Structural and optical properties of chemically synthesized monodispersed CdCr2S4 films

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Journal of Physics and Chemistry of Solids 69 (2008) 1802–1807 www.elsevier.com/locate/jpcs

Structural and optical properties of chemically synthesized monodispersed CdCr2S4 films V.V. Todkara, R.S. Manea, C.D. Lokhandea, Habib M. Pathanb, Oh-Shim Joob, Hoeil Chunga, Moon-Young Yoona, Sung Hwan Hana, a Department of Chemistry, Hanyang University, Sungdong-Ku, Haengdang-dong 17, Seoul 133-791, Republic of Korea Clean Energy Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 130-650, Republic of Korea

b

Received 6 December 2006; received in revised form 28 November 2007; accepted 29 December 2007

Abstract Alkaline chemical synthesis of amorphous CdCr2S4 (CCS) thin films of different thicknesses using cadmium chloride, chromic acid, disodium salt of ethylenediaminetetra acetic acid and thiourea precursors is reported, and the structural and surface morphological properties of CCS using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) techniques are discussed. Films of aggregated grains with some void spaces are obtained. Change in band gap energy and electrical resistivity of CCS films are discussed as a function of film thickness. n-type conductivity is confirmed from the sign of thermally generated voltage across the cold and hot junctions. r 2008 Elsevier Ltd. All rights reserved. Keywords: A. Amorphous materials; A. Thin films; B. Chemical growth; D. Optical properties

1. Introduction As the trend of fabricating smaller devices continued toward nanoscale technology, new effects related to the small size were realized in producing novel devices, like non-volatile memory devices in magneto-electronics, magneto-optical and opto-electronics [1]. The properties of nanocrystalline materials could be altered by changing the crystallite size or thickness of the film [2]. Ferromagnetic chalcogenide spinels (FCS), e.g., CdCr2S4 (CCS), CdCr2Se4, HgCr2Se4 and CuCr2Se4 have acquired scientific interest and importance since the sixties due to their outstanding properties like giant magnetoresistance, photoferromagnetic effects, red shift of the optical absorption edge, giant faraday rotation and others, providing the basis for the design of devices with various applications [3]. During the last few years, many researchers have studied structural, electrical, and magnetic properties [4–6] of said materials. Lade et al. [7] have electro-synthesized CCS thin Corresponding author. Tel.: +82 2 2292 5212; fax: +82 2 2290 0762.

E-mail address: [email protected] (S.H. Han). 0022-3697/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.12.017

films and studied structural, optical, and electrical properties. The band gap energy of 2.6 eV for CCS is reported by Oguchi et al. [8]. Band structure of CCS film consists of relatively narrow valence bands, fairly wide conduction bands, and very narrow d bands. Electric insulating (1015 O cm) nature of CCS films is earlier [9]. Single crystals and pellets of CCS have been prepared using hightemperature methods, such as vacuum deposition, sputtering, etc., and the structural, magnetic, and electrical properties have been studied [9–12]. The effect of structural defects and crystal growth on the physical properties of CCS has been reported [13]. We still believe there is much empty space where the amorphous FCS films have not been extensively studied. In this work, chemical deposition method was applied for the preparation of amorphous CCS thin films of different thicknesses and tuned surface morphological, optical, and electrical, properties are studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), atomic force microscopy (AFM), UV–vis spectrophotometery, electrical resistivity

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techniques, respectively. Type of conductivity is confirmed from the sign of thermally generated voltage keeping one junction at hot place and another at cold.

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end of film of definite area at cold and other at hot junction, type of conductivity was evaluated. 3. Results and discussion

2. Experimental details 3.1. Reaction mechanism and growth process 2.1. Preparation of CCS films The 0.25 M CdCl2, CrO3 and (NH2)2CS solution was used. For the deposition of CCS thin films, the reaction bath was prepared in 100 mL beaker, taking 10 mL of CdCl2 as a starting solution. Ten milliliters of ammonium hydroxide solution was added to form a complex of cadmium tetraammonium ion followed by 20 mL of chromic acid solution. In the same bath, required amount of disodium salt of ethylenediaminetetraacetic (Na2EDTA) acid was added as a complexing agent and finally, the pH was adjusted to 1070.2 by the addition of 25% excess liquid ammonia. The micro-slides of the dimension 25 mm  75 mm  2 mm were boiled in 1 M chromic acid for 2 h washed with detergent and finally, ultrasonically cleaned with double distilled water before deposition of the film. Cleaned microslides were mounted on a specially designed substrate holder and kept rotating in the reaction mixture at a constant speed of 75 rpm. Thiourea solution (40 mL) was added dropwise by using special arrangement and the reaction was carried out at 343 K with automatically controlled digital water bath. The substrates were taken out from the reaction bath after successive deposition time period (30, 60 and 90 min) and washed with distilled water, dried in air and preserved in air-tight container. The films obtained were highly reflecting, pink-yellow in color and well adherent to the substrate. The precipitated powder collected from the reaction bath washed thoroughly with distilled water several times and dried until a constant weight was obtained. 2.2. Characterization of thin films

