueous solution of CoCl2 improves the PL emission intensity of PS in three times [6]. The Ni deposition on the PS surface by electroless deposition improves the PL intensityin 11 times [7]. The Li ions deposition on the PS surface by immersion plating improves the PL intensity of PS layer in two times [8]. The rare earth ions as dopant of PS layer have been reported, the PS doped with Yb3+ by electrochemical process enhanced the PL intensity and this enhancement was improved when it was used the PS micro cavity, the authors suggest that this enhancement was due to radiative center increasing after Yb3+ adsorption on the surface area of PS layer [9,10]. In general, the transition metal ions have been used for silicon etching process, in order to obtain PS layers or silicon nanowire structures [11,12]. However, some transition ions metals such as Cr3+, Ti3+, V2+, Mn5+, Fe2+, Co2+ and Ni2+ have been used successfully in the development of solid-state lasers [13–15]. The electronic configurations of these ions metals involves all 18 electrons in a filled core ending with 6 electrons in 3p orbital. They differ the number of electrons, inasmuch as they have in the outermost 3d electrons turned into active electrons [15]. Trivalent chromium ions have played a central role in the development of solid-state laser, since it was used as the
Corresponding author. E-mail address: [email protected] (W.J. Salcedo).
https://doi.org/10.1016/j.jlumin.2018.03.027 Received 21 December 2016; Received in revised form 30 June 2017; Accepted 12 March 2018 Available online 13 March 2018 0022-2313/ © 2018 Published by Elsevier B.V.
Journal of Luminescence 199 (2018) 109–111
W.J. Salcedo et al.
active ion in the first laser (ruby) and has been the most successful transition metal ion used for laser application in several host crystals [15]. The key idea of this work was to use the excellent optical properties of Cr3+ ion in order to enhance the PL emission from porous silicon. In this sense, the present work reports the high photoluminescence (PL) enhancement from the PS films doped with Cr+3 ions by electrochemical process. This highly intense photoluminescent emission from the Cr3+ in the PS, that acts as a host substrate, is reported at the first time in this work. 2. Experimental procedure Porous silicon (PS) layers were prepared by the electrochemical anodization method as describes elsewhere [16]. The PS films were doped with Cr+3 ions by two different electrochemical processes. In the first one, the Cr3+ ions were incorporated into PS structure by polarizing the PS film cathodically and fixing the current density at 0.1 mA/ cm2 in 0.1 mol L−1 and 0.01 mol L−1 aqueous solutions of Cr(NO3)3, respectively. This electrochemical procedure was applied after PS formation. In the second process, the Cr3+ ions incorporation was achieved during PS formation, i.e, the PS was formed by anodization process in HF: Ethanol: Cr(NO3)3 0.1 mol L−1 aqueous solution (1:1:1), as it will be discussed onward, this last process promote a surprising high photoluminescent emission enhancement. The PL emission spectra were obtained at backscattering setup with Renishaw Raman spectrometer. A detailed description of the optical setup for polarization measurement of PL spectra is described elsewhere [16]. The PL spectra were recorded using the laser lines at 488.0 nm (from an Ar+ laser) and 632.8 nm (from a HeNe laser). It is worth to mention that anodization process for PS formation was made for 10 min originating the PS layer with 10 µm of thickness containing silicon nano crystals of c.a. 3 nm of size as reported in our early work [16].
Fig. 2. The PL spectra of sample I and II excited by 488 nm laser source.
