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Peculiarities of photoluminescence of porous silicon with luminescent liquid crystal fillers
Optik 125 (2014) 5738–5740
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Peculiarities of photoluminescence of porous silicon with luminescent liquid crystal fillers T.D. Ibragimov a,∗ , G.M. Bayramov b , N.G. Darvishov b a b
H. Abdullayev Institute of Physics of Azerbaijan National Academy of Sciences, H. Javid Avenue, 33, AZ114 Baku, Azerbaijan Baku State University, Z. Khalilov Street, 23, AZ1148 Baku, Azerbaijan
a r t i c l e
i n f o
Article history: Received 4 October 2013 Accepted 28 May 2014 Keywords: Porous silicon Photoluminescence Liquid crystal Filler
a b s t r a c t Photoluminescence spectra of porous silicon filled by luminescent liquid crystals 5CB and H109 were investigated. It was observed that there were photoluminescence bands corresponding to both porous silicon and liquid crystal in experimental spectra. In addition, the band corresponding to porous silicon increases in comparison with photoluminescence of porous silicon without the filler. Experimental results are explained by the radiating and nonradiating energy transfer from liquid crystal to porous silicon. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction The discovery of visible luminescence from porous Si (PS) [1] has stimulated a large interest to this material. It has shown itself as an effective emitter of visible light which can be fabricated on silicon substrates and included in microelectronic devices. It is known that parameters of emission of the porous Si strongly depend on the pore size and are very sensitive to a surrounding medium [2]. Substances filled into the porous Si can influence not only on pore properties but also on emitting abilities of this material [3–8]. One of the possible fillers may be a substance having liquid crystal (LC) state. A choice of similar filler is stipulated by the length and the form of its molecules which can penetrate into the cylindrical PS pores. Besides, it is possible an influence on dielectric and optical LC parameters by means of electric field and temperature manipulation. Particularly, application of electric field results in a change of emitting ability of luminophors inside the LC matrix [9] owing to a change of the LC order parameter. It is logical to assume that the LC embedded into porous Si may also affect on its emission parameters. The first similar filler 4-n-pentyl-4 -cyanobiphenyl (5CB) was used in the work [10]. It was shown that this LC influenced on the PS luminescence parameters. Increasing of an amount of LC molecules introduced into the porous Si shifted the luminescence band to the short-wave spectral region [11].
∗ Corresponding author. E-mail address: [email protected] (T.D. Ibragimov). http://dx.doi.org/10.1016/j.ijleo.2014.06.026 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
In this work, we present the results of investigation of photoluminescence (PL) of porous Si filled by luminescent liquid crystals 5CB and H109.
2. Experiment Samples of porous Si have been obtained by the method of anode electrochemical etching on the single p-Si substrate alloyed by boron and having orientation (1 1 1), specific resistance of 10 Ohm cm, and the thickness of 380 m. In order to carry out electrochemical anodization, the backside of silicon plate was covered by a layer of an aluminum film with the method of thermal evaporation. Then it is exposed by the heat treatment at temperature 450 ◦ C during 40 min. An area of layers was about 1 cm2 . A wire of the alloy Pt–Rh served as the second electrode. An electrolyte on the base of the fluoric acid diluted in isopropyl alcohol was chosen as an etchant. The etching time was 1–30 min and the current density was 20 mA/cm2 . A layer of porous Si differed by iridescent color from the basic substrate (Fig. 1). The nematic liquid crystal (LC) 4-n-pentyl-4 -cyanobiphenyl (5CB) was used as filler. It has nematic phase at 22.5–35.5 ◦ C. Besides, the liquid crystal mixture H109 with temperature interval of the nematic phase 5–56 ◦ C was also used for this purpose. This LC mixture consists of following LC components: 4-n-hexyloxyphenyl ester-4 -n-butyl benzoic acid (H-21), 4n-hexyloxyphenyl ester-4 -n-pentanoyloxy-benzoic acid (H-22), 4-n-butylphenyl ester-4 -n-hexyloxy-benzoic acid (H-44), and 4n-pentylphenyl ester-4 -n-methoxy-benzoic acid (H-86). The PS
T.D. Ibragimov et al. / Optik 125 (2014) 5738–5740
5739
Fig. 1. The image of the silicon sample with a porous layer.