Chemical bath deposition (CBD) of CCS thin film involves the controlled dissociation of arrested metal ions using Na2EDTA as a complexing agent. The deposition of CCS thin films is basically based on the slow release of Cd2+, Cr3+ and S2 ions in the solution. When the ratio (IP/SP41; SP—solubility product and IP—ionic product) of ionic product to solubility products is greater than one, the ions, viz., Cd2+, Cr3+ and S2 combine on the substrate and in the solution to form nuclei, which grow with time to form film and precipitate, respectively. Initial rapid growth of films is due to relatively high concentrations Cd2+, Cr3+ and S2 ions. As more and more CCS is formed, the solution becomes depleted of ions, resulting in a lower rate of deposition. The rate of deposition becomes zero, resulting in a terminal thickness when IP/SPX1. The kinetics of film formation can be understood from the following proposed chemical reaction. Thiourea undergoes hydrolysis in alkaline medium and releases sulfide ions [14] as

(1)

H2 O þ HS ! H3 Oþ þ S2 :

(2)

Ammonium hydrolyzes in water to give OH according to the equation



NH3 þ H2 O ! NH4 þ þ OH ;

ions (3)

5

For measuring film thickness, ellipsometry was referred after standardizing with Si single crystal using 630 nm monochromatic light source. The XRD study of CCS thin films of different thicknesses was carried out in the range of the scanning angle 10–801 with Cu Ka radiation (l ¼ 1.5406 A˚) using Philips PW-1710 diffractometer operated under accelerating voltage of 40 kV, and emission current of 100 mA. The surface morphology of CCS films on the glass substrate was studied by SEM, AFM, and TEM, respectively. TEM was performed at an accelerating voltage of 300 kV. The copper mesh was used as the substrate. Optical spectra were measured using UV–vis spectrometer (Hitachi U-2000) in the wavelength range 350–850 nm. Semiconducting behavior was confirmed from the electrical resistivity measurement employing dc twoprobe technique in the temperature range 300–500 K. From the thermally generated voltage, obtained by keeping one

with hydrolysis constant, Khyd ¼ 1.8  10 . During the addition of ammonia to Cd-salt solution, Cd(OH)2 starts precipitating when solubility product of Cd(OH)2 is exceeded, i.e. Cd2þ þ 2OH ! CdðOHÞ2 ;

(4) 2+

 2

with solubility product, Ksp ¼ [Cd ][OH ] ¼ 2.2  1014. The Cd(OH)2 precipitate dissolves in excess ammonia solution to form a stable cadmium tetra-ammonium complex, [Cd(NH3)4]2+: Cd2þ þ 4NH3 ! ½CdðNH3 Þ4 2þ :

(5)

While in case of chromium, the Cr–Na2EDTA complex having more stability constant (24.0) slowly releases Cr3+ ions: 2Cr3þ þ 2½ðNa2 EDTAÞ4  ! 2½Cr2Na2 EDTA :

(6)

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The overall reaction mechanism for CCS thin film formation is proposed in our earlier work [5] as 2½Cr2Na2 EDTA þ CdðNH3 Þ4 2þ þ 4ðNH2 Þ2 CS þ 8OH ! CdCr2 S4 þ 4CH2 N2 þ 8H2 O þ 4NH3 þ 2½ðNa2 EDTAÞ4 :

(7)

This mechanism is nearly similar to that of reported by Nair et al. [15] for the formation of Sb2S3 thin film by using thiourea from alkaline bath. The justification for metal–Na2EDTA complex can be given as stability constants of metal–Na2 EDTA complexes reported [14] as Cd2+ ¼ 16.6 and Cr3+ ¼ 24.0 [15]. However, the solution also contains ammonia other than Na2EDTA, which can be complexed with the metal ion (Cd2+), and then the whole of this ion uncombined with Na2EDTA may not longer be present as the simple hydrated ion. Thus, in practice, the stability of Cd–Na2 EDTA complex may be altered by the presence of ammonia as other complexing agent [15]. In CBD, the deposition of thin film is reported to proceed through a nucleation or incubation period followed by a growth phase and a terminal phase [17,18]. Here, brown color of CCS film supported the formation single phase rather than either CdS (yellow) or Cr2S3 (red) phases. In the present case, the first indication of film formation was observed after 30 min, as shown in Fig. 1, which gives the growth nature of CCS films with time period. From the figure, it is clearly seen that the film thickness of 224 nm can be obtained in 90 min deposition time. It corresponds to the setting of maximum in this case, as film thickness is nearly constant after this period of deposition. This is due to the decrease in number of metal as well as sulfide ions in the solution. 3.2. Structural analysis Fig. 2 shows the XRD patterns of CCS thin film of thickness 224 nm (lower) and that of precipitated powder

