- Email: [email protected]
A comparison between optical properties of TiO2 nanowires obtained by EMA method and experiment
Physica B 406 (2011) 3383–3388
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
A comparison between optical properties of TiO2 nanowires obtained by EMA method and experiment S. Ramezani Sani a,n, A. Morteza Ali a, R. Jafari b a b
Laboratory of Surface Physics, Department of Physics, Alzahra University, Tehran 19938, Iran Department of Physics, Shahid Beheshti University, G.C., Evin, Tehran 19839, Iran
a r t i c l e i n f o
abstract
Article history: Received 26 October 2010 Received in revised form 18 May 2011 Accepted 1 June 2011 Available online 7 June 2011
Optical properties of TiO2 nanowires, synthesized by two-step thermal evaporation process, have been studied experimentally and theoretically. Based on the theoretical method optical constants of nanowires have been calculated with the use of the effective medium approximation (EMA). As evidenced by X-ray diffraction patterns our synthesized nanowires, whose diameters and lengths were within the ranges of 50–90 nm and 500–1500 nm, respectively, were found to be crystalline rutile TiO2 with the major refraction being along the (1 1 0) direction. The experimental data of the reflectance of TiO2 nanowires has been obtained using spectrometer in wavelength 250–800 nm, and then, compared with the spectrum of reflectance predicted by the EMA theoretical model. Our measured experimental optical data has been found to be in good accord with our predicted results spectrum with the use of the EMA modeling; this agreement indicates that our estimation of the volume fraction from atomic force microscopy (AFM) data was accurate. Published by Elsevier B.V.
Keywords: Optical properties TiO2 nanowires Effective medium approximation Reflectance
1. Introduction In recent years, TiO2 one-dimensional (1D) nanostructures have attracted considerable attention because of their various applications such as gas sensors [1], optical devices [2,3], photocatalysis [4–6] and solar cells [7]. The technological significance of TiO2 nanowires is a direct consequence of their 1D shape which gives them some new optical and electrical properties, making them notably beneficial for many applications. It is difficult to synthesize TiO2 nanowires by PVD (physical vapor deposition) method. Thus, there are some reports on the synthesis of TiO2 nanowires by chemical methods such as sol–gel electrophoresis and hydrothermal methods [8–11]. However, improving the crystallinity of TiO2 nanowires synthesized by these chemical methods requires further heat treatment, which increases the cost of the synthesis process and is time-consuming. Moreover, high amounts of contaminations in these methods adversely affect optical and electrical properties of the synthesized nanowires. Another method of synthesizing TiO2 nanowires is thermal evaporation process for which there have been only a few reports thus far [12–16]. Optical properties of 1D nanostructures are important for both traditional and emerging technologies. To design an optical coating it is indispensable to have a precise model of the variation of its optical properties. One of these models is the effective medium
n
Corresponding author. Tel.: þ9821 22940335; fax: þ9821 88047861. E-mail address: [email protected] (S. Ramezani Sani).
0921-4526/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.physb.2011.06.003
approximation (EMA), which is used to calculate optical constants of discontinues thin film. Coatings composed of particles much smaller than the wavelength of light can be modeled by EMA [17–19]. However, there are only a few reports on the use of EMA to calculate optical properties of other shapes of nanostructures. For example, Baxter and Schmuttenmaer [20] studied electrical conductivity of ZnO nanowires, nanoparticles and nanofilms through using time-resolved terahertz spectroscopy, and reached the conclusion that it is appropriate to use EMA. In this study, TiO2 nanowires were synthesized by a thermal evaporation process, and their optical properties have been calculated with the use of the EMA analysis. Moreover, our experimentally measured reflectance data of TiO2 nanowires have been compared with the spectrum predicted reflectance by the EMA model.
2. Effective media approximation (EMA) When the size of particles is much smaller than the wavelength of light, optical properties can be modeled by EMA [21,22]. In our EMA analysis, we used Bruggeman’s method which is given by [21] ep e em e f þ ð1f Þ ¼0 ð1Þ ep þ ke em þ ke where f is the particle’s volume fraction, e is the composite permittivity, em is the matrix permittivity, ep is the particle’s permittivity, and k is a geometric factor which is: (i) k¼1 for an
3384
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
array of cylinders with their axes collinear with the incident radiation, and (ii) k ¼2 for spherical nanoparticles. In Eq. (1), if we consider em for air to be equal to unity, and for nanowires assume k¼1, then we have ep e 1e f þð1f Þ ¼0 ð2Þ ep þ 1e 1þ 1e
where E is photon energy, _ is the Plank’s constant divided by 2p, and c is the speed of light. Using the real and imaginary parts of the refractive index the normal reflectance can be calculated as below
where ep is the permittivity of the bulk TiO2, and e is the TiO2/air composite’s permittivity. The real and imaginary parts of the permittivity, i.e. e1 and e2, respectively, can be obtained through using Eq. (2). The refractive index n, and extinction coefficient k, can be obtained in terms of e
Consequently, the optical parameters of a material can be determined from the EMA analysis.
