Influence of SOP modes on Raman spectra of ZnO(Fe) nanoparticles

Optical Materials 42 (2015) 118–123

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Influence of SOP modes on Raman spectra of ZnO(Fe) nanoparticles B. Hadzˇic´ a, N. Romcˇevic´ a,⇑, M. Romcˇevic´ a, I. Kuryliszyn-Kudelska b, W. Dobrowolski b, U. Narkiewicz c, D. Sibera c a

Institute of Physics, University of Belgrade, Pregrevica 118, 11 080 Belgrade, Serbia Institute of Physics, Polish Academy of Science, al. Lotnikow 32/46, 02-668 Warszawa, Poland c West Pomeranian University of Technology, Institute of Chemical and Environment Engineering, Pulaskiego 10, 70-322 Szczecin, Poland b

a r t i c l e

i n f o

Article history: Received 15 September 2014 Received in revised form 29 November 2014 Accepted 3 December 2014 Available online 9 January 2015 Keywords: Nanostructured materials Optical properties Light Absorption Reflection

a b s t r a c t Nanocrystaline samples of ZnO(Fe) were synthesized by traditional wet chemical method followed by calcinations. Samples were characterized by X-ray diffraction to determine composition of the samples (ZnO, ZnFe2O4 and Fe2O3) and the mean crystalline size (from 8 to 51 nm). In this paper we report the experimental spectra of Raman scattering (from 200 to 1600 cm1) with surface optical phonons (SOP) in range of 500–550 cm1. The phonon of registered phase’s exhibit effects connected to phase concentration, while the SOP phonon mode exhibit significant confinement effect. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Due to the wide band gap (3.37 eV) and a relatively large exciton binding energy of 60 meV, zinc oxide (ZnO), as well as materials based on it, has attracted significant attention. It could be used in many fields, such as light-emitting diodes (LEDs), photodetectors, transparent conducting electrodes in solar cells and flat panel displays, photocatalyst, luminescence, ultraviolet protection and low-voltage and short-wavelength electro-optical devices [1–3]. Transition metal (Fe, Co, Ni, Cr, V, etc.) doped ZnO based diluted magnetic semiconductors (DMSs) have been widely studied due to its application in magnetoelectronic and spintronic devices. There are many reports about high temperature ferromagnetism in these materials, especially in Fe-doped ZnO [1,4–6], although the nature of the ferromagnetism was still unclear. Raman spectroscopy is one powerful technique, as well as sensitive and non-destructive tool, which has often been used to identify the microscopic vibrations caused by the slight structural distortion in bulk, thin films and nanostructure samples, pure and doped. In ZnO and ZnO-related compounds with Raman scattering has been study local atomic arraignment, dopant incorporation, electron–phonon coupling, multi phonon process and others [7–12]. For nanostructures of ZnO is expected appearance of surface optical phonon (SOP) modes in Raman spectra because of their

⇑ Corresponding author. http://dx.doi.org/10.1016/j.optmat.2014.12.029 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

large surface-to-volume ratio. This is the reason why the state of surface atoms have important role in determining their properties. Surface modes are only modes that persist when dimensions becomes extremely small. With this we can say that SOP modes are Raman forbidden modes whose presence is related to loss of long-range order and symmetry breakdown in ZnO shell [13]. All this can be found in many papers predicted theoretically and/or detected experimentally for ZnO nanostructures [14]. In our previous paper [9] we have done analysis of vibrational modes and low-frequency acoustic modes in ZnO dopped with Fe and detaily study SOP mode in ZnO (Co) and ZnO (Mn) (prepared by whet chemical method) samples [15,16], while with this paper we want to continue completing the picture about the impact of doping with the transition elements on vibrational properties of doped ZnO as well as influence of preparation method. The aim of this work is to study influence of preparation method on samples characteristics, by applying micro-Raman spectroscopy to study SOP, the Fe ion position in ZnO lattice, the formation of existing phases, and the samples quality in dependence of Fe2O3 concentration. 2. Samples and characterization The nanocrystalline samples of ZnO doped with Fe2O3 were synthesized by use of the wet chemical method. First, the mixture of iron and zinc hydroxides was obtained by addition of an ammonia solution to the 20% solution of proper amount of Zn(NO3)⁄6H2O and Fe(NO3)⁄4H2O in water:

B. Hadzˇic´ et al. / Optical Materials 42 (2015) 118–123

ZnðNO3 Þ2 þ 2NH4 OH ¼ ZnðOHÞ2 þ 2NH4 NO3

Z-ZnO F1 - Fe2O3 F2 -ZnFe 2O4

FeðNO3 Þ2 þ 2NH4 OH ¼ FeðOHÞ2 þ 2NH4 NO3

3. Results and discussion The micro-Raman spectra were taken in the backscattering configuration and analyzed using Jobin Yvon T64000 spectrometer, equipped with nitrogen cooled charge-coupled-device detector. As excitation source we used the 514.5 nm line of an Ar-ion laser. The measurements were performed at 20 mW laser power. For analysis of Raman spectra we have assumed that all phonon lines are of Lorentzian type which is one of common type of lines for this kind of analysis, other common type of line is Gaussian [18].

Z Z

Z

Z

Z

Z

5% Fe2O3

Z ZZ

Intensity [arb. un.]

20% Fe2O3 F 2

F

F 2

F 2

F 2

2

40% Fe2O3

F 2

50% Fe2O3 60% Fe2O3

Z F 1

F 1

80% Fe2O3

F 1

ZF 1

90% Fe2O3

F 1 F 1

F 1

F 1

F

F 1

20

1

F F 1 1

F 1

40

100% Fe2O3

60

80

100

2θ [deg] Fig. 2. XRD spectras of representative samples all register crystalline phases are marked and evident.

Z-ZnO F1 - Fe2O3 Z F2 -ZnFe2O4

Z Z

5% Fe2O3 20% Fe2O3

Intensity [arb. un.]

Next, the obtained hydroxides were filtered, dried at 70 °C and calcined at 300 °C during one hour. Nanopowders obtained on this way were pressed into indium panel. This method allowed obtaining the series of samples with nominal concentration of Fe2O3 from 5% to 95%. In this paper we present the results of micro-Raman spectroscopy for most of the samples (from 5% to 90%) as well as changes of relative intensity of modes with concentration of Fe2O3. Morphology of the samples was investigated using scanning electron microscope (SEM). SEM images for three representative samples doped with 20, 50 and 80 wt% of Fe2O3 are shown in Fig. 1. In Fig. 1(a), for lower concentration of Fe2O3, it can be easily distinguished bigger particles that belong to ZnO phase whose size exceeds 100 nm and much smaller particles that belong to ZnFe2O4 phase with approximate size of 10 nm, which will be confirmed latter. With increase of doping element Fe2O3 difference in size of particles decrease. This is most evident in Fig. 1(c), for 80% of doped Fe2O3, where it can be notice that these particles have approximately equal size around 25 nm, which is much less than 100 nm. The phase composition of the samples was determined by X-ray diffraction (Co Ka radiation, X’Pert Philips). The detailed phase composition investigation revealed the presence of hexagonal ZnO, spinel structures of ZnFe2O4 and rhombohedral Fe2O3. In the purpose to demonstrate this fact, the characteristically X-ray diffractogram for representative samples is shown in Fig. 2 while on Fig. 3 is given enlarged part of Fig. 2 with highest concentration of peaks of all registered phases. XRD data allowed determining a mean crystallite size in prepared samples by use of Scherrer’s formula [17]. The mean crystalline size ã of these phases are between 26 and 51 nm for ZnO phases, from 8 to 12 nm for ZnFe2O4 phases and at approximately 23, 24 nm for Fe2O3 phases. The results of XRD measurements are gather in Table 1. Sign ‘‘+’’ in this table means that its been register presence of these particles but it was not possible to determinate their size, while ‘‘⁄’’ means that these particles should be registered but they have not and ‘‘’’ means that the presence of these particles have not been register.

