Optical phonon confinement in ZnO nanorods and nanotubes

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 48, October 010, pp Optical phonon confinement in ZnO nanorods and nanotubes Soosen Samuel M, Jiji Koshy, Anoop Chandran & K C George* Department of Physics, S B College, Changanassery, Kerala , India * drkcgeorge@gmail.com Received 18 May 010; accepted 6 August 010 The structure and morphology of ZnO nanorods and nanotubes synthesized by hydrothermal method were studied by TEM, XRD and Raman spectroscopy. The Raman scattering spectra at room temperature were excited by He-Ne laser with wavelength 633 nm. The perfect wurtzite structures of ZnO nanorods and nanotubes were verified by the intense E (high) mode. The strong red shifts of phonon frequencies, broadening of the FWHM of Raman peaks and asymmetry of line shapes were attributed to phonon confinement effect. An optical phonon confinement model was used for calculating the theoretical line shapes, asymmetric broadening and shift of the optical phonons. The results were in good agreement with the experimental values. Besides the fundamental phonon modes, overtone and combination modes were also observed because of the anharmonicity effect. From this study, it was found that the phonon confinement effect was more obvious in ZnO nanotubes as compared to nanorods. Keywords: ZnO nanorods, ZnO nanotubes, Raman spectroscopy, Optical phonon confinement 1 Introduction One dimensional (1D) semiconductors are of a burgeoning and fascinating area of materials research that has significant technological implications 1. Recently, 1D structure of ZnO has generated great interest worldwide. An outstanding feature of ZnO is its large excitonic binding energy (60meV), which leads to extreme stability of excitons at room temperature and enables devices to function at a low threshold voltage. ZnO is a wide gap (3.3 ev) semiconductor with a number of unique properties that render the material interesting, especially, for the development of short wavelength optical devices as well as for applications in optics and opto-electronics such as transparent conducting electrodes for flat panel displays and solar cells 3,4. The tubular structure of ZnO becomes particularly important because high porosity and large surface area are required to fulfill the demand for high efficiency and activity in numerous applications. Its great potential has motivated a great deal of studies on the synthesis and optical properties of lots of ZnO nanomaterials such as nanoparticles 5, nanofilms 6, nanorods 7, nanowires 8, nanotubes 9 etc. Their optical properties are strongly influenced by the confinement of electrons and holes. In analogy with electron confinement, phonon confinement has also been found to show interesting changes in the vibrational spectra. Raman spectroscopy is a non-destructive characterization method on the vibrational properties of ZnO crystals, thin films and micro and nanostructures The confined optical phonons within the grains of ZnO nanostructures lead to interesting changes in its vibrational spectra as compared to that of their bulk counterparts. For crystals of reduced dimensionality, some peak shifts and broadenings in the Raman spectra may occur. The Raman spectra of ZnO nanostructures always show such a shift and broadening from the bulk phonon frequencies. In previous works on ZnO nanocrystals, some researchers attribute these changes to confinement effects 1,13 whereas others claim that the shifts are due to local heating, compressive stress or strain rather than to spatial confinement 14,15. Understanding the nature of the observed shift is important for interpretation of the Raman spectra and understanding properties of ZnO nanostructures. In this paper, the phonon spectra of the ZnO nanorods and nanotubes with that of the bulk ZnO have been compared and the effect of confinement on phonons of different symmetry are examined theoretically. Then, the calculated line shapes, peak position and FWHM are compared with the theoretical result. Experimental Details All reagents were analytical grade and used without further purification. The 1D nanostructures (nanorods and nanotubes) of ZnO were synthesized by hydrothermal process. The reaction solution was prepared by adding the appropriate quantity of