samples were filled by LC with the same amount in the isotropic phase. Luminescence excitation was carried out by xenon lamp DKSL1000-1, whose radiation was passed through monochromator SPM-2 for separation of light of specific wavelength. The PL spectra were recorded by a spectrometer MDR-23. Resolution and accuracy of wavelengths was 0.5–1.0 nm. The signal from the detector FEU-79 (multiplier) was transferred on the computer which took account of characteristics of the lamp and the detector. The spectral measurements were carried out at room temperature. 3. Results and discussion The PL spectra of the PS samples are presented in Fig. 2. Spectra were recorded at excitation of 330 nm light selected from the xenon lamp radiation. These samples are the characteristic ones obtained by various technological groups. We can see that PL practically is absent in short-wave region. A wide unstructured band for each sample is observed at long-wave spectral region. Spectral position and intensity of this band are defined by morphology of the sample. Band maxima are equal to 582 nm and 661 nm for samples from parties 1 and 2, accordingly. In addition, the PL measurements from samples of the same party show that these maxima change only within 578–585 nm and 650–665 nm for samples from 1 and 2 parties, respectively. Similar character of PL occurs owing to quantum-dimensional effect in silicon pores of various profiles [12]. The PL spectra of samples from party 1 with and without filler, and only the liquid crystal 5CB are shown in Fig. 3. Evidently, the liquid crystal filled into porous Si enhances the PL intensity of the band corresponding to it. Meanwhile, the PL maximum of this band is slightly displaced to the long-wave region, and the intensity of
Fig. 2. The photoluminescence spectra of porous Si from different parties: 1(a) and 2(b).
Fig. 3. The photoluminescence spectra: porous silicon of the sample from party 1 without a filler (a), only the liquid crystal 5CB (b), and porous silicon of the sample from party 1 filled by 5CB (c).
the band (a maximum of PL intensity at 380 nm) corresponding to the liquid crystal decreases. Similar results are observed for samples of porous Si with liquid crystal H109, too (Fig. 4). But the PL maximum of this liquid crystal is situated at 405 nm and the effect of an increase in the band intensity of porous Si is weaker. The analogical peculiarities are observed for the PS samples from party 2. The energy transfer from LC to porous Si can be radiating or nonradiating. For definition of character of energy transfer process, porous Si with the LC filler is exposed to pumping of radiation with various wavelengths. Dependence of the band intensities (height maximum) on exciting wavelength for samples of porous Si from party 1, filling with liquid crystals 5CB and H109 are shown in Fig. 5. We see that an approach of the wavelength of exciting radiation to the LC luminescence band decreases the intensity of the band corresponding to porous Si. But there is a local maximum at this wavelength. A reduction of the PL intensity points to a radiating character of the channel of the energy transfer from LC to porous Si. It is obvious that the emission of LC filled into porous Si, in turn, excites porous Si from the inside. But at the same time, local maxima of the PL intensity of porous Si near the wavelength of the LC luminescence maximum indicates on nonradiating character of the channel of energy transfer from the LC to porous Si. The mechanism of such the nonradiating pumping of the energy was presented in work [10]. It is known that the dominant energy carriers in porous Si are excitons whose properties depend on the state of pore walls owing to small diameters of Si threads. The properties
Fig. 4. The photoluminescence spectra of the sample from party 1: porous Si without (a) filler, (b) only liquid crystal H109, and (c) porous Si with the H109 filler.
