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Synthesis and Raman spectra of hammer-shaped ZnO nanostructures via thermal evaporation growth
Materials Science in Semiconductor Processing 15 (2012) 258–263
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Synthesis and Raman spectra of hammer-shaped ZnO nanostructures via thermal evaporation growth Jinfang Kong a,n, Donghua Fan b, Yufu Zhu c a b c
Department of Physics, Shanghai Institute of Technology, Shanghai 201418, PR China School of Applied Physics and Materials, Wuyi University, Jiangmen 529020, PR China Faculty of Mechanical Engineering, Huaiyin Institute of Technology, Jiangsu 223003, PR China
a r t i c l e in f o
abstract
Available online 13 November 2011
Hammer-shaped ZnO nanostructures were synthesized on silicon substrate via a simple thermal evaporation process without catalysts or additives. Scanning electron microscopy results shows that ordered ZnO nanohammers grow from the Si substrate. Transmission electron microscopy and selected area electron diffraction analysis indicate that a single nanohammer is a single crystal and grows along (0001) direction. X-ray diffraction patterns for prepared samples are consistent with a wurtzite ZnO structure. The effect of temperature on Raman scattering of single crystal ZnO nanohammers in the temperature range from 83 to 523 K was determined. Temperature-dependent Raman spectra of E2(high frequency or hf) exhibit phonon frequency redshift and linewidth broadening with increasing temperature, which can be explained by a model taking into account contributions of thermal expansion and anharmonic phonon processes. Results show that decay into three phonons is the probable channel for the E2(hf) mode. & 2011 Elsevier Ltd. All rights reserved.
Keywords: ZnO nanostructure Thermal evaporation process Raman spectra
1. Introduction One-dimensional nanostructures of semiconducting metal oxides have attracted increasing interest due to their potential application in electro-optical devices [1]. ZnO, a wide direct band gap of 3.33 eV and large exciton binding energy of 60 meV, is a versatile material and has been used considerably for its catalytic, electrical, optical, and photochemical properties [2]. The application of nanostructures depends on their morphology. It is predicated that reducing the dimensions of ZnO could enhance gas sensing, electronic, and optical properties due to the surface area increase and the quantum confinement effect [3].
n
Corresponding author: Tel.:þ 86 21 60873193. E-mail addresses: [email protected] (J. Kong), [email protected] (D. Fan), [email protected] (Y. Zhu). 1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.10.010
Structurally, zinc and oxygen atoms alternatively stacking along the c axis induce two polar surfaces of ZnO, in addition, ZnO has three fast growth directions, including [0001], [101¯0], and [2 1¯ 1¯ 0] [4]. These structural properties are helpful in the realization of many kinds of ZnO nanostructures. Many methods have been used to fabricate various ZnO nanostructures, such as the metal-organic chemical vapor deposition [5], chemical vapor deposition process [6], template method [7], solution-phase synthesis [8], and so on. Compared with these methods, thermal evaporation is also an effective method for realizing synthesis of various nanostructures. So far, various ZnO nanostructures have been prepared by the thermal evaporation process, such as nanobelts [9], nanoneedles [10], nanorods [11], nanoprisms [12], nanonails [13], nanotubes [14], and so on. However, the synthesis of a variety of ZnO nanostructures is using the metal catalyst, which affects the optical and electronic properties to restrict their application in electro-optical devices.
J. Kong et al. / Materials Science in Semiconductor Processing 15 (2012) 258–263
On the contrary, the catalyst-free growth process benefits the crystalline quality and thus electro-optical properties of the as-grown ZnO nanostructure [10]. On the other hand, it has been recognized that the electronic and optical properties of ZnO nanostructures exhibit morphology dependence, which may be employed for different fields of application. Raman scattering, as a nondestructive characterization method of choice for many recent studies of the vibrational properties of ZnO nanostructures [15], can directly probe the lattice vibration of semiconductors, which strongly affects the band structure and behavior of carriers. In this paper, we report the successful synthesis of ZnO nanohammers without any catalyst by a thermal evaporation process. Scanning electron microscopy and transmission electron microscopy research the growth of nanostructures in detail. A formation mechanism of ZnO nanohammers is also speculated. Furthermore, a comprehensive Raman investigation of temperature-dependent phonon property of E2(hf) mode in ZnO nanohammers in the temperature range from 83 to 523 K. In combination with a detailed theoretical modeling for the frequency and linewidth broadening, we have clearly illustrated the temperature effect on the phonon frequencies and linewidths in ZnO nanohammers. 