e1 ¼ n2 k2 , e2 ¼ 2nk
With the use of a two-step thermal evaporation method TiO2 nanowires were successfully synthesized on a p-type silicon substrate with the orientation [1 0 0] and the resistivity of 2–5 O cm. Si wafers were ultrasonically cleaned using acetone and ethanol for 10 min. In the first step a Ti buffer layer with a thickness of 50 nm was deposited on each substrate by the e-beam evapora˚ under a pressure tion technique with the evaporation rate of 1.5 A/s of 10 5 mbar. In this step, voltage and current were held to be 4 kV and 100 mA, respectively. A gold thin film with the thickness of 3 nm was deposited as a catalyst on the Ti layer by a sputtering system. In the second step a mixture of Ti and graphite powder with the ratio of 1:1 was placed in a quartz boat as the source material. The boat was located at the center of a horizontal tube furnace in the high-temperature (HT) zone, and the Si substrate was placed over the quartz boat in the low-temperature (LT) zone (800–850 1C). The tube was heated with the heating rate of 20 1C/min up to a temperature 1050 1C, and then, held for 1 h at the same temperature under the atmospheric pressure. The vapor species were generated from the source material by reaction; consequently, they were transported to the substrate and finally they condensed on the surface of the substrate. As a result, this condensed vapor followed up the lower index planes of the seeds to form a 1-D product in order to decrease its surface energy. Then, the substrate was cooled down to room temperature. Thereafter, the sample was taken out for characterization. The structure and morphology of the sample were analyzed by X-ray diffraction (XRD: JEOL JDX-8030), by scanning electron microscopy (SEM: XL130, Philips) and a scanning probe microscopy (SPM: Dualscope/Rasterscope C26, DME, Denmark). The reflectance spectrum of the TiO2 nanowires was examined by spectrophotometer (Ocean Optics-HR4000 CG-UV-NIR). In Section 4.2 we will calculate optical constants by
ð3Þ
The absorption coefficient, a, is defined as
a¼
2E k _c
ð4Þ
Fig. 1. X-ray diffraction pattern shows TiO2 nanowires to be in rutile phase. Au diffraction peaks are the results of the diffraction from the catalysts.
R¼
1n2 þ k2 1þ n2 þ k2
ð5Þ
3. Experimental details
Fig. 2. SEM images of as synthesized TiO2 nanowires. Inset is a close-up view of nanowires. Scale bar for the inset is 500 nm.
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
3385
the EMA theoretical model, and then compare them with optical characteristics obtained from spectrophotometer.
4. Results and discussion 4.1. Structural properties Fig. 1 shows typical XRD patterns of TiO2 nanowires grown on the Si substrate. Our X-ray diffraction results show that the TiO2 nanowires were mainly composed of rutile phase (JCPDS: 73-1765) with a dominant reflectance of the (1 1 0) plane. This observation indicates that the (1 1 0) plan is the most thermodynamically stable plane in TiO2 rutile structure. The unit cell constants of ˚ The Au diffraction TiO2 rutile phase are a¼ b¼4.52 A˚ and c¼2.98 A. peaks (JCPDS: 04-0784) were caused by the diffraction from the catalyst. The scanning electron microscopy (SEM) images of the TiO2 nanowires have been represented in Fig. 2. The SEM image of the nanowires grown on the Au/Ti/Si substrate shows the growth of the nanowires with diameters of 50–90 nm and lengths of 500–1500 nm. Since Au was used as a catalyst the growth of the nanowires was mainly dominated by the vapor–liquid–solid (VLS) mechanism. The Ti buffer layer was partially oxidized into a TiO2 layer by the O2 gas of atmosphere at an elevated temperature. The Ti layer deposited on the silicon substrate diffused into the solid Au catalyst; then, a liquid alloy of Au–Ti was formed. The interfacial region between the TiO2 layer and the liquid Au catalyst formed by the buffer layer-assisted VLS mechanism was in a high-energy state; hence, it provided an active nucleation site for the TiO2 nanowires. The Ti buffer was introduced onto the surface of the Si substrate to prevent any undesirable diffusion reaction between the Ti vapor source and the Si substrate, and also to provide a good nucleation site for the growth of the nanowires. The morphology of TiO2 nanowires was characterized by AFM (Fig. 3). A three-dimensional view of the surface roughness and the height of the surface in the Z-direction are illustrated in the left and right columns of the figure, respectively. It can be inferred from the figure that the roughness is 83.96 nm. 4.2. Optical properties by the EMA method In the EMA method the complex dielectric constant of TiO2/air composite can be calculated from Eq. (2). In this equation it is necessary to know the real and imaginary parts of the dielectric constant of bulk TiO2 and the volume fraction (f) of its nanostructures. The latter is determined by using roughness and correlation
Fig. 4. Dielectric constants of TiO2 nanowires: (a) real part of dielectric constants; (b) imaginary part of dielectric constants.