2 704 INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 010 ammonia into zinc chloride solution (ZnCl, 0.1 M) to adjust the ph value to 10 and then the as prepared solution was poured equally into two bottles with autoclavable screw caps. Copper strips, cleaned with acetone and deionised water in an ultrasonic cleaner were vertically immersed into the reaction solution. The bottles were heated at a constant temperature of 95 C in an oven. The first bottle was taken out of the oven after 90 min. Then the oven was turned off and cooled down naturally. The second bottle was taken out of the oven after that oven had been turned off for 6 hours. Samples were thoroughly washed with deionized water and dried in air for further characterization. The morphologies of the samples were examined by Transmission Electron Microscope (TEM) of model Morgagni 68 D 80 KV, Company Fei Netherlands and the crystal structures of the samples were characterized by X-ray diffraction (XRD) using CuKα radiation. Sample (a) referred to as ZnO nanorods and sample (b) was ZnO nanotubes. The Raman spectrum of the samples was measured using a Jobin-Yvon Horibra LABRAM-HR visible ( nm) confocal micro Raman spectrometer equipped with a charge coupled detector. The He-Ne laser with wavelength 633 nm was used as a source of excitation. 3 Results and Discussion 3.1 Structure and morphology The size and morphology of the as prepared samples analyzed by TEM are shown in Fig. 1 (a and b). Figure1 (a) shows rod likes morphology of ZnO with an average diameter of 37 nm and length is in the range nm. Figure 1(b) clearly shows the tubular structure of ZnO with inner and outer diameter of and nm, respectively and length around 50 nm. The average wall thickness of ZnO nanotubes is about 3.9 nm. Figure (a and b) shows the XRD patterns of ZnO nanorods and nanotubes. All the diffraction peaks coincide with the earlier reported values with lattice 16 parameters a = 3.49Å and c = 5.06Å (JCPDS Card No ). In comparison with the standard card, the peak intensity of (00) is stronger than that of the other peaks which reveals the preferential growth of [0001] orientation along c-axis. The mean crystallite size of the sample was calculated using Scherer s formula 17 L = 0.9λ/βcosθ where L is the average crystallite size, λ the X-ray wavelength (1.5405Å) and θ is full width at half maximum in radians. The average crystallite sizes of ZnO nanorods and nanotubes estimated from (101) plane is and 3.66 nm, respectively. Fig. 1(a-b) TEM images of ZnO (a) nanorods (b) ZnO nanotubes 3. Raman spectra The wurtzite ZnO belongs to the space group C 6υ 4 with two formula units in the primitive cell. Each primitive cell of ZnO has four atoms, each occupying C 3υ sites, leading to 1 phonon branches, 9 optical modes and 3 acoustic modes 18. At the centre of the Brillouine zone (Γ point), group theory predicts the following lattice optical phonons: Γ opt = 1A 1 + B 1 + 1E 1 + E where A 1 and E 1 are polar modes and are both infrared and Raman active, while E modes are nonpolar and only Raman active. The non-polar E modes have two wave numbers, namely, E (high) and E (low) associated with the motion of oxygen and Zn sub lattice, respectively 18. Strong E (high) mode is characteristic of the wurtzite lattice and indicates good crystallinity. The vibrations of A 1 and E 1 modes can polarize in unit cell, which creates a long range

