Magneto-optical Faraday rotation of semiconductor nanoparticles embedded in dielectric matrices Andriy I. Savchuk,* Ihor D. Stolyarchuk, Vitaliy V. Makoviy, and Oleksandr A. Savchuk Department of Physics of Semiconductors and Nanostructures, Chernivtsi National University, 2 Kotsyubynsky Str., 58012 Chernivtsi, Ukraine *Corresponding author: [email protected] Received 15 November 2013; accepted 9 December 2013; posted 24 December 2013 (Doc. ID 201394); published 3 February 2014
Faraday rotation has been studied for CdS, CdTe, and CdS:Mn semiconductor nanoparticles synthesized by colloidal chemistry methods. Additionally these materials were prepared in a form of semiconductor nanoparticles embedded in polyvinyl alcohol films. Transmission electron microscopy and atomic force microscopy analyses served as confirmation of nanocrystallinity and estimation of the average size of the nanoparticles. Spectral dependence of the Faraday rotation for the studied nanocrystals and nanocomposites is correlated with a blueshift of the absorption edge due to the confinement effect in zero-dimensional structures. Faraday rotation spectra and their temperature behavior in Mn-doped nanocrystals demonstrates peculiarities, which are associated with s, p-d exchange interaction between Mn2 ions and band carriers in diluted magnetic semiconductor nanostructures. © 2014 Optical Society of America OCIS codes: (160.4236) Nanomaterials; (160.6000) Semiconductor materials; (160.4760) Optical properties; (230.3810) Magneto-optic systems. http://dx.doi.org/10.1364/AO.53.000B22
1. Introduction
The rotation of the polarization of light after passing a medium in a longitudinal magnetic field is a magnetooptical phenomenon known as Faraday effect [1]. Different types of media in different aggregate states (gases, liquids, solid states) can exhibit Faraday rotation. In the past three decades, the study of the magneto-optical properties of II–VI semiconductor compounds and related diluted magnetic semiconductors (DMSs) with inserted magnetic dopants (Mn, Fe, Co, Ni) have become an attractive area of research due to the revealed unique phenomenon of so-called giant Faraday rotation [2–8]. In DMS type II1−x Mnx VI, the observed enhancement of the Faraday rotation is mainly associated with strong s, p-d exchange interaction between band carriers (s, p) and Mn local moments (d). Therefore, magneto-optical spectroscopy,
first of all, is useful for studying this kind of interaction in DMS materials. On the other hand, going from bulk crystals to nanocrystals opens new avenues for flexible “Faraday rotation engineering.” Magnetooptical properties of DMSs in different forms (bulk crystals, thin films, and nanocrystals) make these materials promising for optoelectronic applications, such as in current sensors, magnetic-field sensors, optical modulators, and optical isolators. In our previous works, different physical and chemical techniques were applied for preparation of II–VI semiconductor low-dimensional structures and their characterization was demonstrated [9–13]. In this study, we examine the wavelength dependence of the Faraday rotation in colloidal CdS, CdTe, and CdS:Mn nanocrystals and nanocomposites with polymer matrices. 2. Experiment
1559-128X/14/100B22-05$15.00/0 © 2014 Optical Society of America B22
APPLIED OPTICS / Vol. 53, No. 10 / 1 April 2014
Synthesis of CdS, CdTe, and CdS:Mn nanocrystals was performed in aqueous solution at room