119

F

40% Fe2O3

2

50% Fe2O3 60% Fe2O3 80% Fe2O3 90% Fe2O3 F

30

32

F

1

34

1

100% Fe2O3

36

38

40

2θ [deg] Fig. 3. A part of XRD spectra’s with peaks of all register crystalline phases which are marked and evident.

In Figs. 4 and 5 are presented all obtained Raman spectra of nanocrystalline ZnO doped with Fe2O3. As we already have mention nanoparticles of ZnO, ZnFe2O4 and Fe2O3 are registered by XRD. For analysis of vibration properties of nanoparticles is crucial understanding of vibration properties of bulk material. That is why we start analysis of obtained Raman spectra with brief report about structural and vibration properties of all potentially present phases in the samples. We expect that bulk modes will be shifted and broadening as a consequence of miniaturization.

Fig. 1. SEM images of three samples of nanosized ZnO doped with Fe2O3, (a) corresponds to sample doped with 20 wt% of Fe2O3, (b) corresponds to sample doped with 50 wt.% of Fe2O3, (c) corresponds to sample doped with 80 wt.% of Fe2O3.

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120

Table 1 XRD analysis results for all identified crystalline phases in the samples and mean crystalline size ã has been determined by use of Scherrer’s formula. wt.% of ZnO

wt.% of Fe2O3

ã (nm) ZnO phase

ã (nm) ZnFe2O4 phase

ã (nm) Fe2O3 phase

95 90 80 70 60 50 40 30 20 10 5

5 10 20 30 40 50 60 70 80 90 95

+ + + 51 26 ⁄ ⁄ ⁄ ⁄ – –

10 8 8 11 12 8 12 12 12 ⁄ –

– – – – – – – – ⁄ 24 23

In our previous papers [15,16] we have mention characteristics of ZnO and his Raman active modes. Here we will, for clarity, repeat only frequencies of ZnO Raman active modes and their

assignation 102 cm1 (E(1) (low)), 379 cm1 (A1(TO)), 410 cm1 2 1 (2) (E1(TO)), 437 cm (E2 (high)), 541 cm1 (A1(LA)), 577 cm1 (A1(LO)) and 592 cm1 (E1(LO)), while modes on 330, 660 and 1153 cm1 are multi phonon modes [19,20]. ZnFe2O4 spinel has cubic structure that belongs to the space group O7h(Fd3m). The full unit cell contains 56 atoms (Z = 8), the smallest Bravais cell only consists of 14 atoms (Z = 2). The factor group analysis predicts the following modes in ZnFe2O4 spinel [9]:

Copt ¼ A1g þ Eg þ F1g þ 3F2g þ 2Au þ 2Eu þ 4F1u þ 2F2u The A1g, Eg and three F2g modes is Raman active. Modes F1u are infrared-active. In the cubic spinel’s, including ferrites, the modes at above 600 cm1 mostly correspond to the motion of oxygen atoms in tetrahedral AO4 group. The other lower frequency modes represent the characteristic of the octahedral BO6 sites. Frequencies and assignation of Raman active modes in ZnFe2O4 presented in paper [9] are given in Table 2. Fe2O3 crystallize in the rhombohedric (trigonal) system with space group D63d. Primitive unit cell contains two formula units

Fig. 4. Raman spectra of nanocrystalline ZnO doped from 5% till 40% of Fe2O3.

Fig. 5. Raman spectra of nanocrystalline ZnO doped from 50% till 90% of Fe2O3.