3 SAMUEL et al.: OPTICAL PHONON CONFINEMENT IN ZnO NANORODS AND NANOTUBES 705 Fig. (a-b) XRD patterns of ZnO (a) nanorods (b) ZnO nanotubes electrostatic field splitting the polar modes into longitudinal optical (LO) and transverse optical (TO) component. The E 1 (LO) mode is associated with the presence of oxygen vacancies, interstitial Zn or their complexes. The B 1 modes are Raman and infrared inactive (silent modes). Figure 3 shows the Raman spectra of ZnO nanorods and nanotubes excited using 633 nm UV line of He-Cd laser. The spectrum of ZnO nanorods shows 5 prominent peaks at 09, 331, 376, 394 and 437 cm 1 in addition to weak and broad peaks at 54 and 654 cm 1. The spectrum of ZnO nanotubes is similar with that of nanorods except that two modes related to multiphonon process are absent. For comparison, a compilation of the reported frequencies of the Raman active phonon modes 18 in bulk ZnO is presented in Table 1. By comparing the obtained optical phonon modes of ZnO nanostructures with that of the bulk, one can assign that the modes at 376 and 438 cm 1 of nanorods, and 375 and 437 cm 1 of nanotubes peaks to A 1 (TO) and E (high) modes, Fig. 3 Raman spectra of ZnO nanorods and nanotubes respectively. 416 cm 1 of ZnO nanorods and 418 cm 1 of nanotubes are assigned to E 1 (TO) mode. While 09, 331 and 54 cm 1 peaks of ZnO nanorods are related to multi-phonon process and have been

4 706 INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 010 Table 1 Wave number and symmetries of the modes found in Raman spectrum of ZnO nanorods and nanotubes and their assignments Mode of symmetry Peak position Peak position Peak position ZnO bulk (cm 1 ) ZnO Nanorods (cm 1 ) ZnO Nanotubes (cm 1 ) TA (M) 09 E (M) A 1 (TO) Quasi A 1 (TO) E 1 (TO) E (high) [E (high) +E (low)] Acoust overtone assigned to the second order Raman spectrum arising from the zone boundary phonons, TA (M), E (M) or [E (high)-e (low)] and [E (high) +E (low)], respectively 19,0. The peak at 394 cm 1 is assigned to quasi A 1 (TO) mode of ZnO nanorods 18. The Raman spectra of ZnO nanotubes also show a combination of both first and second order features. The second order features at 331 and 547 cm 1 are identified as in the ZnO nanorods E (M) and [E (high) +E (low)] due to overtones or combination of first order modes. An acoustic overtone with A 1 symmetry is located at 654 and 646 cm 1 for ZnO nanorods and nanotubes, respectively 19,0. The peak at 56 cm 1 is assigned to laser plasma lines. In this geometry, no LO phonon peaks are seen because the incident light is perpendicular to the c axis of the samples. A 1 (LO) phonon can appear only when the c axis of wurtzite ZnO is parallel to the sample surface. When perpendicular to the sample surface, E 1 (LO) phonon is observed 18. Although, each ZnO nanorods/ nanotubes is c axis oriented, all nanorods/nanotubes are randomly placed. The appearance of well resolved Raman peaks related to multi-phonon and resonance processes and the absence of E 1 (LO) peak at 588 cm 1 related to oxygen deficiency indicates that the prepared ZnO nanorods and nanotubes are of very high optical quality. Strong multi-phonon scattering reveals quantum confinement in the samples because of ZnO nanoentities on the surface of the nanostructures. From Fig. 3, it can be found that the E (high) mode is red shifted by 6 cm 1 for ZnO nanorods and 7 cm 1 for ZnO nanotubes. A 1 (TO) mode is also red shifted by 4 cm 1 for ZnO nanorods and 5 cm 1 ZnO nanotubes. It can also be seen that the E (high) mode is not only broadened, but also it shows an asymmetric broadening towards the low frequency side and A 1 (TO) mode also gives the same result. Generally, three possible mechanisms, that may be responsible for the observed phonon peak shifts in the corresponding Raman spectra are: (1) Spatial confinement within the nanorods/nanotubes boundaries; () Phonon localization by defects such as oxygen deficiency, zinc excess, surface impurities etc; (3) Laser induced heating in nanostructure ensembles and tensile strain. Among these, the