temperature using colloidal chemistry methods. To synthesize CdS and CdS:Mn nanocrystals, chemically pure sodium sulfide (Na2 S), cadmium and manganese chlorides (CdCl2 and MnCl2 ) were used as the precursors. In the case of CdTe nanocrystals as a source of Te, H2 Te was also passed through the solution [14]. The role of stabilizing agent during preparation of sulfide nanocrystals was played by an aqueous solution of polyvinyl alcohol (PVA), whereas for telluride nanocrystals, thioglycol acid (TGA) was used. The films of the PVA∕CdS:Mn nanoparticle composites were formed by the adsorptive dessication method [12]. Transmission electron microscopy (TEM) was used to confirm the nanocrystallinity of the grown samples, to estimate the shape, and to determine the average size of nanoparticles. A Tecnai Osiris X-FEG S/TEM microscope was used for recording high-resolution (HR) TEM images. In order to examine the surface morphology of CdS:Mn∕PVA composite material, atomic force microscopy (AFM) analysis was performed. AFM analysis was performed at ambient conditions with commercial instruments (Nanotec Cervantes AFM). Faraday rotation measurements were carried out at room temperature. As the rotation angle is proportional to the distance traveled by the light, experimental measurements of this effect in lowdimensional semiconductor structures is a problematic task [15–18]. A home-designed setup for Faraday rotation measurements is shown in Fig. 1. An iodine– halogen lamp (L) is used as the radiation source and a grating monochromator (MD) with high resolution is used to yield monochromatic light in a range of 200–2200 nm. After passing through a focusing lens (L1) and a polarizer (P1, Roshon prism), monochromatic light becomes linearly polarized and then propagates through the studied sample placed in the electromagnet (EM) with magnetic field of up to a maximum induction of 5 T. Behind the electromagnet, the analyzer (P2, Wollaston prism) splits the beam into two, which then pass through
Fig. 1. Experimental setup for Faraday rotation measurements. L, radiation source; L1, L2, and L3, focusing lenses; MD, grating monochromator; P1, polarizer; EM, electromagnet; DCG, direct current generator; P2, analyzer; CH, chopper; PMT, photomultiplier tube; LA, lock in amplifier; PC, personal computer. The first inset shows two kinds of studied samples (colloidal nanocrystals and nanocomposite film); in the second inset, detail of the two beams passing through the chopper is shown.
a chopper, as is shown in the inset of Fig. 1. In such a manner these two beams are modulated by a phase shift of 180°. In the absence of a magnetic field, a balance was achieved by equalizing the intensities of the two light beams, and when the electromagnet was switched on, the resultant unbalanced signal could be recorded by using a photodetector system, which includes a photomultiplier tube (PMT), a lock-in amplifier (LA), and a computer (PC). The described setup allows registering a rotation angle of 10−4 rad. 3. Results and Discussion
A HR TEM image of colloidal CdS:Mn nanoparticles is shown in Fig. 2. For this kind of microscopic analysis, a drop of colloidal suspension is placed on a special carbon-coated copper grid. As can be seen, the shape of the nanoparticles is close to spherical with little agglomeration and good crystallinity, similar to the reported data [19]. An average particle diameter of 3 nm is determined. 2D and 3D AFM images (1.2 μm × 1.2 μm area) of surface morphology of the composite CdS:Mn∕PVA film are shown in Fig. 3. Imaging was done in the noncontact dynamic mode at ambient conditions with humidity of 30%–40%. The image shows the regular fluctuation profile of the studied film due to the dispersed CdS:Mn nanoparticles. The maximum height fluctuation is about 20 nm, and the root-mean-square surface roughness is about 4 nm. The nanoparticles are hill shaped and their diameter cannot be read from the AFM images because we have to take into account the shape of the AFM probe. Faraday rotation as a function of photon energy for colloidal CdTe nanocrystals is shown in Fig. 4. For comparison, spectral dependence of the Faraday rotation for empty quartz cuvette is presented as well. The observed increase of the Faraday rotation angle in the spectral region of 2.2–2.8 eV can be
Fig. 2. HR TEM image of a typical sample of colloidal CdS:Mn nanocrystals. 1 April 2014 / Vol. 53, No. 10 / APPLIED OPTICS
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Fig. 3. 2D and 3D AFM images (1.2 μm × 1.2 μm) of CdS:Mn∕PVA nanocomposite film.