2. Experimental ZnO nanohammers were synthesized in a horizontal tube furnace system by the thermal vapor deposition process. Pure Zn powder (99.99% purity) was placed into an alumina boat located at the center of a horizontal quartz tube. N-type Si wafer cleaned by sonication in ethanol and acetone was used as the substrate and was placed at 10 cm away from Zn source along the downstream position of the carrier gas. A mechanical rotary was used to evacuate from quartz tube to remove the residual oxygen before heating. Before the vapor is introduced into furnace, the pressure after evacuation by the rotary pump is about 1 Pa. After the introduction of vapor, the working press is around 160 Pa. The temperature of the furnace was raised to 510 1C for 1 h. At 480 1C, pure argon and oxygen was introduced into the quartz tube at a flow rate of 260 sccm (standard cubic centimeters per minute) and 80 sccm, respectively. After 40 min, the supplement of oxygen increases to 100 sccm. After termination of the reaction, the furnace was cooled naturally to room temperature under the atmosphere mentioned above, and the products were found on the surface of substrate. The morphology and crystalline structure of the assynthesized samples were characterized using the fieldemission scanning electron microscopy (FESEM; Philips XL30FEG) with an accelerating voltage of 5 kV, and X-ray diffraction (XRD) (Bruker/D8 Discover diffractometer with ˚ GADDS) equipped with a CuKa source (l ¼1.5406 A). High-resolution transmission electron microscope (HRTEM) (JEOL JEM-2100 F) was used to study the structural characterization. Temperature-dependent Raman spectra were recorded by a Jobin Yvon LabRAM HR 800 UV micro-Raman system under a Ar þ (514.5 nm)
259
laser excitation. The employment of a 50x optical microscope objective with a numerical aperture of 0.75 will yield a laser spot size of 2 mm. The scattered light was detected in a backscattering geometry of zðx,Þz configuration using an Andor DU420 classic charge-coupled device detector. The sample temperature was varied between 83 and 523 K with Linkam THMS600 cooling/ heating stage containing a nitrogen atmosphere. 3. Results and discussion XRD analysis has been employed to determine the structure and crystallinity of the products. Fig. 1(a) shows XRD patterns of the prepared samples. It displays relatively strong ZnO (100), (002), and (101) diffraction peaks and weak (102), (110), and (103) ones in order of increasing of degrees, revealing that prepared samples have the wurtzite ZnO structures [16]. It is found that there are no other ones besides ZnO diffraction peaks, which indicates that no impurity is introduced into ZnO nanohammers in the whole experiment. The general morphology of prepared sample is observed by FE-SEM. Fig. 1(b) displays a large-scale top view, where the formed nanostructures with high density are quasi vertical growth onto the substrate surface. SEM images in Fig. 1(c) show the high-magnification morphology of the sample. It can be clearly observed that the single nanostructure exhibits a hammer-shaped ZnO nanostructure, which consists of hammerhead and handle. Each nanohammer has a hexagonal shape, suggesting partially that the prepared nanostructures are preferentially grow along c-axis direction. To reveal the growth mechanism of nanostructures, Fig. 1(d) shows the side view of nanohammers. There is ZnO layer between nanohammers and substrate (shown by the white arrows in the Fig. 1(d)), which suggests that ZnO films firstly grow from the surface of Si substrate before forming ZnO nanohammers. The detailed structure of individual nanostructure can be characterized by TEM and selected area electron diffraction (SAED). Fig. 2(a) shows the TEM morphology of a single nanostructure, where it also displays hammershaped nanostructures and consists of hammerhead and handle. Those observations are consistent with FE-SM observation. Fig. 2(b) shows the medium-magnification image of nanohammer, indicated by the rectangle in Fig. 2(a). The inset in Fig. 2(b) clearly reveals the fringe of ZnO [0001] plane with an interplanar spacing of about 0.52 nm, which shows the perfect lattice structure, indicating that the nanorod is single crystalline and grows along the [0001] direction [17]. Fig. 2(c) shows the corresponding SAED pattern, which also testifies HRTEM observation. Vapor–liquid–solid (VLS) and vapor–solid (VS) processes are conventionally used to interpret the growth of one-dimension nanostructures [18–20]. Present experimental results cannot be explained by VLS mechanism because a typical characteristic of VLS processes is the existence of nanoclusters on the end of nanostructure [19,20]. However, SEM and TEM results do not display the corresponding nanoparticles at the end of ZnO
J. Kong et al. / Materials Science in Semiconductor Processing 15 (2012) 258–263
20
30
40 50 2 (degree)
(103)
(110)
(102)
(100) (002)
Intensity (a. u.)
(101)
260
500nm
60
200nm
600nm
Fig. 1. (a) XRD pattern of the prepared sample. SEM images of the prepared sample: (b) low-magnification top view, (c) high-magnification morphology, and (d) the side view.
Fig. 2. (a) TEM images of the single ZnO nanohammer, (b) medium-magnification image indicated by the rectangle in (a) and the corresponding HRTEM one in the inset, and (c) SAED pattern of nanorod.
[0001] (0001)
(0001)
(0010)
ZnO Films
(0001)
(0011)
Fig. 3. Schematic illustration of the growth process for the formation of ZnO nanohammers.