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T.D. Ibragimov et al. / Optik 125 (2014) 5738–5740
intensity corresponding to porous Si increases in comparison with a photoluminescence of pure porous Si. Experimental results are explained by radiating and nonradiating energy transfer from liquid crystal to porous silicon. Acknowledgments The authors are very grateful to Prof. T.D. Dzafarov for samples of party 1. This work was done with the support of the Science and Technology Center in Ukraine (Grant no. 5352). References
Fig. 5. Dependence of the photoluminescence band intensity of the porous Si sample from party 1 with liquid crystalline filler on irradiating wavelength: (a) filler 5CB and (b) filler H109.
of excitons for low-dimensional systems are strongly subjected to the environment influence. When the dielectric constant of surrounding medium is higher than one of porous Si, the binding energy is small and excitons dissociate without luminescence. In the case, when the dielectric constant of the semiconductor is more than the dielectric constant of surrounding medium, their energy increases and there occur radiating disintegration. The LC dielectric permeability is equal to 10 at visible region while the dielectric permeability of the porous Si changes from 11 to 4 at reduction of the pore sizes. It is obvious that the presence of wide size distribution generates both the mechanism of the channel of energy transfer from LC to porous Si. 4. Conclusions The study of photoluminescence spectra of porous silicon filled with liquid crystals 5CB and H109 having more energy of emission than photoluminescence of porous silicon shows that there are photoluminescence bands corresponding to both porous silicon and liquid crystal. At this case, the photoluminescence band
[1] L.T. Canham, Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers, Appl. Phys. Lett. 57 (1990) 1046–1048. [2] R. Hérino, Pore size distribution in porous silicon, in: L. Canham (Ed.), Properties of Porous Silicon, Short Run Press Ltd., London, 1997, pp. 89–96. [3] P.Yu. Piryatinskii, V.A. Semenovich, Distinescence features of time-resolved photoluminescence of porous silicon coated with a layer of diamondlike carbon, Tech. Phys. 43 (1998) 423–426. [4] N.I. Kluyi, A.G. Rozhin, Yu.P. Piryatinskii, V.A. Semenovich, Photoluminescence of porous-silicon/diamondlike-carbon-film structures subjected to rapid thermal annealing, Tech. Phys. Lett. 25 (1999) 304–306. [5] V.A. Makara, N.I. Kluyi, A.G. Rozhin, V.G. Litovchenko, Yu.P. Piryatinski, O.B. Korneta, Porous silicon photoluminescence modification by surface treatments and impregnation of carbon based nanoclusters, Phys. Stat. Sol. (a) 197 (2003) 355–359. [6] H. Jung, G.N. Shin, van den Hoven, A. Polman, Origin of the 1.54 mm luminescence of erbium-implanted porous silicon, Appl. Phys. Lett. 66 (1995) 18–22. [7] Yu.P. Piryatinskii, O.V. Yaroshchuk, L.A. Dolgov, T.V. Bindna, D. Enke, Photoluminescence of liquid-crystal azo derivatives in nanopores, Opt. Spectrosc. 97 (2004) 537–542. [8] S.M. Weiss, J. Zhang, P.M. Fauchet, V.V. Seregin, J.L. Coffer, Tunable silicon-based light sources using erbium doped liquid crystals, Appl. Phys. Lett. 90 (2007), 031112-1–33112-3. [9] L.M. Zarova, M.I. Serbina, N.A. Kasian, Yu.A. Gurkalenko, V.M. Shershukov, L.N. Lisetski, Effects of liquid crystalline ordering on the luminescence spectra of model compounds with packing-sensitive molecular structure, Funct. Mater. 16 (2009) 470–474. [10] Yu.P. Piryatinski, L.O. Dolgov, O.V. Yaroshchuk, S. Lazarouk, Fluorescence of porous silicon filled with liquid crystal 5CB, Mol. Cryst. Liq. Cryst. 467 (2007) 195–202. [11] Q.L. Ma, R. Xiong, Y.M. Huang, Tunable photoluminescence of porous silicon by liquid crystal infiltration, J. Luminesc. 131 (2011) 2053–2057. [12] A.G. Cullis, L.T. Canham, P.D.J. Calcott, The structural and luminescence properties of porous silicon, J. Appl. Phys. 82 (1997) 909–965.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Peculiarities of photoluminescence of porous silicon with luminescent liquid crystal fillers T.D. Ibragimov a,∗ , G.M. Bayramov b , N.G. Darvishov b a b