interpreted in the framework of an appropriate increase of absorption coefficient at the absorption edge. However, the energy position of this absorption edge for CdTe semiconductor nanoparticles is significantly shifted toward higher photon energy as compared with bulk crystals due to the confinement effect. Moreover, in the absorption spectrum of the colloidal CdTe nanoparticles, we have observed an exciton absorption band at 2.34 eV, which is also blueshifted. Similar results on the spectral dependence of the Faraday rotation for colloidal CdS:Mn (3 mol. %) nanoparticles are shown in Fig. 5. The Faraday rotation angle θF normalized by field B and sample thickness d is known as the Verdet constant: V
θF : Bd
(1)
The Verdet constant of the colloidal solution with CdTe nanocrystals is found to vary in the range of 3.0–7.85 deg ∕T m at photon energies of 1.6–2.8 eV. 12
SiO2
12
nc-CdTe B=1T
8
SiO2
10
Faraday rotation (deg)
Faraday rotation (deg)
10
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8
6
4
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Fig. 4. Faraday rotation as a function of photon energy for an empty quartz container and with colloidal CdTe nanocrystals. Total thickness of the two container walls is 3 mm, the inner thickness of the container is 10 mm, and the induction of the magnetic field is 1 T. B24
The value of the Verdet constant related to semiconductor material only is unknown because of the absence exact data on the Faraday rotation of a basic solution and the concentration of semiconductor phase in colloidal samples. Recently, oscillatory behavior of the Faraday rotation as a function of magnetic field has been revealed for CdS, CdSe, and CdTe quantum dots embedded in a glass matrix [20,21]. However, our experiments with colloidal CdTe nanocrystals do not confirm this kind of oscillation in the magnetic field dependence of the Faraday rotation up to B 3T. In order to elucidate the influence of the amount of CdCl2 , MnCl2 , and Na2 S precursors on the growth kinetics, the Faraday rotation spectra of CdS:Mn nanocrystals have been recorded in situ during the growth process in PVA solution. The decrease of the Faraday rotation is clearly observed with increased amounts of CdCl2, MnCl2 , and Na2 S precursors. Such a decrease without a doubt is associated with the increase of the content of the semiconductor phase into the solution and the presence of Mn impurity inside the CdS:Mn nanoparticles. It is well known that, for DMS materials, the large exchange interaction between the band states of electrons
APPLIED OPTICS / Vol. 53, No. 10 / 1 April 2014
2 1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
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3.2
Photon energy (eV)
Fig. 5. Faraday rotation as a function of photon energy for an empty quartz container and with colloidal CdS:Mn nanocrystals.
0.2 0.1
Faraday rotation (deg)
(holes) and the localized d electrons of magnetic ions leads to strong enhancement of Faraday rotation [2–6]. In the studied DMS nanocrystals, competition between diamagnetic and paramagnetic states should be exhibited, as well. In fact, the observed decrease of Faraday rotation and changes in its spectral dependence are associated with a positive and a negative part due to pure Zeeman and s, p-d exchange interaction contributions, respectively. According to the microscopic model of the Faraday rotation in bulk DMS [7,8] the Verdet constant as a function of photon energy hν can be expressed as
0.0 -0.1 4.2 K
-0.3 -0.4
-0.6
1.6
Faraday rotation (deg/mm)
2.4
(2)
where Z, C, and Y are fitting parameters. The f hν function has a positive sign, whereas the ghν function is negative. As a result, competition between these two contributions with opposite signs leads to the observed decrease of Faraday rotation in the CdS:Mn nanocrystals. Obviously, a similar explanation is applicable also in the cases of CdS/PVA and CdS:Mn∕PVA composite films. Figure 6 shows a significant decrease of Faraday rotation in the Mn-doped nanocrystals as compared with undoped CdS nanocrystals. Once again, this is because of the negative sign of the contribution from CdS:Mn nanoparticles. In addition, such a sign reversal can be seen immediately in low-temperature Faraday rotation spectra, which are shown in Fig. 7. The observed anomalies in the spectral dependence of the Faraday rotation are common to the entire DMS class of materials [6]. They indicate that the Faraday effect in DMS is determined at least by two mechanisms whose relative contributions can vary strongly (both in magnitude and sign) with temperature and Mn content. On the other hand, the not so large absolute value of the Verdet constant for the CdS:Mn∕PVA nanocomposite should be noted. The calculated value of the Verdet constant in this sample is 0.016 deg ∕mT cm at
1.4
np CdS:Mn/PVA B = 0.035 T d = 150 µm
-0.5
2.3
Vhν Z · f hν C · Y · ghν;
200 K
-0.2
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
Photon energy (eV)
Fig. 7. Faraday rotation as a function of photon energy for CdS:Mn∕PVA nanocomposite film at different temperatures.