J. Kong et al. / Materials Science in Semiconductor Processing 15 (2012) 258–263
nanohammers and no metal catalyst in the experiment was used during the formation of nanostructure. During the experiment, there is the thin ZnO layer between ZnO nanohammers and silicon substrate. So a growth mechanism has been presumed as follows. Firstly, the yielded Zn gases by heating Zn powder are oxidized into ZnOx (x r1) due to the introduction of oxygen in the reaction system. The formed Zn and ZnOx molecules or clusters are transported on the surface of the substrate at the lowtemperature area to form ZnO layer at an early stage, as shown in Fig. 3(a). Secondly, with further evaporation, oxidization, and nucleation, there is homogeneous epitaxial growth of small ZnO nanorods from the surface of ZnO layer. The origin of various shapes of ZnO nanostructures is attributed to the relative growth rates of different crystal [4]. In the ZnO crystal, the growth rates in the different directions are diverse and are found in the order of [0001] 4½0 1 1 1 4½0 11 04½0 1 1 14½0 0 0 1 under hydrothermal condition [17]. Thus, the maximum growth rate in the ZnO crystal is along the [0001] direction. Therefore, Zn or O atoms tend to be adsorbed to the (0001) face, resulting in a preferential growth of nanorods bounded with the six crystallographic equivalentf0 1 1 0g facets along the [0001] direction, as shown in Fig. 3(b). With the increase of oxygen’s supplement from 80 to 100 sccm, the formation of more ZnOx will cause the growth of nanorod top along both the transverse direction and longitudinal one due to the adsorption of ZnOx on ZnO (0001) face, leading to the formation of nanostructures in Fig. 3(c). After the supplement of oxygen is constant, the continuous absorption of ZnOx gases on the top of nanorods can prompt the growth of nanorods along [0001] direction, as shown in Fig. 3(d). ZnO (0001) plane as a fast growing one generally tends to disappear
Raman Shift (cm-1) 400
440
and leave behind the slower growing f0 1 1 1g surface. After the growth of f0 1 1 1g surface, the (0001) plane is the most likely remaining facet [17], leading to the structural formation in Fig. 3(e). In the end, with continuous supplementation of ZnOx atoms, Zn and O tend to be adsorbed to the [0001] face, resulting in a preferential growth of nanorods with hexagonal morphology along the (0001) direction, as shown in Fig. 3(f). In addition, a formation of aligned nanostructures is generally related with the epitaxial growth of nanostructure, the assist of catalyzer, or the cowing effect [21,22]. In the present experiment, the additive also has not been used, and the thin ZnO layer can be observed between nanohammers and Si substrate. Therefore, we prefer to believe that the crowing effect and epitaxial growth should be the main factor for the formation of aligned ZnO nanostructures. Finally, micro-Raman has also been carried out to characterize the optical properties of ZnO nanohammers. We have randomly measured the Raman shift distribution from the different points of the substrate, and observed that Raman spectra are same. Under the backscattering geometry of zðx,Þz, the Raman spectra display one of the fundamental optical modes in wurtzite ZnO [23]. Fig. 4(a) shows the Raman spectra of the ZnO nanohammers from 83 to 523 K. It is noted that there are two apparent ZnO phonon modes appearing at 380 and 440 cm 1 in the spectral range of 350–500 cm 1 with a temperature of 83 K, which has been assigned to the A1(transverse optical) and E2(hf), respectively [24]. Now, we focus our attention on the temperature dependence of E2(hf) mode in ZnO nanohammers. The E2(hf) mode shift to lower frequencies and broaden with the increase of temperature in Fig. 4(a). In order to determine accurately the Raman frequencies and linewidths of the E2(hf) mode, we have used the origin
Temperature (K) 480 0
150
300
450
600 440 438
83 K
436
223 K 363 K
434
483 K
440 12 438
9
ω ω +Δω ω +Δω ω +Δω +Δω
6 3 0
150
300
450 150 600 Temperature (K)
300
436 434 450
Raman Shift (cm-1)
432
15 Linewidth (cm-1)
Raman Shift (cm-1)
Intensity (a. u.)
360
261
600
Fig. 4. Clockwise from top left: (a) temperature-dependent Raman spectra of the E2(hf) mode from a ZnO nanohammers, (b) measured Raman shifts of E2(hf) mode as a function of temperature. The solid curves are the theoretical calculation results with Eqs. (1)–(3). (c) The contributions from the different terms in Eq. (1) are displayed separately, (d) measured Raman linewidths of the E2(hf) mode as a function of temperature. The solid curves are calculated using Eq. (4).
262
J. Kong et al. / Materials Science in Semiconductor Processing 15 (2012) 258–263
Table 1 Parameters for fitting the Raman shifts and linewidths of the E2(hf) mode in ZnO nanohammers. Mode g
o0 (cm 1)
E2(hf) 1.66a 439.7 a
M1 (cm 1)
M2 (cm 1)
G0
N1 N2 (cm 1) (cm 1) (cm 1)
0.03
0.06
3.33
0.19
0.44
¨ Gruneisen parameters taken from Ref. [29].
