Fabrication and characterisation of SbI3-opal structures

Materials Letters 130 (2014) 17–20

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Fabrication and characterisation of SbI3-opal structures Mirosława Kępińska a,n, Anna Starczewska a, Iwona Bednarczyk b, Janusz Szala b, Piotr Szperlich a, Krystian Mistewicz a a b

Institute of Physics – Center for Science and Education, Silesian University of Technology, Krzywoustego 2, 44-100 Gliwice, Poland Department of Materials Science, Silesian University of Technology, Krasińskiego 8, 40-019 Katowice, Poland

art ic l e i nf o

a b s t r a c t

Article history: Received 7 February 2014 Accepted 10 May 2014 Available online 17 May 2014

This work is focused on forming opal-antimony triiodide (SbI3) structures with direct SiO2 opal. SbI3 is a semiconductor having relatively high refractive index so potentially is very useful in fabrication of photonic crystal. Additionally, crystalline SbI3 exhibits the second-harmonic generation so obtained structures provide a wide range of opportunities for optoelectronic devices. Presented structures are fabricated by infiltration opal with SbI3 dissolved in ethanol and drying in room temperature. The morphology of the samples was characterized by scanning electron microscopy (SEM). The chemical composition of the samples was analyzed using energy dispersion spectroscopy (EDS). Optical properties were investigated by reflectance spectroscopy for wavelengths from 380 nm to 1050 nm. & 2014 Elsevier B.V. All rights reserved.

Keywords: Photonic crystals Opal Optical spectroscopy Antimony triiodide

1. Introduction Photonic crystals are structures with a periodically modulated dielectric constant [1] resulting in the formation of a photonic band gap (PBG). These structures can be used in fabrication of different photonic devices [2]. A good example of a threedimensional photonic crystal is synthetic opal composed of monodisperse nanoscale SiO2 spheres. When the size of SiO2 spheres (period of the structure) is varied, the PBG position for opals can be tuned in a wide spectral range from ultraviolet to near infrared [3]. The fulfilment of free space between spheres in opals with various substances allows changing the effective refractive index and the optical contrast parameter, thereby changing the PBG position and its width [4]. Furthermore embedding the substance in the opal structure can improve the existing properties and even reveal the new ones e.g. photoluminescence [5–7] or the second-harmonic generation [8]. One of the most promising materials not yet used in the production of photonic crystals is antimony triiodide (SbI3). It is a semiconductor having relatively high refractive index [9], which exhibits the secondharmonic generation. Photosensitive films of SbI3 have found applications among others in high-resolution image microrecording and in information storage [10]. 3D nanocomposites fabricated on the based on opal matrices can also be used as active elements of the amplifying or generating systems, as control systems in fiber optics laser, in semiconducting nanoelectronic apparatus, and in

n

Corresponding author. Tel.: þ 48 32 603 41 88. E-mail address: [email protected] (M. Kępińska).

http://dx.doi.org/10.1016/j.matlet.2014.05.063 0167-577X/& 2014 Elsevier B.V. All rights reserved.

other devices [11]. The fabrication of SbI3-opal structure should be useful in optimization of optoelectronic devices based on SbI3. It may also open up new application possibilities. In this work we have demonstrated a very simple and low cost method of fabrications of SbI3-opal structure by infiltration of SiO2 opal template with SbI3 dissolved in ethanol. The morphology, chemical composition and optical properties of obtained SbI3-opal structures have been investigated.

2. Experiment Monodisperse silica spheres with diameter of several hundred nanometers range have been prepared using procedure described in [6] based on the Stö ber method [13]. Their size (DSEM) has been determined from electron micrographs as it was described in [14]. For opal fabrication we have used the gravity sedimentation method [15]. The obtained plates of opals had thickness of
0.3 mm. Two kinds of opals composed of spheres of different diameters (see Table 1) have been chosen. A difference in sizes of the spheres building opals is related to the color variation. Antimony triiodide solution has been prepared with ethanol as a solvent. Mixture of 0.268 g of SbI3 single crystalline platelets and 0.5 ml of ethanol has been ultrasonically mixed for about 30 min. Opals have been infiltrated with the SbI3 solution and dried in room temperature for 24 h. This procedure has been repeated five times. Formed SbI3-opal structures have a shiny surface without the need of cutting and/or polishing of the sample after infiltration.