2.61 eV, which is the same order of magnitude for the bulk diamagnetic CdS crystals. Our suggestion is that this is associated with low magnetization of CdS:Mn nanocrystals at room temperature, small content of Mn inside nanocrystallites, and probably surface distribution of Mn2 ions, which create a shell around the CdS core. 4. Conclusion
In summary, Faraday rotation of CdS, CdTe, and CdS:Mn nanocrystals and nanocomposites as a function of photon energy has been comparatively investigated. It is shown that the Faraday rotation spectra of the studied nanocrystals and nanocomposites in the spectral region near the absorption edge are blueshifted due to exhibition of the confinement effect. Compared with undoped nanocrystals, in Mn-doped DMS nanomaterials, the Faraday rotation exhibits a negative sign of rotation angle, which is associated with an appropriate contribution from the s, p-d exchange interaction. Significant enhancement of the absolute value of the Verdet constant type of giant Faraday rotation in bulk DMS was not found because of the small content of Mn ions inside nanoparticles and their surface distribution. Funding support from the Ministry of Education and Science of Ukraine (contract no. 12-800) is acknowledged. The authors would like to acknowledge Mrs. E. Gomez from Nanotec Electronica for help with AFM measurements.
nc CdS/PVA nc CdS:Mn/PVA
1.2 1.0 0.8
References
0.6 0.4 0.2 1.8
1.9
2.0
2.1 2.2 2.3 2.4 Photon energy (eV)
2.5
2.6
2.7
Fig. 6. Faraday rotation as a function of photon energy for CdS/ PVA and CdS:Mn∕PVA nanocomposite films at room temperature with induction of the magnetic field of 1 T. The thickness of the first film is 140 μm and that of the second one is 150 μm.
1. M. Faraday, “On the magnetization of light and the illumination of magnetic lines of force,” Phil. Trans. R. Soc. London 136, 104–123 (1846). 2. A. V. Komarov, S. M. Ryabchenko, O. V. Terletskij, I. I. Zheru, and R. D. Ivanchuk, “Magneto-optical studies and double optic-magnetic resonance of exciton band in CdTe:Mn2+,” Sov. Phys. JETP 46, 318–326 (1977). 3. J. A. Gaj, R. R. Galazka, and M. Nawrocki, “Giant exciton Faraday rotation in Cd1-xMnxTe mixed crystals,” Solid State Commun. 25, 193–195 (1978). 4. A. V. Komarov, A. V. Los, S. M. Ryabchenko, and S. M. Romanenko, “Diluted magnetic semiconductor effects in 1 April 2014 / Vol. 53, No. 10 / APPLIED OPTICS
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Mn-implanted silicon carbide,” J. Appl. Phys. 109, 083936 (2011). Y. K. Furdyna, “Diluted magnetic semiconductors,” J. Appl. Phys. 64, R29–R64 (1988). P. I. Nikitin and A. I. Savchuk, “The Faraday effect in semimagnetic semiconductors,” Sov. Phys. Usp. 33, 974–989 (1990). C. Buss, S. Huggonard-Bruyere, R. Frey, and C. Flytzanis, “Theory of Faraday rotation in semimagnetic semiconductors,” Solid State Commun. 92, 929–933 (1994). S. Huggonard-Bruyere, C. Buss, F. Vouilloz, R. Frey, and C. Flytzanis, “Faraday-rotation spectra of semimagnetic semiconductors,” Phys. Rev. B 50, 2200–2207 (1994). A. I. Savchuk, I. D. Stolyarchuk, S. V. Medynskiy, V. I. Fediv, Ye. O. Kandyba, A. Perrone, M. I. DeGiorgi, and P. I. Nikitin, “Technological aspects of fabrication of semimagnetic semiconductor nanocrystals,” Mater. Sci. Eng., C 15, 79–81 (2001). A. I. Savchuk, V. I. Fediv, I. D. Stolyarchuk, T. A. Savchuk, and A. Perrone, “Magneto-optical Faraday effect in II-VI based semimagnetic semiconductor nanocrystals,” Phys. Status Solidi C 3, 1160–1163 (2006). A. I. Savchuk, V. I. Fediv, A. G. Voloshchuk, T. A. Savchuk, Yu. Yu. Bacherikov, and A. Perrone, “New approach to synthesis of semimagnetic semiconductor nanoparticles,” Mater. Sci. Eng., C 26, 809–812 (2006). A. I. Savchuk, G. Yu. Rudko, V. I. Fediv, A. G. Voloshchuk, E. G. Gule, S. A. Ivanchak, and V. V. Makoviy, “Evolution of CdS:Mn nanoparticle properties caused by pH of colloid solution and ultrasound irradiation,” Phys. Status Solidi C 7, 1510–1512 (2010). V. I. Fediv, A. I. Savchuk, V. M. Frasunyak, V. V. Makoviy, and O. A. Savchuk, “Magnetic and magnetooptical properties of