contributions of Dod(T) and Doe(T) to the Raman shift, as shown in Fig. 4(c). It is clear that the dominant contribution comes from the effect of the linear thermal expansion of the lattice, especially in the high temperature range. Similar to the temperature-dependent Raman shift, the linewidth can be calculated by assuming again the symmetric decays into two and three phonons [27,28]:
GðTÞ ¼ G0 þ N1 ½1 þ2nðT, o0 =2Þ þ N2 ½1 þ 3nðT, o0 =3Þ 8.0 software with a Lorentz profile fitting to perform a curve fitting of the Raman spectra peaks throughout the measured temperature, since the true Raman line is Lorentzian [25]. The temperature-dependent frequencies and linewidths of the E2(hf) mode in ZnO nanohammers are presented as open triangle in Fig. 4(b) and (d). The redshift of the Raman frequency with the increase of temperature is mainly due to the effects of thermal expansion, and anharmonic coupling to other phonons [26]. In modeling the Raman shift, we can write the temperature-dependent Raman frequency o(T) as [27,28]
oðTÞ ¼ o0 þ Doe ðTÞ þ Dod ðTÞ
ð1Þ
where o0 is the harmonic frequency, Doe(T) is the contribution of thermal expansion or volume change, and Dod(T) is the one due to the anharmonic coupling to phonons of other branches. The term Doe(T) can be given by [27,28] Z T Doe ðTÞ ¼ o0 g ½ac ðT 0 Þ þ2aa ðT 0 ÞdT 0 ð2Þ 0
¨ parameter, which has been experiwith g the Gruneisen mentally determined under hydrostatic pressure recently for hexagonal ZnO [29], ac and aa the temperature-dependent coefficients of linear thermal expansion parallel and perpendicular to the hexagonal c axis, respectively [30]. Taking into account the cubic and quartic terms in the anharmonic Hamiltonian, we have the term Dod(T) as [27,28]
Dod ðTÞ ¼ M1 ½1þ nðT, o1 Þ þ nðT, o2 Þþ M2 ½1 þ3nðT, o0 =3Þ þ 3n2 ðT, o0 =3Þ
ð3Þ
where nðT, oÞ ¼ ½expð_o=kB TÞ11 is the Bose–Einstein function. In Eq. (3), the first term corresponds to the decay into two phonons of frequency o1 and o2 (three-phonon process), with o1 þ o2 ¼ o0; while the second term accounts for the decay into three phonons (four-phonon process). M1 and M2 are anharmonic constants, which are related to the relative probability of the occurrence of each process. For simplicity, in the fitting process, we only take into account the symmetric decays of the zonecenter phonons into two (three-phonon process) or three (four-phonon process) identical phonons [31]. Eqs. (1)–(3) have been used to fit the temperature dependence of Raman frequencies o(T) for the E2(hf) in ZnO nanohammers with the fitting parameters o0, M1, and M2. Fig. 4(b) shows good agreement between the theoretical fit (solid curves) and the experimental data (open triangle), and the fitting parameters are summarized in Table 1. In order to identify the different temperature-dependent behaviors of two effects, we have also presented the net
þ3n2 ðT, o0 =3Þ
ð4Þ
where G0 denotes a damping contribution due to inherent defect or impurity scattering. Anharmonic constants of N1 and N2 are the relative probability of the decay into either two or three phonons, respectively. In Fig. 4(d), we have shown the least-squares fit of Eq. (4) (solid curves) for the temperature-dependent linewidths of the E2(hf) mode in ZnO nanohammers. The fitting parameters G0, N1, and N2 have also been listed in Table 1. Good fitting has been obtained using Eq. (4) for lindwidths with temperature. From the ratios M1/M2 and N1/N2, the relative contributions of the three-phonon and four-phonon processes to the total phonon decay can be estimated. We have M1/M2 E0.50 and N1/N2 E0.43 for E2(hf) mode, indicating that the decay into four phonons is the dominating process, while the three-phonon process makes minor contribution in the anharmonic coupling of the E2(hf) mode. This observation is consistent with the calculated ZnO phonon-dispersion curves given by Serrano et al. [29], where the weak phonon density of states at o0/2 220 cm 1 represents less probability of the three-phonon process. Consequently, the probability for the E2(hf) mode to decay into two phonons is considerably reduced. The above observations permit us to have a clear understanding of the temperature effect on the phonon frequency and linewidth in ZnO nanohammers.
4. Conclusions In summary, hammer-shaped ZnO nanostructures were synthesized on silicon substrates via a simple thermal evaporation process without catalysts or additives. SEM, TEM, and SAED results show that the single crystal ZnO nanohammers grow along (0001) direction from the Si substrate. A formation mechanism of ZnO nanohammers is suggested based upon experimental observations. XRD patterns prove that the prepared samples have wurtzite ZnO crysral structure. Based on this, we have used the Raman spectroscopy to study in detail the E2(hf) mode in ZnO nanohammers under the temperature range of 83–523 K. The observed temperaturedependent phonon frequencies and linewidths of the E2(hf) mode can be well described by a model, which has taken into account the thermal expansion of the crystal lattice, and symmetric decays of phonons into two and three phonons with lower energies. It is found that the four-phonon process dominates the decay of E2(hf) mode in the measured temperature.