most important mechanism is the spatial confinement in ZnO nanorods and nanotubes. The broadening of the spectra, the asymmetry of line shape and strong red shifts of the E (high) and A 1 (TO) mode towards lower frequency side may be attributed to optical phonon confinement 1. It is well known that the Raman spectra of ZnO nanorods and nanotubes may contain numerous combination and overtone bands because of anharmonicity effect. The spatial confinement of optical phonons showed that the Raman spectra are red shifted (in wave numbers) and broadened due to the relaxation of the q-vector selection rule in finite size nanocrystals. When the crystallite is reduced to nanometer scale, the momentum selection rule will be relaxed. This allows the phonon with wave vector k = k ±π/l to participate in the first order Raman scattering, where k is the wave vector of the incident light and L is the size of the crystal. The phonon scattering will not be limited to the center of the Brillouin zone, and the phonon dispersion near the zone center must be considered. As a result, the shift, broadening and the asymmetry of the first order optical phonon can be observed. Therefore, the phenomenological phonon confinement model proposed by Richter et al. 1 which was extended to rod like geometry by Cambell and Fauchet 13, has been used to characterize and interpret the observed Raman spectra of ZnO nanorods and nanotubes. According to this Gaussian confinement model, the first order Raman spectrum, I(ω), is: 3 ω d q C(0, q) I ( ) = (1) [ ω ω( q)] + ( Γ / ) 0 where, ω(q) is the phonon dispersion curve Γ 0 the natural line width, and C (0, q) the Fourier co-efficient that describes the phonon confinement. Here, we consider a column shaped crystal i.e., ZnO nanorods and nanotubes, the Fourier co-efficient can be written as: C (0, q1, q) = exp( q1 L1 /16π ) exp( q L /16π (1/) 1 erf { iql / (3 π ) } () )

5 SAMUEL et al.: OPTICAL PHONON CONFINEMENT IN ZnO NANORODS AND NANOTUBES 707 Fig. 4 Calculated and experimental Raman spectra of ZnO nanorods Scattered line: Experimental Raman spectra. Solid line: calculated spectra, it is the sum of all contribution Fig. 6 Calculated and experimental peak of E (high) mode Fig. 7 Calculated and experimental peak of A 1 (TO) mode Fig. 5 Calculated and experimental Raman spectra of ZnO nanotubes. Scattered line: Experimental Raman spectra. Solid line: calculated spectra, it is the sum of all contribution where L 1 and L are the diameter and length of the ZnO nanorods and nanotubes. The length of the nanostructures is much larger than the diameter, which means that the confinement effect mainly occurs along the diameter direction. This confinement model is used for calculating the spectral line shapes of confined optical phonons of different symmetries such as E (high) and A 1 (TO) modes. Figures 4 and 5 show the calculated and experimental Raman spectra of ZnO nanorods and nanotubes. Fitting Eq. (1) to the experimental E (high) and A 1 (TO) modes of ZnO nanorods and nanotubes are shown in Figs 6 and 7. As the intensity of E 1 (TO) mode is extremely small, due to the little orientation deviation of the crystal lattice from the axis of the rods/tubes, fitting of confined phonon line shape and consequent estimation of unambiguous parameters is rather difficult. Three parameters are used to describe the Raman line shape: (a) (b) (c) Γ a /Γ b, the ratio of the half width at half maximum (HWHM) on the low energy side to HWHM on the high energy side, which is the measure of asymmetric broadening. Γ, the full width at half maximum ω, the difference between the zone centre and zone boundary frequencies of the phonon dispersion curve of interest. The results of the peak position, FWHM and Γ a /Γ b of ZnO nanorods and nanotubes obtained by this model are presented in Table. From which, it is