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CdS:Mn quantum dots in PVA matrix,” J. Phys. Conf. Ser. 244, 012084 (2010). D. V. Korbutyak, S. M. Kalytchuk, and I. I. Geru, “Colloidal CdTe and CdSe quantum dots: technology of preparing and optical properties,” J. Nanoelectron. Optoelectron. 4, 1–6 (2009). D. Pagliero, Y. Li, S. Fisher, and C. A. Meriles, “Approach to high-frequency, cavity-enhanced Faraday rotation in fluids,” Appl. Opt. 50, 648–654 (2011). A. Arnaud, F. Silveira, E. M. Frins, A. Dubra, C. D. Perciante, and J. A. Ferrari, “Precision synchronous polarimeter with linear response for the measurement of small rotation angles,” Appl. Opt. 39, 2601–2604 (2000). M.-H. Kim, V. Kurz, G. Achas, C. T. Ellis, and J. Cerne, “Measurements of the infrared complex Faraday angle in semiconductors and isolators,” J. Opt. Soc. Am. B 28, 199–207 (2011). I. Crassee, J. Levallois, A. L. Walter, M. Ostler, A. Bostwick, E. Rotenberg, T. Seyller, D. van der Marel, and A. B. Kuzmenko, “Giant Faraday rotation in single- and multilayer graphene,” Nat. Phys. 7, 48–51 (2011). S. K. Mishra, R. K. Srivastava, S. G. Prakash, R. S. Yadav, and A. G. Panday, “Structural, optical and photoconductivity characteristics of manganese doped cadmium sulfide nanoparticles synthesized by co-precipitation method,” J. Alloys Compd. 513, 118–124 (2012). J. H. Kratzer and J. Schroeder, “Magnetooptic properties of semiconductor quantum dots in glass composites,” J. NonCryst. Solids 349, 299–308 (2004). J. Schroeder and J. H. Kratzer, “Magneto-optic studies of rare earth containing sodium silicate glasses and semiconductor quantum dots in glass composites: nonlinear effects,” J. Non-Cryst. Solids 358, 3517–3523 (2012).
Faraday rotation has been studied for CdS, CdTe, and CdS:Mn semiconductor nanoparticles synthesized by colloidal chemistry methods. Additionally these materials were prepared in a form of semiconductor nanoparticles embedded in polyvinyl alcohol films. Transmission electron microscopy and atomic force microscopy analyses served as confirmation of nanocrystallinity and estimation of the average size of the nanoparticles. Spectral dependence of the Faraday rotation for the studied nanocrystals and nanocomposites is correlated with a blueshift of the absorption edge due to the confinement effect in zero-dimensional structures. Faraday rotation spectra and their temperature behavior in Mn-doped nanocrystals demonstrates peculiarities, which are associated with s, p-d exchange interaction between Mn2 ions and band carriers in diluted magnetic semiconductor nanostructures. © 2014 Optical Society of America OCIS codes: (160.4236) Nanomaterials; (160.6000) Semiconductor materials; (160.4760) Optical properties; (230.3810) Magneto-optic systems. http://dx.doi.org/10.1364/AO.53.000B22
1. Introduction
The rotation of the polarization of light after passing a medium in a longitudinal magnetic field is a magnetooptical phenomenon known as Faraday effect [1]. Different types of media in different aggregate states (gases, liquids, solid states) can exhibit Faraday rotation. In the past three decades, the study of the magneto-optical properties of II–VI semiconductor compounds and related diluted magnetic semiconductors (DMSs) with inserted magnetic dopants (Mn, Fe, Co, Ni) have become an attractive area of research due to the revealed unique phenomenon of so-called giant Faraday rotation [2–8]. In DMS type II1−x Mnx VI, the observed enhancement of the Faraday rotation is mainly associated with strong s, p-d exchange interaction between band carriers (s, p) and Mn local moments (d). Therefore, magneto-optical spectroscopy,