J. Kong et al. / Materials Science in Semiconductor Processing 15 (2012) 258–263
Acknowledgments This work is partly supported by the Natural Science Foundation of China (Contract no. 10804071), a Start-up Grant from Wuyi University, and the Special Fund of Training Outstanding Young Teachers of Shanghai University (Contract no. yyy10045). References [1] M. AbuDakka, A. Qurashi, P. Hari, M.W. Alam, Materials Science in Semiconductor Processing 13 (2010) 115. [2] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, et al., Science 292 (2001) 1897. [3] L. Cao, M.K. Li, M. Lu, W. Zhang, Q. Wei, B.Z. Liu, Materials Science in Semiconductor Processing 11 (2008) 25. [4] Z.L. Wang, X.Y. Kong, Y. Ding, P.X. Gao, W.L. Hughes, R. Yang, et al., Advanced Functional Materials 14 (2004) 943. [5] W.I. Park, G.C. Yi, J.W. Kim, S.M. Park, Applied Physical Letters 82 (2003) 4358. [6] J.J. Wu, S.C. Liu, Journal of Physical Chemistry B 106 (2002) 9546. [7] J.S. Jie, G.Z. Wang, Q.T. Wang, Y.M. Chen, X.H. Han, X.P. Wang, et al., Journal of Physical Chemistry B 108 (2004) 11976. [8] L. Vayssieres, Advanced Materials 15 (2003) 464. [9] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [10] W.I. Park, G.C. Yi, M. Kim, S.J. Pennycook, Advanced Materials 14 (2002) 1841. [11] H. Chik, J. Liang, S.G. Cloutier, N. Koukin, J.M. Xu, Applied Physical Letters 84 (2004) 3376. [12] D.F. Liu, Y.J. Xiang, Z.X. Zhang, J.X. Wang, Y. Gao, L. Song, et al., Nanotechnology 16 (2005) 2665.
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Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Synthesis and Raman spectra of hammer-shaped ZnO nanostructures via thermal evaporation growth Jinfang Kong a,n, Donghua Fan b, Yufu Zhu c a b c
Department of Physics, Shanghai Institute of Technology, Shanghai 201418, PR China School of Applied Physics and Materials, Wuyi University, Jiangmen 529020, PR China Faculty of Mechanical Engineering, Huaiyin Institute of Technology, Jiangsu 223003, PR China
a r t i c l e in f o
abstract
Available online 13 November 2011
Hammer-shaped ZnO nanostructures were synthesized on silicon substrate via a simple thermal evaporation process without catalysts or additives. Scanning electron microscopy results shows that ordered ZnO nanohammers grow from the Si substrate. Transmission electron microscopy and selected area electron diffraction analysis indicate that a single nanohammer is a single crystal and grows along (0001) direction. X-ray diffraction patterns for prepared samples are consistent with a wurtzite ZnO structure. The effect of temperature on Raman scattering of single crystal ZnO nanohammers in the temperature range from 83 to 523 K was determined. Temperature-dependent Raman spectra of E2(high frequency or hf) exhibit phonon frequency redshift and linewidth broadening with increasing temperature, which can be explained by a model taking into account contributions of thermal expansion and anharmonic phonon processes. Results show that decay into three phonons is the probable channel for the E2(hf) mode. & 2011 Elsevier Ltd. All rights reserved.
Keywords: ZnO nanostructure Thermal evaporation process Raman spectra
1. Introduction One-dimensional nanostructures of semiconducting metal oxides have attracted increasing interest due to their potential application in electro-optical devices [1]. ZnO, a wide direct band gap of 3.33 eV and large exciton binding energy of 60 meV, is a versatile material and has been used considerably for its catalytic, electrical, optical, and photochemical properties [2]. The application of nanostructures depends on their morphology. It is predicated that reducing the dimensions of ZnO could enhance gas sensing, electronic, and optical properties due to the surface area increase and the quantum confinement effect [3].
n
Corresponding author: Tel.:þ 86 21 60873193. E-mail addresses: [email protected] (J. Kong), [email protected] (D. Fan), [email protected] (Y. Zhu). 1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.10.010
Structurally, zinc and oxygen atoms alternatively stacking along the c axis induce two polar surfaces of ZnO, in addition, ZnO has three fast growth directions, including [0001], [101¯0], and [2 1¯ 1¯ 0] [4]. These structural properties are helpful in the realization of many kinds of ZnO nanostructures. Many methods have been used to fabricate various ZnO nanostructures, such as the metal-organic chemical vapor deposition [5], chemical vapor deposition process [6], template method [7], solution-phase synthesis [8], and so on. Compared with these methods, thermal evaporation is also an effective method for realizing synthesis of various nanostructures. So far, various ZnO nanostructures have been prepared by the thermal evaporation process, such as nanobelts [9], nanoneedles [10], nanorods [11], nanoprisms [12], nanonails [13], nanotubes [14], and so on. However, the synthesis of a variety of ZnO nanostructures is using the metal catalyst, which affects the optical and electronic properties to restrict their application in electro-optical devices.