Fig. 3. AFM images of TiO2 nanowires. Left column shows 3D image rough surface and the right column shows surface height in Z-direction. It shows roughness to be 83.96 nm.
3386
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
function. It was estimated that the nanowires volume fraction in our prepared sample was 70%. Thereafter, with the use of the bulk TiO2 optical constant [23] and f, optical parameters were determined from Eqs. (2)–(5). Fig. 4 shows the spectra e1 and e2 of TiO2/air composite. Both e1 and e2 constants show significant decrease with respect to the bulk TiO2, duo to the depression of the transparency coefficient of TiO2 nanowires and also from scattering effects. For the light incident in the range of E44.2 eV, we have e1 o0; that is, there is a high amount of reflection in this range. It can be seen that the TiO2 nanowires have less reflectance in comparison with the bulk TiO2. These results are in good agreement with the reflectance spectrum shown in Fig. 6. The appearance of the peak energy at 4 eV indicates an interband transition in the energy level (d) of Ti. The decrease of the peak energy at 4 eV confirms the nanostructural properties of the TiO2 nanowires, which is in accordance with the results of Ref. [24]. From Eq. (3), the refractive index was estimated to be around 2.5 at 3 eV (Fig. 5(a)). It can be inferred from Fig. 5(a) that the refractive index of the nanowires decreases from its corresponding value for the bulk material. This reduction can be attributed to
the variation of the packing density [25]. The packing density of TiO2 nanowires is less than the bulk TiO2. The decrease of the refractive index and packing density in the TiO2 nanowires can be ascribed to their porous structure as shown in the SEM images.
Fig. 6. Reflectance for TiO2 nanowires as the function of wavelength.
Table 1 Charecterization and volume fraction nanowires under different system pressure.
Fig. 5. (a) Refractive index and (b) absorption coefficient of TiO2 nanowires.
System pressure (mbar)
Nanowire diameters (nm)
Nanowire lengths (nm)
Volume fractions of Porosity nanowires (f) (1 f)
60 60 30
50–90 45–90 40–80
500–1500 300–1100 300–2500
0.71 0.65 0.63
0.29 0.35 0.37
Fig. 7. Reflectance for TiO2 nanowires as a function wavelength under different system pressure.
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
3387
the nanowires and also an increase in voids’ roughness. This result is in agreement with the results reported by Keis and Roos [27]; they studied optical characteristics of ZnO and TiO2 nanostructures, and observed the same reflectance spectrum for TiO2 nanostructures [27]. It can be seen from Fig. 6 that our measured experimental optical data are in excellent agreement with the predictions made by the EMA modeling, which indicates that our estimation of the volume fraction is correct. In the other experiments, with other parameters (i.e., reaction time, heating and cooling rate, substrate temperature, etc.) unchanged, system pressure was varied from 1000 to 30 mbar. Thereafter, characterization and volume fraction samples were analyzed that listed in Table 1. Fig. 7 shows the measured reflection spectra using spectrophotometer for samples with different volume fraction of TiO2 nanowires. As we can see in Fig. 7, with a decrease in the volume fraction of nanowires, reflectance spectrum also decreases duo to the increase in the porosity and decrease in the nanowire size. It can be seen from Table 1 that with decrease in the system pressure from 1000 to 30 mbar, nanowire diameter decreases, thus indicating the porosity increase. Fig. 8 shows the reflectance spectra (dashed lines) calculated by EMA and the measured reflection spectra (solid lines) by spectrophotometer for two samples. By comparing the results of theoretical and experimental reflection spectra, it seems that our estimation of the volume fraction of the nanowires is good.