first of all, is useful for studying this kind of interaction in DMS materials. On the other hand, going from bulk crystals to nanocrystals opens new avenues for flexible “Faraday rotation engineering.” Magnetooptical properties of DMSs in different forms (bulk crystals, thin films, and nanocrystals) make these materials promising for optoelectronic applications, such as in current sensors, magnetic-field sensors, optical modulators, and optical isolators. In our previous works, different physical and chemical techniques were applied for preparation of II–VI semiconductor low-dimensional structures and their characterization was demonstrated [9–13]. In this study, we examine the wavelength dependence of the Faraday rotation in colloidal CdS, CdTe, and CdS:Mn nanocrystals and nanocomposites with polymer matrices. 2. Experiment
1559-128X/14/100B22-05$15.00/0 © 2014 Optical Society of America B22
APPLIED OPTICS / Vol. 53, No. 10 / 1 April 2014
Synthesis of CdS, CdTe, and CdS:Mn nanocrystals was performed in aqueous solution at room
temperature using colloidal chemistry methods. To synthesize CdS and CdS:Mn nanocrystals, chemically pure sodium sulfide (Na2 S), cadmium and manganese chlorides (CdCl2 and MnCl2 ) were used as the precursors. In the case of CdTe nanocrystals as a source of Te, H2 Te was also passed through the solution [14]. The role of stabilizing agent during preparation of sulfide nanocrystals was played by an aqueous solution of polyvinyl alcohol (PVA), whereas for telluride nanocrystals, thioglycol acid (TGA) was used. The films of the PVA∕CdS:Mn nanoparticle composites were formed by the adsorptive dessication method [12]. Transmission electron microscopy (TEM) was used to confirm the nanocrystallinity of the grown samples, to estimate the shape, and to determine the average size of nanoparticles. A Tecnai Osiris X-FEG S/TEM microscope was used for recording high-resolution (HR) TEM images. In order to examine the surface morphology of CdS:Mn∕PVA composite material, atomic force microscopy (AFM) analysis was performed. AFM analysis was performed at ambient conditions with commercial instruments (Nanotec Cervantes AFM). Faraday rotation measurements were carried out at room temperature. As the rotation angle is proportional to the distance traveled by the light, experimental measurements of this effect in lowdimensional semiconductor structures is a problematic task [15–18]. A home-designed setup for Faraday rotation measurements is shown in Fig. 1. An iodine– halogen lamp (L) is used as the radiation source and a grating monochromator (MD) with high resolution is used to yield monochromatic light in a range of 200–2200 nm. After passing through a focusing lens (L1) and a polarizer (P1, Roshon prism), monochromatic light becomes linearly polarized and then propagates through the studied sample placed in the electromagnet (EM) with magnetic field of up to a maximum induction of 5 T. Behind the electromagnet, the analyzer (P2, Wollaston prism) splits the beam into two, which then pass through
Fig. 1. Experimental setup for Faraday rotation measurements. L, radiation source; L1, L2, and L3, focusing lenses; MD, grating monochromator; P1, polarizer; EM, electromagnet; DCG, direct current generator; P2, analyzer; CH, chopper; PMT, photomultiplier tube; LA, lock in amplifier; PC, personal computer. The first inset shows two kinds of studied samples (colloidal nanocrystals and nanocomposite film); in the second inset, detail of the two beams passing through the chopper is shown.