Additionally, the peak position of sample II take place at 709 nm showing the blue-shift effect in relation of PL from the simple PS film (747 nm). The PL enhancement in PS film with Cr3+ ions is directly related to ions concentration because the sample I that was obtained by electrochemical cathodic polarization in low concentration aqueous solution of Cr(NO3)3 salts (0.01 mol L−1) showed less emission intensity (Fig. 2) than of sample II obtained in 0.1 mol L−1 concentration. Just like it was mentioned in the experimental procedure, the Cr3+ ions incorporation in the sample III have been obtained during anodic polarization of silicon substrate for PS film formation. This process was a completely different than process used in samples I and II, as described above. The PL emission of sample III showed an amazing so high emission intensity reaching enhancement of 6 orders of magnitude relative to the emission intensity of normal PS sample (Fig. 3). It is important to point out that PL intensity from sample III have been showed 3 orders of magnitude more intense than sample II. Additionally, the peak position of PL emission from sample III was taken place at 655 nm showing a blue-shift effect in relation to PL emission peak from samples I and II, respectively (Fig. 2). Considering the above results, it is clear that high PL intensity from the samples treated with Cr3+ ions is related to Cr3+ ions incorporation into the PS films, suggesting that emission was happen from the active optical electrons of Cr3+ ions. It is well known that the active electron in Cr3+ ions is related to 3d3 electrons. These are the electrons that play the dominant role in determining the optical properties of the ions. The energy levels of these electrons could be splitting by electron-electron interaction and crystal-field effect. The inter-electron interaction is related to B and C Racah parameter [17]. The crystal-field intensity is related to Dq parameter that dependents on the r−5, where r is the distance between ions-ligand moieties in the crystal structures [18]. In the case of Ruby crystal, the ions take place in the octahedral symmetry center splitting the energy levels because of crystal-field effect. From
3. Results and discussion The Samples that were obtained after cathodic polarization in Cr (NO3)3 aqueous solution of 0.1 mol L−1 and 0.01 mol L−1 were labeled as sample I and II, respectively. The PS sample that the Cr3+ ions incorporation was obtained during PS formation was labeled as sample III. The Fig. 1 shows the PL emission spectra of sample II and of the PS samples without Cr3+ ions after excitation with laser source of 632.8 nm. From the spectra results it can be observed that the sample II (with Cr3+ ions) showed highest PL emission relative to of the PS sample. This PL intensification showed enhancement of 3 orders of magnitude in relation to emission intensity from PS sample.
Fig. 3. PL emission spectra of sample III (with Cr3+) and the normally PS samples after excitation with 632.8 nm laser source.
Fig. 1. The PL spectra of sample II (with Cr3+) and the PS samples without Cr3+ ions.
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Journal of Luminescence 199 (2018) 109–111
W.J. Salcedo et al.
4. Conclusions From the results obtained in the present work, we can draw the following conclusions. The Cr3+ ions that were incorporated into PS structures promote the so high PL emission and the intensity enhancement depends on the experimental procedure used for ions incorporation. Sample that was submitted to cathodic polarization in Cr(NO3)3 aqueous solution immediately after PS formation showed PL enhancement of 3 order of magnitude in relation to emission intensity from PS sample. Sample that ions incorporation was achieved during PS formation showed PL enhancement of 6 orders of magnitude. The peak positions of PL band emission from the sample with Cr3+ ions take place below 710 nm suggesting that the ions have been under strong crystal-field action in the PS structure. However, the coordination of ions in the PS film should be different than in Ruby crystal (octahedral symmetry) since the sample with highest PL emission showed its peak at 655 nm. The Raman spectra results suggest that Cr3+ ions have been situated at the surface of porous structure forming the oxide complex. The experimental process used for Cr3+ ions incorporation into PS films and it's so high PL enhancement emission is reported at the first time in the present work.
Fig. 4. Raman spectra of PS sample (without Cr3+ ions) and sample III containing Cr3+ ions.
the Tanabe-Sugano diagrams [17], the 4T2 level that is proportional to Dq parameter crossover the 2E level (that predominately depends from the B and C Racah parameter) at the Dq/B ~ 2.3, this point defined the limit between weak and strong crystal-field regions. For Dq/B > 2.3 the crystal-field is considered to be strong field region and weak field region in another case, respectively. Ruby is the standard example of a specific example of a strong-field Cr3+ laser material; in this case the host crystal is α-phase corundum, generally referred to as sapphire (Al2O3) [15]. The fluorescence spectrum of Ruby at room temperature consist the R lines zero phonon transition situated at 693 and 695 nm respectively and broad band (714 nm) associated with multiphonon vibronic transition [15]. All of these transitions are related to 2E → 4A transition [15]. In the case of weak field crystal (Dq/B < 2.3) the radiative transition could be happened from the 2T2 → 4A transition, the existence of this transition has been observed in some glasses that contain the Cr3+ ions and where the Cr-O bonds showed a highly disperse sizes showing a broad band emission with its maximum situated at 850 nm [19]. In the present work, the peaks of bands emission take place below to 710 nm and in the case of sample III was situated at 655 nm suggesting that the Cr3+ ions into the PS films have been under a strong crystal-field action since in addition no band emission was observed in the 850 nm region. The 2E excited state is slowly dependent on the Dq parameter in the sense of Tanabe-Sugano diagrams so the increasing in the crystal-field intensity do not change the energy of 2E → 4A transition energy could be expected. However, the sample III showed its peak at 655 nm that is blue shift in about of 59 nm in comparison with the Ruby phonon assisted emission band (714 nm). These results suggest that the coordination of Cr3+ ions in the PS samples could had different structural features than in Ruby crystal, where the ion coordination has octahedral symmetry feature. Now, the question is where the Cr3+ ions have been incorporated into PS structure? In this sense, the sample III have been studied by Raman scattering technique and comparing with the Raman spectrum from the non doped PS sample (Fig. 4). From the Raman spectra, it can be observed that normal PS films and sample III showed the same spectra features. In particular the band in 400–540 cm−1 region (of sample III) that correspond to optical mode vibration of Si-Si bonds in the nano crystallites structures [16] not showed any change in the peak position and FWHM feature. This result suggests that the Cr3+ ions have not been incorporated into the silicon crystallites of the PS film suggesting that ions take place at the surface of porous structure forming the oxide complex as Cr2O3 as it was observed in some glasses [20]. For other side, it is worth to mention that the PL enhancement by Cr3+ ions was only observed in the nano-porous silicon layer.