B. Hadzˇic´ et al. / Optical Materials 42 (2015) 118–123

(Z = 2). The factor group analysis predicts the following modes in Fe2O3

Copt ¼ 2A1g þ 2A1u þ 3A2g þ 2A2u þ 5Eg þ 4Eu 2A1g and 5Eg are Raman active modes. 2A2u and 4Eu modes are infrared active [9]. There is, as addition to these first order Raman spectra, multi phonon peak at 1320 cm1. When symmetry rules are broken can be visible Raman forbidden mode at 660 cm1. Frequencies and assignation of Raman active modes in Fe3O4 presented in paper [9] are given in Table 3. In Fig. 4 are given Raman spectra of ZnO doped from 5% till 40 % of Fe2O3. On this spectra’s we noticed, like it shown XRD, modes that belongs to ZnO and ZnFe2O4. Sharp and narrow peak, clearly visible on spectra doped with 5% of Fe2O3, whose position is in that spectra on 437 cm1 is obviously E(2) 2 mode of ZnO. With increase of doping element, intensity of this peak decrease, due to ZnO ori1 gin. Beside this E(2) 2 mode, also are visible ZnO modes at 379 cm 1 1 A1(TO), 577 cm A1(LO) and 592 cm E1(LO). Multi phonon 2LO ZnO modes at 330 and 1150 cm1 are clearly visible too, while multi phonon mode 660 cm1 with ZnFe2O4 peak at 647 cm1 form one wide structure. Also on these spectra we can notice ZnFe2O4 modes at 246 cm1 Eg, 355 cm1 F2g and 451 cm1 F2g. Beside these modes that belongs to two registered phases by XRD in samples doped from 5% till 40 % of Fe2O3 we can notice peaks that belong to Fe2O3 phase as well, at 294 cm1 Eg and 498 cm1 Ag. Most of center peak positions are on something smaller frequencies, due to nanosized structure of samples, but generally in good agreement with reported Raman frequencies in works [19,20,9]. In Fig. 5 are presented Raman spectra of nanocrystalline ZnO doped from 50% till 90% of Fe2O3. In spectra of samples doped from 50% till 70% of Fe2O3 it can be noticed existence of three wide structures result of more close modes overlapping. Similar situation is obvious in spectra with lower dopant concentration such as 20% and 40% of Fe2O3 shown in Fig. 4. Overlapping modes are, in first wide region whose center is at approximately 330 cm1, Fe2O3 peak at 294 cm1 Eg, multi phonon ZnO peak at 330 cm1 and ZnFe2O4 peak at 355 cm1 F2g. In second wide region, whose center is at approximately 470 cm1, contribute peaks on 1 437 cm1 (E(2) (F2g, ZnFe2O4) and 498 cm1 (Ag, 2 , ZnO), 451 cm Fe2O3), while ZnO peak at 410 cm1 influence in both first and second wide region. Third wide region, whose center is at approximately 660 cm1 is result of three ZnO peaks at 577 cm1 A1(LO),

592 cm1 E1(LO), multi phonon 660 cm1 and one ZnFe2O4 peak at 647 cm1 A1g overlapping. Intensity of ZnO multi phonon peak at 1153 cm1 decrease with increase of Fe2O3 concentration and his existence in these spectra is not so obvious. With further increase of doping element Fe2O3, ZnO peaks lose their intensity while Fe2O3 peaks become dominant. In spectra with 80% and 90% of Fe2O3 we can easily notice Fe2O3 peaks at 225 cm1 Ag, 294 cm1 Eg, 412 cm1 Eg and 498 cm1 Ag along with two multi phonon peaks at 660 cm1 and 1324 cm1. Also here are evident two ZnFe2O4 peaks at 355 cm1 F2g and 451 cm1 F2g, while existence of ZnO modes are represented with four weak peaks at 1 437 cm1 E(2) A1(LO), 592 cm1 E1(LO) and multi phonon 2 , 577 cm peak 660 cm1. With Raman spectroscopy in these spectra have been register existence of phases which have not been register by XRD such as existence of Fe2O3 phase on lower dopant concentration and existence of ZnO and ZnFe2O4 phases for highest dopant concentration. Here are, as well as on spectra’s shown in Fig. 4, most of center peak position on something smaller frequencies as a consequence of nanosized structure of samples but generally in good agreement with previously reported Raman frequencies in works [19,20,9]. Apart all modes that we have mentioned on all presented spectra’s are also evident existence of additional structure at approximately 540 cm1. This wide structures whose range of appearance and peak center position (500–550 cm1) change with dopant concentration is SOP mode. A SOP mode appears as a consequence of breakdown of phonon momentum selection rules [7,8,21,22] as well as in polar crystals where particles size is smaller than the wavelength of incident laser beam [13]. To describe optical properties of surface phonons we use effective medium theory and Bruggeman formula and it mixing rule [23–25]. In Bruggeman model does not exist restrictions for volume fraction that’s why is suitable for high concentration of inclusions, which make it much more appropriate in our case. We used the effective dielectric function for the Bruggeman mixing rule:

ð1  f Þ

F2g(1) Eg F2g(2) F2g(3) A1g

Table 3 Frequencies and assignation of Raman active modes in Fe2O3. Frequencies for Fe2O3 (cm1)

Assignation of modes

225 246 294 300 412 498 613 660 1324

Ag Eg Eg Eg Eg Ag Eg Multi phonon Multi phonon

ð1Þ

100

80

Intensity [arb. un.]

Assignation of modes

221 246 355 451 647

e1  eeff e2  eeff þf ¼0 eeff þ gðe1  eeff Þ eeff þ gðe2  eeff Þ

where g is a geometric factor who depends on the shape of the inclusions and in our samples is g = 1/3 as a consequence of clusterized nanoparticles that occupy a significant volume and are not well-separated in air, clearly visible in Fig. 1, along with e1 ¼ 1. Having in mind that the A1 and E1 symmetry phonon of ZnO are

Table 2 Frequencies and assignation of Raman active modes in ZnFe2O4. Frequencies for ZnFe2O4 (cm1)

121

30% Fe2O3 fit SOP

60

40

20

0 200

400

600

800

1000

1200

1400

1600

Raman shift [cm-1] Fig. 6. Fitted Raman spectra with SOP mode of nanocrystalline ZnO doped with 30% of Fe2O3.

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122

The crystalline phases of ZnO, ZnFe2O4 and Fe2O3 were identified in samples. Crystallite size of all registered phases does not have monotonous dependence.  By Raman spectroscopy are registered peaks from all phases found by XRD in all samples. Raman peaks of these phases are shifted and broadening compared to bulk modes.  Due to nanosize structure of samples in presented Raman spectra’s are also evident surface optical phonon (SOP) modes from ZnO nanoparticles. Relative intensity of ZnO and SOP modes decreases with increases of Fe2O3 concentration.

16 14

Intensity [arb. un.]

12

5% Fe2O3 10

20% Fe2O3

8

40% Fe2O3 70% Fe2O3

6

90% Fe2O3

4 2

Acknowledgments 0 -2 200

400

600

800

1000

1200

1400

1600

Raman shift [cm-1 ] Fig. 7. Change of characteristic SOP modes with concentration of Fe2O3.

References

1,0

SOP 0,8

I/I5%

This work was supported under the Agreement of Scientific Collaboration between Polish Academy of Science and Serbian Academy of Sciences and Arts. The work in Serbia was supported by Serbian Ministry of Education, Science and Technological Development (Project 45003) and in Poland by National Science Center granted under decision No. DEC-2011/01/B/ST5/06602.

0,6

0,4

0,2

0,0 0

20

40

60

80

100

wt% Fe2O3 Fig. 8. Change of intensity of SOP modes with concentration of Fe2O3.

registered in Raman spectra, while there is no A1 (LA) symmetry phonon this can indicate that the A1(LA) symmetry phonon participate in SOP creation. This SOP mode, which peak center position is on 548 cm1, for sample doped with 30% of Fe2O3 is shown in Fig. 6. Change of characteristic SOP modes with concentration of doping element (Fe2O3) is shown in Fig. 7. In this figure we can notice that intensity of SOP modes decrease with increases of Fe2O3 concentration. This change of intensity of SOP modes is similar to the change of intensity of ZnO modes and opposite to the change of intensity of ZnFe2O4 and Fe2O3 modes, which is one more proof that SOP modes originate from ZnO. The change of normalized intensity of SOP modes from concentration of Fe2O3 is shown in Fig. 8. It is clearly visible that the intensity of SOP modes decreases with increase of Fe2O3 concentration, as it has been expected in the case of phonon modes which are another proof that SOP mode is associated with the ZnO phase. 4. Conclusions  The phase composition nanocrystalline samples of ZnO doped with Fe2O3 prepared by traditional wet chemistry method followed by calcinations was determined by X-ray diffraction.

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