a chopper, as is shown in the inset of Fig. 1. In such a manner these two beams are modulated by a phase shift of 180°. In the absence of a magnetic field, a balance was achieved by equalizing the intensities of the two light beams, and when the electromagnet was switched on, the resultant unbalanced signal could be recorded by using a photodetector system, which includes a photomultiplier tube (PMT), a lock-in amplifier (LA), and a computer (PC). The described setup allows registering a rotation angle of 10−4 rad. 3. Results and Discussion
A HR TEM image of colloidal CdS:Mn nanoparticles is shown in Fig. 2. For this kind of microscopic analysis, a drop of colloidal suspension is placed on a special carbon-coated copper grid. As can be seen, the shape of the nanoparticles is close to spherical with little agglomeration and good crystallinity, similar to the reported data [19]. An average particle diameter of 3 nm is determined. 2D and 3D AFM images (1.2 μm × 1.2 μm area) of surface morphology of the composite CdS:Mn∕PVA film are shown in Fig. 3. Imaging was done in the noncontact dynamic mode at ambient conditions with humidity of 30%–40%. The image shows the regular fluctuation profile of the studied film due to the dispersed CdS:Mn nanoparticles. The maximum height fluctuation is about 20 nm, and the root-mean-square surface roughness is about 4 nm. The nanoparticles are hill shaped and their diameter cannot be read from the AFM images because we have to take into account the shape of the AFM probe. Faraday rotation as a function of photon energy for colloidal CdTe nanocrystals is shown in Fig. 4. For comparison, spectral dependence of the Faraday rotation for empty quartz cuvette is presented as well. The observed increase of the Faraday rotation angle in the spectral region of 2.2–2.8 eV can be
Fig. 2. HR TEM image of a typical sample of colloidal CdS:Mn nanocrystals. 1 April 2014 / Vol. 53, No. 10 / APPLIED OPTICS
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Fig. 3. 2D and 3D AFM images (1.2 μm × 1.2 μm) of CdS:Mn∕PVA nanocomposite film.
interpreted in the framework of an appropriate increase of absorption coefficient at the absorption edge. However, the energy position of this absorption edge for CdTe semiconductor nanoparticles is significantly shifted toward higher photon energy as compared with bulk crystals due to the confinement effect. Moreover, in the absorption spectrum of the colloidal CdTe nanoparticles, we have observed an exciton absorption band at 2.34 eV, which is also blueshifted. Similar results on the spectral dependence of the Faraday rotation for colloidal CdS:Mn (3 mol. %) nanoparticles are shown in Fig. 5. The Faraday rotation angle θF normalized by field B and sample thickness d is known as the Verdet constant: V
θF : Bd
(1)
The Verdet constant of the colloidal solution with CdTe nanocrystals is found to vary in the range of 3.0–7.85 deg ∕T m at photon energies of 1.6–2.8 eV. 12
SiO2
12
nc-CdTe B=1T
8
SiO2
10
Faraday rotation (deg)
Faraday rotation (deg)
10
6 4 2 0 1.4
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2.2
2.4
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8
6
4
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Fig. 4. Faraday rotation as a function of photon energy for an empty quartz container and with colloidal CdTe nanocrystals. Total thickness of the two container walls is 3 mm, the inner thickness of the container is 10 mm, and the induction of the magnetic field is 1 T. B24
The value of the Verdet constant related to semiconductor material only is unknown because of the absence exact data on the Faraday rotation of a basic solution and the concentration of semiconductor phase in colloidal samples. Recently, oscillatory behavior of the Faraday rotation as a function of magnetic field has been revealed for CdS, CdSe, and CdTe quantum dots embedded in a glass matrix [20,21]. However, our experiments with colloidal CdTe nanocrystals do not confirm this kind of oscillation in the magnetic field dependence of the Faraday rotation up to B 3T. In order to elucidate the influence of the amount of CdCl2 , MnCl2 , and Na2 S precursors on the growth kinetics, the Faraday rotation spectra of CdS:Mn nanocrystals have been recorded in situ during the growth process in PVA solution. The decrease of the Faraday rotation is clearly observed with increased amounts of CdCl2, MnCl2 , and Na2 S precursors. Such a decrease without a doubt is associated with the increase of the content of the semiconductor phase into the solution and the presence of Mn impurity inside the CdS:Mn nanoparticles. It is well known that, for DMS materials, the large exchange interaction between the band states of electrons
APPLIED OPTICS / Vol. 53, No. 10 / 1 April 2014
2 1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
Photon energy (eV)
Fig. 5. Faraday rotation as a function of photon energy for an empty quartz container and with colloidal CdS:Mn nanocrystals.