Acknowledgments The authors thank CNPq (306962/2015-2) for financial support and thank to Laboratorio de Espectroscopia Molecular LEM of IQ-USP for spectrometer facilities. References [1] V. Parkhutik, Solid State Electron. 43 (1999) 1121. [2] M. Saleem, M. Rafiq, Sung-Yum Seo, K.H. Lee, Acetylcholinesterase immobilization and characterization, and comparison of the activity of the porous silicon-immobilized enzyme with its free counterpart, Biosci. Rep. (2016), http://dx.doi.org/ 10.1042/BSR20150154. [3] W.Y. Tong, M.J. Sweetman, E.R. Marzouk, C. Fraser, T. Kuchel, N.H. Voelcker, Towards a subcutaneous optical biosensor based on thermally hydrocarbonised porous silicon, Biomaterials 74 (2016) 217–230. [4] O. Syshchyk, V.A. Skryshevsky, O.O. Soldatkin, A.P. Soldatkin, Enzyme biosensor systems based on porous silicon photoluminescence for detection of glucose, urea and heavy metals, Biosens. Bioelectron. 66 (2015) 89–94. [5] M. Mizuhataa, Y. Mineyamaa, H. Makia, Fabrication of ZnS/porous silicon composite and its enhancement of photoluminescence, Electrochim. Acta 201 (2016) 86–95. [6] M.B. Bouzourâa, M. Rahmani, M.A. Zaïbi, N. Lorrain, L. Hajji, M. Oueslati, Optical study of annealed cobalt–porous silicon nanocomposites, J. Lumin. 143 (2013) 521–525. [7] S. Amdouni, M. Rahmani, M.A. Zaïbi, M. Oueslati, Enhancement of porous silicon photoluminescence by electroless deposition of nickel, J. Lumin. 157 (2015) 93–97. [8] I. Haddadi, S.B. Amor, R. Bousbih, S.E. Whibi, A. Bardaoui, W. Dimassi, H. Ezzaouia, Metal deposition on porous silicon by immersion plating to improve photoluminescence properties, J. Lumin. 173 (2016) 257–262. [9] S. Di-fei, J. Zhen-hong, Z. Jun, Enhanced photoluminescence from porous silicon microcavities by rare earth doping, Optoelectron. Lett. 12 (1) (2016) 5–6. [10] H.A. Lopez, P.M. Fauchet, Mater. Sci. Eng. B 81 (I-3) (2001) 91. [11] A. Backes, A. Bittner, M. Leitgeb, U. Schmid, Influence of metallic catalyst and doping level on the metal assisted chemical etching of silicon, Scr. Mater. 114 (2016) 27–30. [12] W. McSweeney, H. Geaney, C. O'Dwyer, Metal-assisted chemical etching of silicon and the behavior of nanoscale silicon materials as Li-ion battery anodes, Nano Res. 8 (5) (2015) 1395–1442. [13] J.A. Caird, S.A. Payne, M.J. Weber (Ed.), In the CRC Handbook of Laser Science and Technology, CRC, boca Raton, FL, 1989, p. 23. [14] P.F. Moulton, M. Bass, M.L. Sitch (Eds.), Laser Handbook, Elsevier, Amsterdam, 1985, p. 203. [15] R.C. Powell, W.F. Gordon (Ed.), Physics of Solid State LaserMaterials, SpringVerlag, New York Inc., 1998. [16] W.J. Salcedo, F.J.R. Fernandez, J.C. Rubim, J. Raman Spectrosc. 32 (2001) 151. [17] I.B. Bersuker, Electronic Structures and Properties of Transition Metal Compounds, John Wiley & Sons, INC, 1996, p. 76. [18] K.W. Lee, P.E. Hoggard, Inorg. Chem. 29 (1990) 850–854. [19] G. Quérel, B. Reynard, Chem. Geol. 128 (1996) 65–75. [20] J.R. Sohn, S.G. Ryu, Langmuir 9 (1993) 126.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Huge enhancement of photoluminescence emission from porous silicon film doped with Cr(III) ions
T
⁎
Walter J. Salcedoa, , Mauro S. Bragab, Ruth F.V.V. Jaimesc a
Laboratório de Microeletrônica, Escola Politécnica, Universidade de São Paulo, C.P.61548, 05424-970 São Paulo, SP, Brazil Instituto Federal de Educação, Ciência e Tecnologia de São Paulo – IFSP, CEP:11533-160 Cubatão, SP, Brazil c Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, CEP 09210-580 Santo Andre, SP, Brazil b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous silicon Photoluminescence Cr(III) ion emission
In the present work, it is reported the high photoluminescence (PL) enhancement of porous silicon (PS) films doped with Cr+3 ions by electrochemical process. The experimental results showed that PL emission on these samples have been achieved enhancement of 6 orders of magnitude relative to emission intensity from the normal PS sample. The PL peak position of these samples takes place below 710 nm suggesting that Cr3+ ions in PS structure have been under strong crystal-field action. The Raman spectra results suggest that Cr3+ ions have been incorporated at the surface of porous structure forming the oxide complex.