H. Abdullayev Institute of Physics of Azerbaijan National Academy of Sciences, H. Javid Avenue, 33, AZ114 Baku, Azerbaijan Baku State University, Z. Khalilov Street, 23, AZ1148 Baku, Azerbaijan
a r t i c l e
i n f o
Article history: Received 4 October 2013 Accepted 28 May 2014 Keywords: Porous silicon Photoluminescence Liquid crystal Filler
a b s t r a c t Photoluminescence spectra of porous silicon filled by luminescent liquid crystals 5CB and H109 were investigated. It was observed that there were photoluminescence bands corresponding to both porous silicon and liquid crystal in experimental spectra. In addition, the band corresponding to porous silicon increases in comparison with photoluminescence of porous silicon without the filler. Experimental results are explained by the radiating and nonradiating energy transfer from liquid crystal to porous silicon. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction The discovery of visible luminescence from porous Si (PS) [1] has stimulated a large interest to this material. It has shown itself as an effective emitter of visible light which can be fabricated on silicon substrates and included in microelectronic devices. It is known that parameters of emission of the porous Si strongly depend on the pore size and are very sensitive to a surrounding medium [2]. Substances filled into the porous Si can influence not only on pore properties but also on emitting abilities of this material [3–8]. One of the possible fillers may be a substance having liquid crystal (LC) state. A choice of similar filler is stipulated by the length and the form of its molecules which can penetrate into the cylindrical PS pores. Besides, it is possible an influence on dielectric and optical LC parameters by means of electric field and temperature manipulation. Particularly, application of electric field results in a change of emitting ability of luminophors inside the LC matrix [9] owing to a change of the LC order parameter. It is logical to assume that the LC embedded into porous Si may also affect on its emission parameters. The first similar filler 4-n-pentyl-4 -cyanobiphenyl (5CB) was used in the work [10]. It was shown that this LC influenced on the PS luminescence parameters. Increasing of an amount of LC molecules introduced into the porous Si shifted the luminescence band to the short-wave spectral region [11].
∗ Corresponding author. E-mail address: [email protected] (T.D. Ibragimov). http://dx.doi.org/10.1016/j.ijleo.2014.06.026 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
In this work, we present the results of investigation of photoluminescence (PL) of porous Si filled by luminescent liquid crystals 5CB and H109.
2. Experiment Samples of porous Si have been obtained by the method of anode electrochemical etching on the single p-Si substrate alloyed by boron and having orientation (1 1 1), specific resistance of 10 Ohm cm, and the thickness of 380 m. In order to carry out electrochemical anodization, the backside of silicon plate was covered by a layer of an aluminum film with the method of thermal evaporation. Then it is exposed by the heat treatment at temperature 450 ◦ C during 40 min. An area of layers was about 1 cm2 . A wire of the alloy Pt–Rh served as the second electrode. An electrolyte on the base of the fluoric acid diluted in isopropyl alcohol was chosen as an etchant. The etching time was 1–30 min and the current density was 20 mA/cm2 . A layer of porous Si differed by iridescent color from the basic substrate (Fig. 1). The nematic liquid crystal (LC) 4-n-pentyl-4 -cyanobiphenyl (5CB) was used as filler. It has nematic phase at 22.5–35.5 ◦ C. Besides, the liquid crystal mixture H109 with temperature interval of the nematic phase 5–56 ◦ C was also used for this purpose. This LC mixture consists of following LC components: 4-n-hexyloxyphenyl ester-4 -n-butyl benzoic acid (H-21), 4n-hexyloxyphenyl ester-4 -n-pentanoyloxy-benzoic acid (H-22), 4-n-butylphenyl ester-4 -n-hexyloxy-benzoic acid (H-44), and 4n-pentylphenyl ester-4 -n-methoxy-benzoic acid (H-86). The PS
T.D. Ibragimov et al. / Optik 125 (2014) 5738–5740
5739
Fig. 1. The image of the silicon sample with a porous layer.