0.2 0.1
Faraday rotation (deg)
(holes) and the localized d electrons of magnetic ions leads to strong enhancement of Faraday rotation [2–6]. In the studied DMS nanocrystals, competition between diamagnetic and paramagnetic states should be exhibited, as well. In fact, the observed decrease of Faraday rotation and changes in its spectral dependence are associated with a positive and a negative part due to pure Zeeman and s, p-d exchange interaction contributions, respectively. According to the microscopic model of the Faraday rotation in bulk DMS [7,8] the Verdet constant as a function of photon energy hν can be expressed as
0.0 -0.1 4.2 K
-0.3 -0.4
-0.6
1.6
Faraday rotation (deg/mm)
2.4
(2)
where Z, C, and Y are fitting parameters. The f hν function has a positive sign, whereas the ghν function is negative. As a result, competition between these two contributions with opposite signs leads to the observed decrease of Faraday rotation in the CdS:Mn nanocrystals. Obviously, a similar explanation is applicable also in the cases of CdS/PVA and CdS:Mn∕PVA composite films. Figure 6 shows a significant decrease of Faraday rotation in the Mn-doped nanocrystals as compared with undoped CdS nanocrystals. Once again, this is because of the negative sign of the contribution from CdS:Mn nanoparticles. In addition, such a sign reversal can be seen immediately in low-temperature Faraday rotation spectra, which are shown in Fig. 7. The observed anomalies in the spectral dependence of the Faraday rotation are common to the entire DMS class of materials [6]. They indicate that the Faraday effect in DMS is determined at least by two mechanisms whose relative contributions can vary strongly (both in magnitude and sign) with temperature and Mn content. On the other hand, the not so large absolute value of the Verdet constant for the CdS:Mn∕PVA nanocomposite should be noted. The calculated value of the Verdet constant in this sample is 0.016 deg ∕mT cm at
1.4
np CdS:Mn/PVA B = 0.035 T d = 150 µm
-0.5
2.3
Vhν Z · f hν C · Y · ghν;
200 K
-0.2
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
Photon energy (eV)
Fig. 7. Faraday rotation as a function of photon energy for CdS:Mn∕PVA nanocomposite film at different temperatures.
2.61 eV, which is the same order of magnitude for the bulk diamagnetic CdS crystals. Our suggestion is that this is associated with low magnetization of CdS:Mn nanocrystals at room temperature, small content of Mn inside nanocrystallites, and probably surface distribution of Mn2 ions, which create a shell around the CdS core. 4. Conclusion
In summary, Faraday rotation of CdS, CdTe, and CdS:Mn nanocrystals and nanocomposites as a function of photon energy has been comparatively investigated. It is shown that the Faraday rotation spectra of the studied nanocrystals and nanocomposites in the spectral region near the absorption edge are blueshifted due to exhibition of the confinement effect. Compared with undoped nanocrystals, in Mn-doped DMS nanomaterials, the Faraday rotation exhibits a negative sign of rotation angle, which is associated with an appropriate contribution from the s, p-d exchange interaction. Significant enhancement of the absolute value of the Verdet constant type of giant Faraday rotation in bulk DMS was not found because of the small content of Mn ions inside nanoparticles and their surface distribution. Funding support from the Ministry of Education and Science of Ukraine (contract no. 12-800) is acknowledged. The authors would like to acknowledge Mrs. E. Gomez from Nanotec Electronica for help with AFM measurements.
nc CdS/PVA nc CdS:Mn/PVA
1.2 1.0 0.8
References
0.6 0.4 0.2 1.8
1.9
2.0
2.1 2.2 2.3 2.4 Photon energy (eV)
2.5
2.6
2.7
Fig. 6. Faraday rotation as a function of photon energy for CdS/ PVA and CdS:Mn∕PVA nanocomposite films at room temperature with induction of the magnetic field of 1 T. The thickness of the first film is 140 μm and that of the second one is 150 μm.
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