J. Kong et al. / Materials Science in Semiconductor Processing 15 (2012) 258–263
On the contrary, the catalyst-free growth process benefits the crystalline quality and thus electro-optical properties of the as-grown ZnO nanostructure [10]. On the other hand, it has been recognized that the electronic and optical properties of ZnO nanostructures exhibit morphology dependence, which may be employed for different fields of application. Raman scattering, as a nondestructive characterization method of choice for many recent studies of the vibrational properties of ZnO nanostructures [15], can directly probe the lattice vibration of semiconductors, which strongly affects the band structure and behavior of carriers. In this paper, we report the successful synthesis of ZnO nanohammers without any catalyst by a thermal evaporation process. Scanning electron microscopy and transmission electron microscopy research the growth of nanostructures in detail. A formation mechanism of ZnO nanohammers is also speculated. Furthermore, a comprehensive Raman investigation of temperature-dependent phonon property of E2(hf) mode in ZnO nanohammers in the temperature range from 83 to 523 K. In combination with a detailed theoretical modeling for the frequency and linewidth broadening, we have clearly illustrated the temperature effect on the phonon frequencies and linewidths in ZnO nanohammers. 2. Experimental ZnO nanohammers were synthesized in a horizontal tube furnace system by the thermal vapor deposition process. Pure Zn powder (99.99% purity) was placed into an alumina boat located at the center of a horizontal quartz tube. N-type Si wafer cleaned by sonication in ethanol and acetone was used as the substrate and was placed at 10 cm away from Zn source along the downstream position of the carrier gas. A mechanical rotary was used to evacuate from quartz tube to remove the residual oxygen before heating. Before the vapor is introduced into furnace, the pressure after evacuation by the rotary pump is about 1 Pa. After the introduction of vapor, the working press is around 160 Pa. The temperature of the furnace was raised to 510 1C for 1 h. At 480 1C, pure argon and oxygen was introduced into the quartz tube at a flow rate of 260 sccm (standard cubic centimeters per minute) and 80 sccm, respectively. After 40 min, the supplement of oxygen increases to 100 sccm. After termination of the reaction, the furnace was cooled naturally to room temperature under the atmosphere mentioned above, and the products were found on the surface of substrate. The morphology and crystalline structure of the assynthesized samples were characterized using the fieldemission scanning electron microscopy (FESEM; Philips XL30FEG) with an accelerating voltage of 5 kV, and X-ray diffraction (XRD) (Bruker/D8 Discover diffractometer with ˚ GADDS) equipped with a CuKa source (l ¼1.5406 A). High-resolution transmission electron microscope (HRTEM) (JEOL JEM-2100 F) was used to study the structural characterization. Temperature-dependent Raman spectra were recorded by a Jobin Yvon LabRAM HR 800 UV micro-Raman system under a Ar þ (514.5 nm)
259
laser excitation. The employment of a 50x optical microscope objective with a numerical aperture of 0.75 will yield a laser spot size of 2 mm. The scattered light was detected in a backscattering geometry of zðx,Þz configuration using an Andor DU420 classic charge-coupled device detector. The sample temperature was varied between 83 and 523 K with Linkam THMS600 cooling/ heating stage containing a nitrogen atmosphere. 3. Results and discussion XRD analysis has been employed to determine the structure and crystallinity of the products. Fig. 1(a) shows XRD patterns of the prepared samples. It displays relatively strong ZnO (100), (002), and (101) diffraction peaks and weak (102), (110), and (103) ones in order of increasing of degrees, revealing that prepared samples have the wurtzite ZnO structures [16]. It is found that there are no other ones besides ZnO diffraction peaks, which indicates that no impurity is introduced into ZnO nanohammers in the whole experiment. The general morphology of prepared sample is observed by FE-SEM. Fig. 1(b) displays a large-scale top view, where the formed nanostructures with high density are quasi vertical growth onto the substrate surface. SEM images in Fig. 1(c) show the high-magnification morphology of the sample. It can be clearly observed that the single nanostructure exhibits a hammer-shaped ZnO nanostructure, which consists of hammerhead and handle. Each nanohammer has a hexagonal shape, suggesting partially that the prepared nanostructures are preferentially grow along c-axis direction. To reveal the growth mechanism of nanostructures, Fig. 1(d) shows the side view of nanohammers. There is ZnO layer between nanohammers and substrate (shown by the white arrows in the Fig. 1(d)), which suggests that ZnO films firstly grow from the surface of Si substrate before forming ZnO nanohammers. The detailed structure of individual nanostructure can be characterized by TEM and selected area electron diffraction (SAED). Fig. 2(a) shows the TEM morphology of a single nanostructure, where it also displays hammershaped nanostructures and consists of hammerhead and handle. Those observations are consistent with FE-SM observation. Fig. 2(b) shows the medium-magnification image of nanohammer, indicated by the rectangle in Fig. 2(a). The inset in Fig. 2(b) clearly reveals the fringe of ZnO [0001] plane with an interplanar spacing of about 0.52 nm, which shows the perfect lattice structure, indicating that the nanorod is single crystalline and grows along the [0001] direction [17]. Fig. 2(c) shows the corresponding SAED pattern, which also testifies HRTEM observation. Vapor–liquid–solid (VLS) and vapor–solid (VS) processes are conventionally used to interpret the growth of one-dimension nanostructures [18–20]. Present experimental results cannot be explained by VLS mechanism because a typical characteristic of VLS processes is the existence of nanoclusters on the end of nanostructure [19,20]. However, SEM and TEM results do not display the corresponding nanoparticles at the end of ZnO
J. Kong et al. / Materials Science in Semiconductor Processing 15 (2012) 258–263
20
30
40 50 2 (degree)
(103)
(110)
(102)
(100) (002)
Intensity (a. u.)
(101)
260
500nm
60
200nm
600nm
Fig. 1. (a) XRD pattern of the prepared sample. SEM images of the prepared sample: (b) low-magnification top view, (c) high-magnification morphology, and (d) the side view.
Fig. 2. (a) TEM images of the single ZnO nanohammer, (b) medium-magnification image indicated by the rectangle in (a) and the corresponding HRTEM one in the inset, and (c) SAED pattern of nanorod.