18

M. Kępińska et al. / Materials Letters 130 (2014) 17–20

The morphology of formed structures has been characterized by SEM using Hitachi S-4200 microscope in the case of bare opals and STEM HITACHI HD-2300A microscope in the case of SbI3-opal structures. The chemical composition of the prepared structure has been analyzed using EDS in scanning electron microscope (Hitachi S-4200 with Thermo Scientific EDS). Optical properties have been investigated by reflectance spectroscopy. Spectra of optical reflectance were registered in room temperature using PC2000 (Ocean Optics Inc.) spectrophotometer with master card with 600 lines grating blazed at 500 nm. Investigations have been performed for wavelengths (λ) from 380 to 1050 nm. Spectrophotometer was equipped with appropriate reflection probe R7  400-2-LOH and the deuterium-halogen light source DH2000FHS from Ocean Optics Inc. The illuminated opals have been mounted horizontally, perpendicular to the light beam. For angular investigations samples have been mounted on the table of GUR-5 (LOMO) goniometer. Using Glan–Thomson polarizer (LOT-Oriel) the incoming radiation beam was linear polarized with the electric vector parallel (p) or perpendicular (s) to the plane of incidence. The multiple averaged spectral characteristics of optical reflectance (R) containing 2048 data points for various wavelengths (λ) have been registered using the OOI-Base program from Ocean Optics Inc.

3. Results and discussion Fig. 1a presents TEM image of SbI3 crystals obtained from ethanol solution. When the crystallization takes place in the

interior of the opal, the crystallite size is limited by the dimensions of the space between the balls [11], which favours the formation of nanocrystallites. Typical SEM micrographs of bare opal as well as both SbI3-opals structures are presented in Fig. 1b–e. They show view of top surface of SbI3-opal structure consisting of spheres with diameter DSEM ¼244(7) nm (Fig. 1c), and DSEM ¼333(12) nm (Fig. 1d,e) obtained with different magnifications. Fig. 1f shows the crosssection of one of SbI3-opal structures. They confirm the rather good quality of the obtained opals. One can see hexagonal network structure typical for the (111) layers in the terms of the fcc lattice. The pores of the opal-SbI3 composite are filled uniformly (see Fig. 1c–f) with SbI3. This fact was confirmed by EDS investigations. Fig. 2a and b presents the EDS spectra of the surface and interior (after cutting the sample) of SbI3-opal structure, respectively. The characteristic peaks for antimony, iodine, silicon and oxygen are observed. The source of silicon and oxygen peaks is silica spheres of opal template. The source of antimony and iodine peaks is material surrounding and filling empty spaces between spheres. The elemental atomic ratios of 0.247:0.753 for Sb and I averaged over the surface of SbI3-opal structure and 0.278:0.722 for Sb and I averaged over the interior of structure have been obtained. These last values are only a little different from the appropriate for a stoichiometric SbI3. Prepared opals have been investigated by the optical method described in [12], which is based on measuring of Bragg's peak position for opals infiltrated with liquids of known refractive indices. This method allowed determining not only the diameter of the spheres building opals (Dopt) but also the filling factor (f). Results of obtained parameters are presented in Table 1 in

Table 1 Parameters of bare opals and of SbI3-opal structures evaluated using reflectance spectroscopy and analysis of SEM micrographs (DSEM). Sample

DSEM (nm)

for bare opal

for SbI3-opal structure

λc (nm)

Dopt (nm)

f

λcðSbI 3 Þ (nm)

nef f

nSbI3

ff

ð1 f  f f Þ

♯1

244(7)

508(1)

234(1)

0.6792(3)

632(2) for θ¼ 01 λcðSbI 3 Þ ðθÞ

1.651(6) 1.668(3)

2.60(1)

0.165(2) 0.176(3)

0.149(3) 0.144(3)

♯2

333(12)

689(1)

325(1)

0.6049(6)

878(2) for θ¼ 01 λcðSbI 3 Þ ðθÞ

1.652(6) 1.648(2)

2.43(1)

0.214(3) 0.212(2)

0.177(3) 0.182(4)

Fig. 1. Typical SEM micrographs of bare opal (a), SbI3-opal structure consisted of spheres of diameter DSEM ¼244(7) nm (b,c), and DSEM ¼ 333(12) nm (d,e,f) obtained with different magnifications.