1. Introduction Porous Silicon (PS) films have been used as an excellent photonic material for optical devices fabrication, e.g. wave guides, photonic band gap crystal, optical filter and electro luminescence devices [1]. The tunable optical properties of porous silicon have been successfully used for sensor and biosensor device fabrication, for example the mesoporous silicon was used as reusable bio catalytic device after functionalization by enzyme molecules [2]. The corrugated porous silicon filter with its surface hidrocarbonized was tested as a biocompatible optical sensor applied for microenvironmental control in cells cultures assay [3]. The porous silicon photoluminescence change was used as sensor response for glucose and urea detection reverting to initial intensity after interference of some ions metals [4]. In order to improve the sensors sensitivity, the photoluminescence (PL) emission from porous silicon have been used as principal response parameter in sensor and bio-sensor application. In this sense many effort of PL enhancement has been reported, for example the PL emission features have been tuned by changing the forming electrochemical parameters such as current density, etching time and electrolyte concentration. These experimental procedure tuned the peaks position and the high porosity zones showed highest PL intensity [3,12]. The supercritical drying process at the final stage of porous silicon formation increases the PL intensity in two times [5]. The silicon wafer excitation with He-Ne laser, during of PS electrochemical formation process, also improve the PL intensity [10]. Otherwise, PS fabrication and post-fabrication process have been suggested such as PS based nanocomposite
⁎
formation or metal ions doped PS.For example,the PS/Ce-oxide nanocomposite enhanced the PL emission in 30 times; this enhancement was assigned to efficient energy transfer from Ce-oxide to Si nano-crystal into PS layer [4]. The PS/ZnS nano composite,obtainedby electrochemical process, showed high PL, the authors suggest that the PL emission come out predominantly from ZnS emission [5]. The Co deposition on the PS surface by simple immersion in the aqueous solution of CoCl2 improves the PL emission intensity of PS in three times [6]. The Ni deposition on the PS surface by electroless deposition improves the PL intensityin 11 times [7]. The Li ions deposition on the PS surface by immersion plating improves the PL intensity of PS layer in two times [8]. The rare earth ions as dopant of PS layer have been reported, the PS doped with Yb3+ by electrochemical process enhanced the PL intensity and this enhancement was improved when it was used the PS micro cavity, the authors suggest that this enhancement was due to radiative center increasing after Yb3+ adsorption on the surface area of PS layer [9,10]. In general, the transition metal ions have been used for silicon etching process, in order to obtain PS layers or silicon nanowire structures [11,12]. However, some transition ions metals such as Cr3+, Ti3+, V2+, Mn5+, Fe2+, Co2+ and Ni2+ have been used successfully in the development of solid-state lasers [13–15]. The electronic configurations of these ions metals involves all 18 electrons in a filled core ending with 6 electrons in 3p orbital. They differ the number of electrons, inasmuch as they have in the outermost 3d electrons turned into active electrons [15]. Trivalent chromium ions have played a central role in the development of solid-state laser, since it was used as the
Corresponding author. E-mail address: [email protected] (W.J. Salcedo).
https://doi.org/10.1016/j.jlumin.2018.03.027 Received 21 December 2016; Received in revised form 30 June 2017; Accepted 12 March 2018 Available online 13 March 2018 0022-2313/ © 2018 Published by Elsevier B.V.