5. Conclusion
Fig. 8. Comparing the experimental reflectance with the calculated reflectance for nanowires under different system pressures: (a) system pressure 60 mbar, (b) system pressure 30 mbar.
TiO2 nanowires were grown by thermal evaporation on a silicon substrate coated with a catalytic Au-coated Ti layer. The nanowires analyzed with XRD, SEM and SPM were found to be in rutile phase with the major reflection along the (1 1 0) direction. Diameters and lengths of TiO2 nanowires were in the ranges of 50–90 nm and 500–1500 nm, respectively. The growth of the nanowires was dominantly governed by the vapor–liquid–solid (VLS) mechanism because Au was used as a catalyst. Optical properties of TiO2 nanowires were studied experimentally and theoretically. In the theoretical method optical constants were deduced by using the effective medium approximation (EMA). The reflectance spectrum of the TiO2 nanowire is lesser than that of the corresponding bulk material, as the roughness and scattering of surface of a nanowire are more than those of the corresponding bulk material. An excellent agreement has been found between our experimentally obtained optical data and the theoretical results of the EMA modeling. References
The absorption spectrum of the TiO2 nanowires calculated from Eq. (4) has been shown in Fig. 5(b). It can be found that in comparison with the bulk TiO2 the absorption edge in the TiO2 nanowires shifted to higher energies (blue shift). The blue shift in the absorption coefficient of TiO2 nanowires has been claimed to be a consequence of exciton confinement with the decrease in nanowires’ size [26]. Fig. 6 shows the optical reflectance curves of the TiO2 nanowires in the wavelength range of 250–800 nm. The dashed line shows the reflectance spectrum calculated by Eq. (5), and the solid line is the reflectance curve measured by spectrophotometer. The amount of the reflection in the visible region is about 16–24% for the TiO2 nanowires, which is less than the reflectance spectrum of the bulk TiO2 (30–34% in the dotted line) in the same region. The smaller amount of the reflection of the nanowires than that of the bulk is due to the scattering from the surface of
[1] A. Rothschild, A. Evakov, Y. Shapira, N. Ashkenasy, Y. Komen, Surf. Sci. 419 (2003) 532. [2] E.G.J. Wijnhoven, W.L. Vos, Science 281 (1998) 802. [3] A. Richel, N.P. Johnson, D. McComb, Appl. Phys. Lett. 76 (2000) 1816. [4] S.P. Albu, A. Ghicov, J.M. Macak, R. Hahn, P. Schmuki, Nano Lett. 7 (2007) 862. [5] A.R. Armstrong, G. Armstrong, J. Canales, R. Garcia, P.G. Bruce, Adv. Mater. 17 (2005) 862. [6] G. Armstrong, A.R. Armstrong, J. Canales, R. Garcia, P.G. Bruce, Electrochem. Solid Lett. 9 A (2006) 139. [7] N.G. Park, J. Van de Lagemaat, A.J. Frank, J. Phys. Chem. B. 104 (2000) 8989. [8] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 69. [9] K.Y. Dong, Y.K. Dong, Nano Lett. 3 (2003) 207. [10] G.H. Du, Q. Chen, R.C. Che, Z.Y. Yuan, L.M. Peng, Appl. Phys. Lett. 79 (2001) 3702. [11] L. Miao, S. Tanemura, S. Toh, K. Kaneko, M. Tanemura, J. Cryst. Growth 264 (2004) 246. [12] J. Wu, H. Shin, W. Wu, Y. Tseng, I. Chen, J. Cryst. Growth 281 (2005) 384. [13] J. Wu, W. Wu, H. Shin, J. Electrochem. Soc. 152 (2005) G613. [14] J. Wu, H. Shin, W. Wu, J. Vac. Sci. Technol. B 23 (2005) 2122. [15] H. Zhang, X. Luo, J. Xu, B. Xiang, D. Yu, J. Phys. Chem. B 108 (2004) 14866. [16] J. Wu, H. Shin, W. Wu, Nanotechnology 17 (2006) 105.