M. Kępińska et al. / Materials Letters 130 (2014) 17–20

comparison with DSEM. For calculations it was assumed that the opal has fcc structure [4]. The difference between values of Dopt and DSEM are smaller than 3%. Typical spectra of R(λ) registered for bare opals and SbI3-opal composites are presented in Fig. 3a and b. One can identify the Bragg's diffraction peaks. The intensity of the diffraction peak of the infiltrated opals presents lower reflectivity respect to the bare one. This phenomenon is commonly observed in opals after infiltration (e.g. [4]) because the refractive index contrast between spheres and surrounding media diminishes [16]. Large red-shift of the Bragg's peaks is related to change of effective refractive indices. The Bragg's peaks positions are shifted to shorter wavelengths with the increase of the angle of light incidence (θ) according to the modified form of Bragg's law: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

λc ¼ 2d n2ef f  sin 2 θ

ð1Þ

19

pffiffiffiffiffiffiffiffi where d ¼ 2=3Dopt is the interplanar spacing between (111) planes in photonic crystal, Dopt is the diameter of silica spheres, n2ef f ¼ n2sph Uf þ n2medium U ð1  f Þ is the effective refractive index, f is the filling factor of the spheres, nsph and nmedium are the refractive indices of silica nanospheres and the surrounding medium for appropriate λc, respectively. Fig. 3c presents angular dependencies of Bragg's peaks positions for both SbI3-opal structures for different polarization of light (s and p). One can see that for p polarization dependencies λc(SbI3)(θ) have been determined in narrower range of angles than for s polarization. The reason for this is the suppression of the Bragg's peak as the angle of incidence approach to the Brewster angle. Characteristics obtained for both polarizations are comparable within the measurement uncertainty. Experimental results have been fitted with dependence (1) assuming the spectral dependence of the refractive index of the SiO2 spheres (nsph(λ)) presented in [17]. Parameters of the fitting are given in Table 1 in comparison with parameters calculated from Equation (1) for θ ¼01.

Fig. 2. EDS spectra of the surface (a) and interior (b) of SbI3-opal structure.

Fig. 3. Spectra of optical reflectance measured for ♯1(a) and ♯2 (b) bare opals (∎,⎕), and appropriate SbI3-opal structures (,○), (c)angular dependencies of Bragg's peaks positions for ♯1(open symbols) and ♯2(filled symbols) SbI3-opal structures obtained for light polarizations: s - (∎,⎕), p - (,○), curves – the least square fitted dependence (1); parameters of the fitting are given in Table 1.

20

M. Kępińska et al. / Materials Letters 130 (2014) 17–20

Taking into account the method of filling the opals medium surrounding silica nanospheres in opals can be composed of several fractions. In this work opals have been repeatedly infiltrated with ethanol-SbI3 solution and then dried. Therefore it can be assumed that after evaporation of the ethanol the remaining medium consists of SbI3 particles and air. In this case, according to [18], expression describing effective refractive index of composite opal structures takes the form: n2ef f ¼ n2sph f þ n2SbI3 f f þ n2air ð1  f  f f Þ

ð2Þ

where f is the filling factor of the spheres, f f is the filling factor of SbI3 particles, nsph ; nSbI 3 ; nair are the refractive indices of silica nanospheres, SbI3 particles and air for appropriate λc, respectively. This formula may be used to determine filling factor of SbI3 if other parameters i.e. nef f ; nsph ; nSbI3 ; f are known. SbI3 is an anisotropic semiconductor having relatively high ordinary and extraordinary refractive indices. Spectra of both parameters are presented in [9]. However, SbI3 particles filling opal template can not be treated as a bulk crystal. It is allowed to treat them rather as an isotropic material due to random packing of particles in opal template. Therefore needed values of nSbI3 have been determined as the average of the ordinary and extraordinary refractive indices for wavelength of Bragg's peak for SbI3-opal structure. Assuming the refractive indices listed in Table 1 the filling factors of SbI3 particles have been calculated using formula: ff ¼