Journal of Luminescence 199 (2018) 109–111
W.J. Salcedo et al.
active ion in the first laser (ruby) and has been the most successful transition metal ion used for laser application in several host crystals [15]. The key idea of this work was to use the excellent optical properties of Cr3+ ion in order to enhance the PL emission from porous silicon. In this sense, the present work reports the high photoluminescence (PL) enhancement from the PS films doped with Cr+3 ions by electrochemical process. This highly intense photoluminescent emission from the Cr3+ in the PS, that acts as a host substrate, is reported at the first time in this work. 2. Experimental procedure Porous silicon (PS) layers were prepared by the electrochemical anodization method as describes elsewhere [16]. The PS films were doped with Cr+3 ions by two different electrochemical processes. In the first one, the Cr3+ ions were incorporated into PS structure by polarizing the PS film cathodically and fixing the current density at 0.1 mA/ cm2 in 0.1 mol L−1 and 0.01 mol L−1 aqueous solutions of Cr(NO3)3, respectively. This electrochemical procedure was applied after PS formation. In the second process, the Cr3+ ions incorporation was achieved during PS formation, i.e, the PS was formed by anodization process in HF: Ethanol: Cr(NO3)3 0.1 mol L−1 aqueous solution (1:1:1), as it will be discussed onward, this last process promote a surprising high photoluminescent emission enhancement. The PL emission spectra were obtained at backscattering setup with Renishaw Raman spectrometer. A detailed description of the optical setup for polarization measurement of PL spectra is described elsewhere [16]. The PL spectra were recorded using the laser lines at 488.0 nm (from an Ar+ laser) and 632.8 nm (from a HeNe laser). It is worth to mention that anodization process for PS formation was made for 10 min originating the PS layer with 10 µm of thickness containing silicon nano crystals of c.a. 3 nm of size as reported in our early work [16].
Fig. 2. The PL spectra of sample I and II excited by 488 nm laser source.
Additionally, the peak position of sample II take place at 709 nm showing the blue-shift effect in relation of PL from the simple PS film (747 nm). The PL enhancement in PS film with Cr3+ ions is directly related to ions concentration because the sample I that was obtained by electrochemical cathodic polarization in low concentration aqueous solution of Cr(NO3)3 salts (0.01 mol L−1) showed less emission intensity (Fig. 2) than of sample II obtained in 0.1 mol L−1 concentration. Just like it was mentioned in the experimental procedure, the Cr3+ ions incorporation in the sample III have been obtained during anodic polarization of silicon substrate for PS film formation. This process was a completely different than process used in samples I and II, as described above. The PL emission of sample III showed an amazing so high emission intensity reaching enhancement of 6 orders of magnitude relative to the emission intensity of normal PS sample (Fig. 3). It is important to point out that PL intensity from sample III have been showed 3 orders of magnitude more intense than sample II. Additionally, the peak position of PL emission from sample III was taken place at 655 nm showing a blue-shift effect in relation to PL emission peak from samples I and II, respectively (Fig. 2). Considering the above results, it is clear that high PL intensity from the samples treated with Cr3+ ions is related to Cr3+ ions incorporation into the PS films, suggesting that emission was happen from the active optical electrons of Cr3+ ions. It is well known that the active electron in Cr3+ ions is related to 3d3 electrons. These are the electrons that play the dominant role in determining the optical properties of the ions. The energy levels of these electrons could be splitting by electron-electron interaction and crystal-field effect. The inter-electron interaction is related to B and C Racah parameter [17]. The crystal-field intensity is related to Dq parameter that dependents on the r−5, where r is the distance between ions-ligand moieties in the crystal structures [18]. In the case of Ruby crystal, the ions take place in the octahedral symmetry center splitting the energy levels because of crystal-field effect. From
3. Results and discussion The Samples that were obtained after cathodic polarization in Cr (NO3)3 aqueous solution of 0.1 mol L−1 and 0.01 mol L−1 were labeled as sample I and II, respectively. The PS sample that the Cr3+ ions incorporation was obtained during PS formation was labeled as sample III. The Fig. 1 shows the PL emission spectra of sample II and of the PS samples without Cr3+ ions after excitation with laser source of 632.8 nm. From the spectra results it can be observed that the sample II (with Cr3+ ions) showed highest PL emission relative to of the PS sample. This PL intensification showed enhancement of 3 orders of magnitude in relation to emission intensity from PS sample.