[0001] (0001)
(0001)
(0010)
ZnO Films
(0001)
(0011)
Fig. 3. Schematic illustration of the growth process for the formation of ZnO nanohammers.
J. Kong et al. / Materials Science in Semiconductor Processing 15 (2012) 258–263
nanohammers and no metal catalyst in the experiment was used during the formation of nanostructure. During the experiment, there is the thin ZnO layer between ZnO nanohammers and silicon substrate. So a growth mechanism has been presumed as follows. Firstly, the yielded Zn gases by heating Zn powder are oxidized into ZnOx (x r1) due to the introduction of oxygen in the reaction system. The formed Zn and ZnOx molecules or clusters are transported on the surface of the substrate at the lowtemperature area to form ZnO layer at an early stage, as shown in Fig. 3(a). Secondly, with further evaporation, oxidization, and nucleation, there is homogeneous epitaxial growth of small ZnO nanorods from the surface of ZnO layer. The origin of various shapes of ZnO nanostructures is attributed to the relative growth rates of different crystal [4]. In the ZnO crystal, the growth rates in the different directions are diverse and are found in the order of [0001] 4½0 1 1 1 4½0 11 04½0 1 1 14½0 0 0 1 under hydrothermal condition [17]. Thus, the maximum growth rate in the ZnO crystal is along the [0001] direction. Therefore, Zn or O atoms tend to be adsorbed to the (0001) face, resulting in a preferential growth of nanorods bounded with the six crystallographic equivalentf0 1 1 0g facets along the [0001] direction, as shown in Fig. 3(b). With the increase of oxygen’s supplement from 80 to 100 sccm, the formation of more ZnOx will cause the growth of nanorod top along both the transverse direction and longitudinal one due to the adsorption of ZnOx on ZnO (0001) face, leading to the formation of nanostructures in Fig. 3(c). After the supplement of oxygen is constant, the continuous absorption of ZnOx gases on the top of nanorods can prompt the growth of nanorods along [0001] direction, as shown in Fig. 3(d). ZnO (0001) plane as a fast growing one generally tends to disappear
Raman Shift (cm-1) 400
440
and leave behind the slower growing f0 1 1 1g surface. After the growth of f0 1 1 1g surface, the (0001) plane is the most likely remaining facet [17], leading to the structural formation in Fig. 3(e). In the end, with continuous supplementation of ZnOx atoms, Zn and O tend to be adsorbed to the [0001] face, resulting in a preferential growth of nanorods with hexagonal morphology along the (0001) direction, as shown in Fig. 3(f). In addition, a formation of aligned nanostructures is generally related with the epitaxial growth of nanostructure, the assist of catalyzer, or the cowing effect [21,22]. In the present experiment, the additive also has not been used, and the thin ZnO layer can be observed between nanohammers and Si substrate. Therefore, we prefer to believe that the crowing effect and epitaxial growth should be the main factor for the formation of aligned ZnO nanostructures. Finally, micro-Raman has also been carried out to characterize the optical properties of ZnO nanohammers. We have randomly measured the Raman shift distribution from the different points of the substrate, and observed that Raman spectra are same. Under the backscattering geometry of zðx,Þz, the Raman spectra display one of the fundamental optical modes in wurtzite ZnO [23]. Fig. 4(a) shows the Raman spectra of the ZnO nanohammers from 83 to 523 K. It is noted that there are two apparent ZnO phonon modes appearing at 380 and 440 cm 1 in the spectral range of 350–500 cm 1 with a temperature of 83 K, which has been assigned to the A1(transverse optical) and E2(hf), respectively [24]. Now, we focus our attention on the temperature dependence of E2(hf) mode in ZnO nanohammers. The E2(hf) mode shift to lower frequencies and broaden with the increase of temperature in Fig. 4(a). In order to determine accurately the Raman frequencies and linewidths of the E2(hf) mode, we have used the origin
Temperature (K) 480 0
150
300
450
600 440 438
83 K
436
223 K 363 K
434
483 K
440 12 438
9
ω ω +Δω ω +Δω ω +Δω +Δω
6 3 0
150
300
450 150 600 Temperature (K)
300
436 434 450
Raman Shift (cm-1)
432
15 Linewidth (cm-1)
Raman Shift (cm-1)
Intensity (a. u.)
360
261
600
Fig. 4. Clockwise from top left: (a) temperature-dependent Raman spectra of the E2(hf) mode from a ZnO nanohammers, (b) measured Raman shifts of E2(hf) mode as a function of temperature. The solid curves are the theoretical calculation results with Eqs. (1)–(3). (c) The contributions from the different terms in Eq. (1) are displayed separately, (d) measured Raman linewidths of the E2(hf) mode as a function of temperature. The solid curves are calculated using Eq. (4).