n2ef f  1  f ðn2sph  1Þ n2SbI3 1

ð3Þ

Values of determined parameters f f have been given in Table 1. Uncertainties of all determined values have been calculated using total differential method. Efficiencies of filling processes (calculated as the percentage ratio of f f toð1  f Þ) are equal to 55% for sample ♯1 and 53,7% for sample ♯2 for neff obtained from angular dependencies of λcðSbI3 Þ ðθÞ. In the case of calculations performed with simplified formula (for θ ¼01) efficiencies of filling are equal to 51,4% for sample ♯1 and 54,2% for sample ♯2. The efficiency of presented method seems to be comparable, regardless of the size of the spheres building opal and hence the size of the space between the spheres. 4. Conclusions Simple procedure of SbI3-opal structure forming by infiltration SiO2 opal with ethanol-SbI3 solution has been presented. This technique is convenient and effective. Presented SEM images as well as reflectance spectroscopy confirm the effectiveness of

multiple infiltration opals with SbI3 dissolved in ethanol as a method of forming SbI3-opal structure. The efficiency of such filling process reaches 55%. The measurements of reflectance exhibit large red-shift of the Bragg's peak connected with the change of effective refractive index of structure. The efficiency of presented method seems to be independent of spheres size building opal and hence the space size between the balls. The fabrication of SbI3-opal structure should be useful in optimization of optoelectronic devices based on SbI3. The particle size usually plays a vital role on the features of materials, so obtaining SbI3 in the form of nanocrystallites inside the opal may also open up new application possibilities. It is therefore appropriate to continue the work related to technology and research SbI3-opal structure.

Acknowledgments This paper was partially supported by the NCN (Poland) under Contract no. NN507250140.

References [1] Yablonovitch E. Phys Rev Lett 1987;58:2059–62. [2] Fenollosa R, Ibisate M, Rubio S, Lopez C, Sanchez-Dehesa J. J Appl Phys 2003;93:671–4. [3] Xia Y, Byron B, Yin Y, Lu Y. Adv Mater 2000;12:693–713. [4] Golubev VG, Kosobukin VA, Kurdyukov DA, Medvedev AV, Pevtsov AB. Semiconductors 2001;35(6):680–3. [5] Gonçalves MC, Fortes LM, Almeida RM, Chiasera A, Chiappini A, Ferrari M. Opt Mater 2009;31:1315–8. [6] Chiappini A, Armellini C, Carpentiero A, Minati L, Righini GC, Ferrari M. Opt Mater 2013;36:130–4. [7] Abrarov SM, Yuldashev ShU, Lee SB, Kang TW. J Lumin 2004;109:25–9. [8] Fedyanin AA, Aktsipetrov OA, Kurdyukov DA, Golubev VG, Inoue M. Appl Phys Lett 2005;87:151111. [9] Kępińska M, Nowak M, Duka P, Kotyczka-Morańska M, Szperlich P. Opt Mater 2011;33:1753–9. [10] Mohan DB, Philip A, Sunandana CS. Vacuum 2008;82:561–5. [11] Samoilovich MI, Samoilovich SM, Guryanov AV, Tsvetkov MYu. Microelectron Eng 2003;69:237–47. [12] Kępińska M, Starczewska A, Szala J. Opt Mater 2014;36:932–5. [13] Stö ber W, Fink A, Bohn E. J Colloid Interface Sci 1968;26:62–9. [14] Starczewska A, Szala J, Kępińska M, Nowak M, Mistewicz K, Sozańska M. Solid State Phenom 2013;197:119–24. [15] Bardyshev II, Mokrushin AD, Pribylov AA, Samarov EN, Masalov VM, Karpov IA, Emel’chenko GA. Colloid J 2006;68(1):20–5. [16] Calvo ME, Sánchez–Sobrado O, Lozano G, Míguez H. J Mater Chem 2009;19:3144–8. [17] Lopez C, Vazquez L, Meseguer F, Mayoral R, Ocana M. Superlattices Microstruct 1997;22(3):399–404. [18] Abrarov SM, Kim TW, Kang TW. Opt Commun 2006;264(1):378–84.