6 708 INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 010 Table Observed and calculated peak position, FWHM and Γ a /Γ b of different symmetries Sample Optical Reported Observed Calculated Observed Calculated Γ a /Γ b Phonon Peak position Peak position peak position FWHM FWHM (cm 1 ) (cm 1 ) (cm 1 ) (cm 1 ) (cm 1 ) (cm 1 ) ZnO A 1 (TO) Nanorods E (high) ZnO A 1 (TO) Nanotubes E (high) evident that there is a good agreement between the experimental and calculated Raman shift and FWHM for E (high) and A 1 (TO) mode of ZnO nanorods and nanotubes. It is obvious that the calculated FWHM of the Raman modes are similar to that of the measured values which mean that the phonon confinement effect is the only physical mechanism that causes the peak broadening. This analysis clearly see the trend that the Raman peaks [E (high) and A 1 (TO)] have a large red shift and broadening as the nanorods become transformed into nanotubes and the asymmetry of the line shapes attributed to optical phonon confinement is more precise in ZnO nanotubes. Large surface to volume ratio and smaller diameter contributed to this enhanced optical phonon confinement effect in the as prepared ZnO nanotubes. 4 Conclusions Raman spectra of ZnO nanorods and nanotubes were recorded and analyzed. The appearance of well resolved Raman peaks related to multi-phonon and resonance processes and the weak feature of E 1 (LO) peak indicate that the prepared ZnO nanorods and nanotubes were of very high optical quality. The strong red shift observed in E (high) and A 1 (TO) modes of ZnO nanorods and nanotubes can be attributed to the phonon confinement effect. The shift and broadening of the Raman peaks were calculated on the basis of the confinement model which was in good agreement with the measured spectra. This analysis clearly showed that the strong shift and broadening of the Raman peaks were dominated by the anharmonic effects originating from quantum phonon confinement effect. The broadening of the Raman peak and the asymmetry of the line shape attributed to optical phonon confinement was found to be more prominent in ZnO nanotubes than the nanorods. The present study on optical phonon confinement effects in ZnO nanorods and nanotubes should be useful to further investigation of physical properties like electron phonon interaction and scattering and be of benefit due to their applications. Acknowledgement This work was supported by Kerala State Council for Science, Technology and Environment (KSCSTE). The authors are thankful to Central Instrumentation Facility (CIF), St Berchmans College and UGC-DAE Consortium for Scientific Research, Indore. References 1 Gulseren O, Furio Ercolessi, & Erio Tosatti, Phys Rev Lett, 80 (1998) Chen Y, Bagnall D M, Koh H, et al., J Appl Phys, 84 (1998) Zin Z C, Hamberg I & Granqvist C G, J Appl Phys, 64 (1988) Yolk, Tompa G S, Liang S, et al., J Vac Sci Technol, 16 (1979) Guo L, Yang S H, Yang C L, et al., Appl Phys Lett, 76 (006) Buinitskaya G, Kulyuk L, Mirovitskii V, et al., Superlattices Microstruct, 39 (006) Umar A Karunagaran B Suh E K & Hahn Y B, Nanotechnology, 17 (006) Hsu H C, Cheng H M, Lee C Y & Hsieh W F, Nanotechnology, 17 (006) Huang M H,Mao S, Feick H, et al., Science, 9 (001) Wang Z, Zhang H, Zhang L, et al., Nanotechnology, 14 (003) Xing Y J, Xi Z H, Zue Z Q, et al., Appl Phys Lett, 83 (003) Zi J, B uscher H, Falter C,Ludwig et al., Appl Phys Lett, 69 (1996) Campbell I H & Fauchet P M, Solid State Commun, 58 (1986) Alim K A, Fonoberov V A, Shamsa M & Balandin A A, J Appl Phys, 97 (005) Demangeot F, Paillard V, Chassaing P M, et al, Appl Phys Lett, 88 (006) Heo Y W, Tien L C, Norton D P, Appl Phys Lett, 85 (004) Cullity B D, Elements of X-Ray Diffraction (Addison- Wesley: Reading, MA) nd edn Arguello A, Rousseau D L & Porto S P S, Phys Rev, 181 (1969) Cusco R, Alarcon- Llado E, Lbanez & Artius L, Phys Rev B, 75 (007) Samanta K, Bhattacharya P & Katiyar R S, Phys Rev B, 73 (006) Richter H, Wang Z P, & Ley, Solid State commun, 39 (1981) 65.