samples were filled by LC with the same amount in the isotropic phase. Luminescence excitation was carried out by xenon lamp DKSL1000-1, whose radiation was passed through monochromator SPM-2 for separation of light of specific wavelength. The PL spectra were recorded by a spectrometer MDR-23. Resolution and accuracy of wavelengths was 0.5–1.0 nm. The signal from the detector FEU-79 (multiplier) was transferred on the computer which took account of characteristics of the lamp and the detector. The spectral measurements were carried out at room temperature. 3. Results and discussion The PL spectra of the PS samples are presented in Fig. 2. Spectra were recorded at excitation of 330 nm light selected from the xenon lamp radiation. These samples are the characteristic ones obtained by various technological groups. We can see that PL practically is absent in short-wave region. A wide unstructured band for each sample is observed at long-wave spectral region. Spectral position and intensity of this band are defined by morphology of the sample. Band maxima are equal to 582 nm and 661 nm for samples from parties 1 and 2, accordingly. In addition, the PL measurements from samples of the same party show that these maxima change only within 578–585 nm and 650–665 nm for samples from 1 and 2 parties, respectively. Similar character of PL occurs owing to quantum-dimensional effect in silicon pores of various profiles [12]. The PL spectra of samples from party 1 with and without filler, and only the liquid crystal 5CB are shown in Fig. 3. Evidently, the liquid crystal filled into porous Si enhances the PL intensity of the band corresponding to it. Meanwhile, the PL maximum of this band is slightly displaced to the long-wave region, and the intensity of
Fig. 2. The photoluminescence spectra of porous Si from different parties: 1(a) and 2(b).
Fig. 3. The photoluminescence spectra: porous silicon of the sample from party 1 without a filler (a), only the liquid crystal 5CB (b), and porous silicon of the sample from party 1 filled by 5CB (c).
the band (a maximum of PL intensity at 380 nm) corresponding to the liquid crystal decreases. Similar results are observed for samples of porous Si with liquid crystal H109, too (Fig. 4). But the PL maximum of this liquid crystal is situated at 405 nm and the effect of an increase in the band intensity of porous Si is weaker. The analogical peculiarities are observed for the PS samples from party 2. The energy transfer from LC to porous Si can be radiating or nonradiating. For definition of character of energy transfer process, porous Si with the LC filler is exposed to pumping of radiation with various wavelengths. Dependence of the band intensities (height maximum) on exciting wavelength for samples of porous Si from party 1, filling with liquid crystals 5CB and H109 are shown in Fig. 5. We see that an approach of the wavelength of exciting radiation to the LC luminescence band decreases the intensity of the band corresponding to porous Si. But there is a local maximum at this wavelength. A reduction of the PL intensity points to a radiating character of the channel of the energy transfer from LC to porous Si. It is obvious that the emission of LC filled into porous Si, in turn, excites porous Si from the inside. But at the same time, local maxima of the PL intensity of porous Si near the wavelength of the LC luminescence maximum indicates on nonradiating character of the channel of energy transfer from the LC to porous Si. The mechanism of such the nonradiating pumping of the energy was presented in work [10]. It is known that the dominant energy carriers in porous Si are excitons whose properties depend on the state of pore walls owing to small diameters of Si threads. The properties
Fig. 4. The photoluminescence spectra of the sample from party 1: porous Si without (a) filler, (b) only liquid crystal H109, and (c) porous Si with the H109 filler.