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Table 1 Parameters for fitting the Raman shifts and linewidths of the E2(hf) mode in ZnO nanohammers. Mode g
o0 (cm 1)
E2(hf) 1.66a 439.7 a
M1 (cm 1)
M2 (cm 1)
G0
N1 N2 (cm 1) (cm 1) (cm 1)
0.03
0.06
3.33
0.19
0.44
¨ Gruneisen parameters taken from Ref. [29].
contributions of Dod(T) and Doe(T) to the Raman shift, as shown in Fig. 4(c). It is clear that the dominant contribution comes from the effect of the linear thermal expansion of the lattice, especially in the high temperature range. Similar to the temperature-dependent Raman shift, the linewidth can be calculated by assuming again the symmetric decays into two and three phonons [27,28]:
GðTÞ ¼ G0 þ N1 ½1 þ2nðT, o0 =2Þ þ N2 ½1 þ 3nðT, o0 =3Þ 8.0 software with a Lorentz profile fitting to perform a curve fitting of the Raman spectra peaks throughout the measured temperature, since the true Raman line is Lorentzian [25]. The temperature-dependent frequencies and linewidths of the E2(hf) mode in ZnO nanohammers are presented as open triangle in Fig. 4(b) and (d). The redshift of the Raman frequency with the increase of temperature is mainly due to the effects of thermal expansion, and anharmonic coupling to other phonons [26]. In modeling the Raman shift, we can write the temperature-dependent Raman frequency o(T) as [27,28]
oðTÞ ¼ o0 þ Doe ðTÞ þ Dod ðTÞ
ð1Þ
where o0 is the harmonic frequency, Doe(T) is the contribution of thermal expansion or volume change, and Dod(T) is the one due to the anharmonic coupling to phonons of other branches. The term Doe(T) can be given by [27,28] Z T Doe ðTÞ ¼ o0 g ½ac ðT 0 Þ þ2aa ðT 0 ÞdT 0 ð2Þ 0
¨ parameter, which has been experiwith g the Gruneisen mentally determined under hydrostatic pressure recently for hexagonal ZnO [29], ac and aa the temperature-dependent coefficients of linear thermal expansion parallel and perpendicular to the hexagonal c axis, respectively [30]. Taking into account the cubic and quartic terms in the anharmonic Hamiltonian, we have the term Dod(T) as [27,28]
Dod ðTÞ ¼ M1 ½1þ nðT, o1 Þ þ nðT, o2 Þþ M2 ½1 þ3nðT, o0 =3Þ þ 3n2 ðT, o0 =3Þ
ð3Þ
where nðT, oÞ ¼ ½expð_o=kB TÞ11 is the Bose–Einstein function. In Eq. (3), the first term corresponds to the decay into two phonons of frequency o1 and o2 (three-phonon process), with o1 þ o2 ¼ o0; while the second term accounts for the decay into three phonons (four-phonon process). M1 and M2 are anharmonic constants, which are related to the relative probability of the occurrence of each process. For simplicity, in the fitting process, we only take into account the symmetric decays of the zonecenter phonons into two (three-phonon process) or three (four-phonon process) identical phonons [31]. Eqs. (1)–(3) have been used to fit the temperature dependence of Raman frequencies o(T) for the E2(hf) in ZnO nanohammers with the fitting parameters o0, M1, and M2. Fig. 4(b) shows good agreement between the theoretical fit (solid curves) and the experimental data (open triangle), and the fitting parameters are summarized in Table 1. In order to identify the different temperature-dependent behaviors of two effects, we have also presented the net
þ3n2 ðT, o0 =3Þ
ð4Þ
where G0 denotes a damping contribution due to inherent defect or impurity scattering. Anharmonic constants of N1 and N2 are the relative probability of the decay into either two or three phonons, respectively. In Fig. 4(d), we have shown the least-squares fit of Eq. (4) (solid curves) for the temperature-dependent linewidths of the E2(hf) mode in ZnO nanohammers. The fitting parameters G0, N1, and N2 have also been listed in Table 1. Good fitting has been obtained using Eq. (4) for lindwidths with temperature. From the ratios M1/M2 and N1/N2, the relative contributions of the three-phonon and four-phonon processes to the total phonon decay can be estimated. We have M1/M2 E0.50 and N1/N2 E0.43 for E2(hf) mode, indicating that the decay into four phonons is the dominating process, while the three-phonon process makes minor contribution in the anharmonic coupling of the E2(hf) mode. This observation is consistent with the calculated ZnO phonon-dispersion curves given by Serrano et al. [29], where the weak phonon density of states at o0/2 220 cm 1 represents less probability of the three-phonon process. Consequently, the probability for the E2(hf) mode to decay into two phonons is considerably reduced. The above observations permit us to have a clear understanding of the temperature effect on the phonon frequency and linewidth in ZnO nanohammers.
4. Conclusions In summary, hammer-shaped ZnO nanostructures were synthesized on silicon substrates via a simple thermal evaporation process without catalysts or additives. SEM, TEM, and SAED results show that the single crystal ZnO nanohammers grow along (0001) direction from the Si substrate. A formation mechanism of ZnO nanohammers is suggested based upon experimental observations. XRD patterns prove that the prepared samples have wurtzite ZnO crysral structure. Based on this, we have used the Raman spectroscopy to study in detail the E2(hf) mode in ZnO nanohammers under the temperature range of 83–523 K. The observed temperaturedependent phonon frequencies and linewidths of the E2(hf) mode can be well described by a model, which has taken into account the thermal expansion of the crystal lattice, and symmetric decays of phonons into two and three phonons with lower energies. It is found that the four-phonon process dominates the decay of E2(hf) mode in the measured temperature.
J. Kong et al. / Materials Science in Semiconductor Processing 15 (2012) 258–263
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