Fig. 3. PL emission spectra of sample III (with Cr3+) and the normally PS samples after excitation with 632.8 nm laser source.
Fig. 1. The PL spectra of sample II (with Cr3+) and the PS samples without Cr3+ ions.
110
Journal of Luminescence 199 (2018) 109–111
W.J. Salcedo et al.
4. Conclusions From the results obtained in the present work, we can draw the following conclusions. The Cr3+ ions that were incorporated into PS structures promote the so high PL emission and the intensity enhancement depends on the experimental procedure used for ions incorporation. Sample that was submitted to cathodic polarization in Cr(NO3)3 aqueous solution immediately after PS formation showed PL enhancement of 3 order of magnitude in relation to emission intensity from PS sample. Sample that ions incorporation was achieved during PS formation showed PL enhancement of 6 orders of magnitude. The peak positions of PL band emission from the sample with Cr3+ ions take place below 710 nm suggesting that the ions have been under strong crystal-field action in the PS structure. However, the coordination of ions in the PS film should be different than in Ruby crystal (octahedral symmetry) since the sample with highest PL emission showed its peak at 655 nm. The Raman spectra results suggest that Cr3+ ions have been situated at the surface of porous structure forming the oxide complex. The experimental process used for Cr3+ ions incorporation into PS films and it's so high PL enhancement emission is reported at the first time in the present work.
Fig. 4. Raman spectra of PS sample (without Cr3+ ions) and sample III containing Cr3+ ions.
the Tanabe-Sugano diagrams [17], the 4T2 level that is proportional to Dq parameter crossover the 2E level (that predominately depends from the B and C Racah parameter) at the Dq/B ~ 2.3, this point defined the limit between weak and strong crystal-field regions. For Dq/B > 2.3 the crystal-field is considered to be strong field region and weak field region in another case, respectively. Ruby is the standard example of a specific example of a strong-field Cr3+ laser material; in this case the host crystal is α-phase corundum, generally referred to as sapphire (Al2O3) [15]. The fluorescence spectrum of Ruby at room temperature consist the R lines zero phonon transition situated at 693 and 695 nm respectively and broad band (714 nm) associated with multiphonon vibronic transition [15]. All of these transitions are related to 2E → 4A transition [15]. In the case of weak field crystal (Dq/B < 2.3) the radiative transition could be happened from the 2T2 → 4A transition, the existence of this transition has been observed in some glasses that contain the Cr3+ ions and where the Cr-O bonds showed a highly disperse sizes showing a broad band emission with its maximum situated at 850 nm [19]. In the present work, the peaks of bands emission take place below to 710 nm and in the case of sample III was situated at 655 nm suggesting that the Cr3+ ions into the PS films have been under a strong crystal-field action since in addition no band emission was observed in the 850 nm region. The 2E excited state is slowly dependent on the Dq parameter in the sense of Tanabe-Sugano diagrams so the increasing in the crystal-field intensity do not change the energy of 2E → 4A transition energy could be expected. However, the sample III showed its peak at 655 nm that is blue shift in about of 59 nm in comparison with the Ruby phonon assisted emission band (714 nm). These results suggest that the coordination of Cr3+ ions in the PS samples could had different structural features than in Ruby crystal, where the ion coordination has octahedral symmetry feature. Now, the question is where the Cr3+ ions have been incorporated into PS structure? In this sense, the sample III have been studied by Raman scattering technique and comparing with the Raman spectrum from the non doped PS sample (Fig. 4). From the Raman spectra, it can be observed that normal PS films and sample III showed the same spectra features. In particular the band in 400–540 cm−1 region (of sample III) that correspond to optical mode vibration of Si-Si bonds in the nano crystallites structures [16] not showed any change in the peak position and FWHM feature. This result suggests that the Cr3+ ions have not been incorporated into the silicon crystallites of the PS film suggesting that ions take place at the surface of porous structure forming the oxide complex as Cr2O3 as it was observed in some glasses [20]. For other side, it is worth to mention that the PL enhancement by Cr3+ ions was only observed in the nano-porous silicon layer.
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