5740
T.D. Ibragimov et al. / Optik 125 (2014) 5738–5740
intensity corresponding to porous Si increases in comparison with a photoluminescence of pure porous Si. Experimental results are explained by radiating and nonradiating energy transfer from liquid crystal to porous silicon. Acknowledgments The authors are very grateful to Prof. T.D. Dzafarov for samples of party 1. This work was done with the support of the Science and Technology Center in Ukraine (Grant no. 5352). References
Fig. 5. Dependence of the photoluminescence band intensity of the porous Si sample from party 1 with liquid crystalline filler on irradiating wavelength: (a) filler 5CB and (b) filler H109.
of excitons for low-dimensional systems are strongly subjected to the environment influence. When the dielectric constant of surrounding medium is higher than one of porous Si, the binding energy is small and excitons dissociate without luminescence. In the case, when the dielectric constant of the semiconductor is more than the dielectric constant of surrounding medium, their energy increases and there occur radiating disintegration. The LC dielectric permeability is equal to 10 at visible region while the dielectric permeability of the porous Si changes from 11 to 4 at reduction of the pore sizes. It is obvious that the presence of wide size distribution generates both the mechanism of the channel of energy transfer from LC to porous Si. 4. Conclusions The study of photoluminescence spectra of porous silicon filled with liquid crystals 5CB and H109 having more energy of emission than photoluminescence of porous silicon shows that there are photoluminescence bands corresponding to both porous silicon and liquid crystal. At this case, the photoluminescence band
[1] L.T. Canham, Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers, Appl. Phys. Lett. 57 (1990) 1046–1048. [2] R. Hérino, Pore size distribution in porous silicon, in: L. Canham (Ed.), Properties of Porous Silicon, Short Run Press Ltd., London, 1997, pp. 89–96. [3] P.Yu. Piryatinskii, V.A. Semenovich, Distinescence features of time-resolved photoluminescence of porous silicon coated with a layer of diamondlike carbon, Tech. Phys. 43 (1998) 423–426. [4] N.I. Kluyi, A.G. Rozhin, Yu.P. Piryatinskii, V.A. Semenovich, Photoluminescence of porous-silicon/diamondlike-carbon-film structures subjected to rapid thermal annealing, Tech. Phys. Lett. 25 (1999) 304–306. [5] V.A. Makara, N.I. Kluyi, A.G. Rozhin, V.G. Litovchenko, Yu.P. Piryatinski, O.B. Korneta, Porous silicon photoluminescence modification by surface treatments and impregnation of carbon based nanoclusters, Phys. Stat. Sol. (a) 197 (2003) 355–359. [6] H. Jung, G.N. Shin, van den Hoven, A. Polman, Origin of the 1.54 mm luminescence of erbium-implanted porous silicon, Appl. Phys. Lett. 66 (1995) 18–22. [7] Yu.P. Piryatinskii, O.V. Yaroshchuk, L.A. Dolgov, T.V. Bindna, D. Enke, Photoluminescence of liquid-crystal azo derivatives in nanopores, Opt. Spectrosc. 97 (2004) 537–542. [8] S.M. Weiss, J. Zhang, P.M. Fauchet, V.V. Seregin, J.L. Coffer, Tunable silicon-based light sources using erbium doped liquid crystals, Appl. Phys. Lett. 90 (2007), 031112-1–33112-3. [9] L.M. Zarova, M.I. Serbina, N.A. Kasian, Yu.A. Gurkalenko, V.M. Shershukov, L.N. Lisetski, Effects of liquid crystalline ordering on the luminescence spectra of model compounds with packing-sensitive molecular structure, Funct. Mater. 16 (2009) 470–474. [10] Yu.P. Piryatinski, L.O. Dolgov, O.V. Yaroshchuk, S. Lazarouk, Fluorescence of porous silicon filled with liquid crystal 5CB, Mol. Cryst. Liq. Cryst. 467 (2007) 195–202. [11] Q.L. Ma, R. Xiong, Y.M. Huang, Tunable photoluminescence of porous silicon by liquid crystal infiltration, J. Luminesc. 131 (2011) 2053–2057. [12] A.G. Cullis, L.T. Canham, P.D.J. Calcott, The structural and luminescence properties of porous silicon, J. Appl. Phys. 82 (1997) 909–965.