220 200 Thickness (nm)

(upper), respectively. Here, it should be noted that for all the selected thicknesses (110, 168, and 224 nm), nature of X-ray pattern was almost same, except small change in relative intensities. As-deposited CCS film were amorphous, whereas the precipitated powder collected from the bath was microcrystalline in nature with preferred orientation along (2 2 0), (4 0 0), (3 3 1), (4 2 2), (5 1 1), (4 4 0), (5 3 3) planes of CCS. An observed broad hump for film sample in between 15 and 401 is because of the glass substrate. The calculated ‘d’ values from XRD patterns of the precipitated powder obtained from the bath showed good agreement with the corresponding ‘d’ values of standard JCPDF data for CCS [19]; thus, this confirms the formation of CCS. In next section, this observation is also supported by SAED pattern. Here, for the phase confirmation of CCS films, electron diffraction X-ray electron analysis was carried out. Film showed 1:2:4 chemical stoichiometric ratio for three said elementals (for structural and chemical analysis details see Ref. [20]). 3.3. Surface morphology

240

180 160 140 120 100

Fig. 2. XRD patterns of as-deposited CCS thin film of 224 nm thickness and that of sedimented dried powder.

40

60 80 Deposition time (min)

100

Fig. 1. Variation of film thickness measured by ellipsometry as a function of deposition time.

The CCS films deposited onto glass substrates with different thicknesses were used for surface morphological studies. Fig. 3(a–c) shows the SEM micrographs of different thicknesses at 50k magnification. Literature reveals that with increase in deposition time, there is a substantial granular growth of films with respect to grain size [21]. The CCS SEM images of different thicknesses exhibits two typical morphologies: distinct monodispersed spherical grains (Fig. 3(a)) and broken monodispersed spheres with well-defined spongy elongated grains (Fig. 3(c)). Unavoidable, small void spaces are also observed in every SEM image. At close observation in SEM image of samples ‘a’ and ‘b’, this morphology in small extend do appeared. Each spongy CCS spherical grain consists of several 8–10 elongated sub-grains. With increase in film thickness, there is slight grain growth. At this stage, it is difficult to

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Fig. 3. . SEM images of different thicknesses: (a) 110 nm, (b) 168 nm, and (c) 224 nm scanned at 15 kV applied voltage and at 50k magnification.

Fig. 4. 3D AFM image of 224 nm as-deposited CCS thin film.

conclude exact direction of elongated grains, as they are in random with 50 nm in diameter and 300– 400 nm in length. Here, the phenomenon of image ‘c’ is not clearly understood but seems to be a breaking of monodispersed spherical grains due to enlargement of grain size beyond the optimum limit. AFM image (3D) of 224 nm CCS film (Fig. 4) shows the clear spherical grains in clusters, separated by valley like architecture (as seen in SEM). The grain size again falls within the nanometer regime. Fig. 5 shows the TEM image of CCS film and its related SAED pattern (inset). Irregular surface morphology with lack of sharp grain boundaries, suggests amorphous nature of CCS. In support to XRD spectrum, inset SAED pattern reveals a broad unrecognized fuzzy spectrum due to its amorphous character [22,23].

Fig. 5. Bright field TEM image of 224 nm CCS thin film. The circled area was considered for recording the selected area electron diffraction pattern (inset).

3.4. Optical absorption and measurement of band gap The optical absorption spectra for different thicknesses were studied at room temperature in the wavelength range of 350–850 nm. Fig. 6(a) shows the variation of absorbance (at) with the wavelength of incident radiation (l). It is

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100

0.6

(αhν)2 X 108 (eV-cm-12)

Absorbance, αt

0.8

a b c

0.4 0.2 0.0

400

600

80

40 20 0 1.5

800

a b c

60

2.0

Wavelength (nm)

2.5

hν/eV

Fig. 6. Variation of absorbance (at) vs. wavelength (l) of CCS thin films for different thicknesses (A): 110 nm (a), 168 nm (b), 224 nm (c), and (B) related band gap energies.

3.5. Electrical resistivity and thermo-EMF measurements Electrical resistivity measurements of CCS films of different thicknesses were carried out using two-point dc probe method. Fig. 7 demonstrates the variation of log of electrical resistivity (r) with reciprocal of temperature (1000/T) for CCS films for different thicknesses. The electrical resistivity measured at room temperature for films of thickness 110, 168, and 224 nm is given in Table 1. It is seen that at 300 K constant temperature, the resistivity of the CCS film decreased with increase in film thickness. The decrease in electrical resistivity with increase in temperature confirmed the semiconducting behavior. Nonlinear nature of graphs is mainly due to presence of defects as commonly observed in thin amorphous films [31]. Decrease in electrical resistivity and activation energy (measured by standard relation [32]) with increase in film

16

a b c

14 12 Log ρ (ohm-cm)

observed that there is a continuous increase in absorbance with decrease in wavelength and is related to the change in thickness of the film. This particular overgrowth on the film surface can increase the absorbance due to scattering losses [22]. The relation between absorption coefficient and incident photon energy (hn) can be written as (ahn) ¼ A(hnEg)n/2, where A is a constant, Eg the separation between the valence and conduction bands, n is a constant equal to 1 for direct gap semiconductors and 4 for indirect gap materials. In the present investigation, the optical absorption coefficient is of the order of 104 cm1, supporting the direct transition of the material [16,24]. The variation of (ahn)2 vs. hn is linear (Fig. 6(b)), which means that the mode of transition in these films is of direct nature. Extrapolation of these curves to energy axis for zero absorption coefficient value gives the optical band gap energy. With increase in film thickness from 110 to 224 nm, Eg is decreased from 2.42 to 2.39 eV. The decrease in band gap energy with increase in film thickness is commonly observed phenomenon in semiconducting thin films [25–29]. Nair et al. [30] have discussed causes for change in optical band gap of semiconductor thin films as the effect of film thickness in terms of quantized energy levels.

10 8 6 4 2 0

2.4

2.8

3.2

1000/T (K-1)

Fig. 7. Log of electrical resistivity (r) vs. reciprocal of temperature (1/T; variation within the 300–500 K range) of CCS films having thicknesses as stated in figure, confirming semiconducting behavior.

Table 1 The effect CCS film thicknesses on electrical and optical studies Thickness (nm)

Band gap energy, Eg (eV)

Electrical resistivity, (  105 O cm)

Activation energy, Ea (eV)

110 168 224

2.47 2.38 2.35

3.5 1.8 0.8

0.3 0.3 0.2

thickness can be attributed to the decrease in defect levels, which are more in ultra thin films than the thick films [33]. The CCS film showed a electrical resistivity of 0.78  105 O cm at 300 K which is closely related to 0.8  105 O cm reported by Lade et al. [7]. From the polarity of thermoelectric voltage generated across the cold and hot junctions, n-type conductivity in support to our photoelectrochemical report [20] is confirmed.

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4. Conclusions In summery, the amorphous CCS films are synthesized using CBD method and structurally elucidated using XRD and SAED patterns. Monodispersed spherical grains are obtained for the 110 and 168 nm film thicknesses, which are broken into spongy elongated grains at 224 nm thickness. Change in optical absorption with increase in film thickness is responsible for decrease in band gap energy from 2.47 to 2.35 eV and decrease in electrical resistivity from 3.55 to 0.76  105 O cm. The n-type conductivity in support to our photoelectrochemical studies is confirmed from thermo-EMF measurement. Acknowledgments The national R&D project for Nano-Science and Technology, Korea, is greatly acknowledged. R.S.M. wish thanks to KOSEF for the ward of Brain Pool fellowship for the year 2006–2008. We acknowledge the grant (#20050401034632) from BioGreen 21 Program, Rural Development Administration, Republic of Korea. References [1] S.S. Kale, C.D. Lokhande, Mater. Chem. Phys. 62 (2000) 103. [2] G.A. Ozin, Adv. Mater. 4 (1992) 612. [3] L. Golik, S.M. Grigorovitch, M.I. Elinson, Z.E. Kunkova, V.M. Ukrainskyi, in: Proceedings of the Third International Conference on Thin Films, Basic Problems, Applications and Trends, Budapest, Hungary, 25–29 August, Paper 7, 1975, p. 17. [4] R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 65 (2000) 1. [5] R.S. Mane, B.R. Sankapal, K.M. Gadave, C.D. Lokhande, Mater. Res. Bull. 34 (1999) 2035. [6] A.M. Salem, M.E. El-Ghazzawi, Semicond. Sci. Technol. 19 (2004) 236. [7] S.J. Lade, M.M. Uplane, M.D. Uplane, C.D. Lokhande, Mater. Chem. Phys. 53 (1998) 185. [8] T. Oguchi, T. Kambara, K.I. Gondaira, Phys. Rev. B 24 (1981) 3441.

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