3388
[17] [18] [19] [20] [21] [22]
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
E. Aspnes, E. Kinsbron, D.D. Bacon, Phys. Rev. B 21 (1980) 3290. G. Granqvist, O. Hunderi, Phys. Rev. B 16 (1997) 3513. H. Savaloni, A.R. Khakpour, Eur. Phys. J. Appl. Phys. 31 (2005) 101. J.B. Baxter, C.A. Schmuttenmaer, J. Phys. Chem. B 110 (2006) 25229. G.A. Nikasson, C.G. Granqvist, J. Appl. Phys. 55 (1984) 3382. G.A. Niklasson, Mater. Sci. Sol. Energy Convers. Syst. (1991) 7–43.
[23] [24] [25] [26] [27]
H.J. Cho, H. Boek, Appl. Opt. 35 (1996) 5552. M.M. Abdel-Aziz, et al., J. Appl. Surf. Sci. 252 (2006) 8163. W. Heitman, Thin Solid Films 5 (1970) 61. L.Q. Jing, Z.L. Xu, X.J. Sun, et al., Appl. Surf. Sci. 180 (2001) 308. K. Keis, A. Roos, J. Opt. Mater. 20 (2002) 35.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
A comparison between optical properties of TiO2 nanowires obtained by EMA method and experiment S. Ramezani Sani a,n, A. Morteza Ali a, R. Jafari b a b
Laboratory of Surface Physics, Department of Physics, Alzahra University, Tehran 19938, Iran Department of Physics, Shahid Beheshti University, G.C., Evin, Tehran 19839, Iran
a r t i c l e i n f o
abstract
Article history: Received 26 October 2010 Received in revised form 18 May 2011 Accepted 1 June 2011 Available online 7 June 2011
Optical properties of TiO2 nanowires, synthesized by two-step thermal evaporation process, have been studied experimentally and theoretically. Based on the theoretical method optical constants of nanowires have been calculated with the use of the effective medium approximation (EMA). As evidenced by X-ray diffraction patterns our synthesized nanowires, whose diameters and lengths were within the ranges of 50–90 nm and 500–1500 nm, respectively, were found to be crystalline rutile TiO2 with the major refraction being along the (1 1 0) direction. The experimental data of the reflectance of TiO2 nanowires has been obtained using spectrometer in wavelength 250–800 nm, and then, compared with the spectrum of reflectance predicted by the EMA theoretical model. Our measured experimental optical data has been found to be in good accord with our predicted results spectrum with the use of the EMA modeling; this agreement indicates that our estimation of the volume fraction from atomic force microscopy (AFM) data was accurate. Published by Elsevier B.V.
Keywords: Optical properties TiO2 nanowires Effective medium approximation Reflectance
1. Introduction In recent years, TiO2 one-dimensional (1D) nanostructures have attracted considerable attention because of their various applications such as gas sensors [1], optical devices [2,3], photocatalysis [4–6] and solar cells [7]. The technological significance of TiO2 nanowires is a direct consequence of their 1D shape which gives them some new optical and electrical properties, making them notably beneficial for many applications. It is difficult to synthesize TiO2 nanowires by PVD (physical vapor deposition) method. Thus, there are some reports on the synthesis of TiO2 nanowires by chemical methods such as sol–gel electrophoresis and hydrothermal methods [8–11]. However, improving the crystallinity of TiO2 nanowires synthesized by these chemical methods requires further heat treatment, which increases the cost of the synthesis process and is time-consuming. Moreover, high amounts of contaminations in these methods adversely affect optical and electrical properties of the synthesized nanowires. Another method of synthesizing TiO2 nanowires is thermal evaporation process for which there have been only a few reports thus far [12–16]. Optical properties of 1D nanostructures are important for both traditional and emerging technologies. To design an optical coating it is indispensable to have a precise model of the variation of its optical properties. One of these models is the effective medium
n
Corresponding author. Tel.: þ9821 22940335; fax: þ9821 88047861. E-mail address: [email protected] (S. Ramezani Sani).
0921-4526/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.physb.2011.06.003
approximation (EMA), which is used to calculate optical constants of discontinues thin film. Coatings composed of particles much smaller than the wavelength of light can be modeled by EMA [17–19]. However, there are only a few reports on the use of EMA to calculate optical properties of other shapes of nanostructures. For example, Baxter and Schmuttenmaer [20] studied electrical conductivity of ZnO nanowires, nanoparticles and nanofilms through using time-resolved terahertz spectroscopy, and reached the conclusion that it is appropriate to use EMA. In this study, TiO2 nanowires were synthesized by a thermal evaporation process, and their optical properties have been calculated with the use of the EMA analysis. Moreover, our experimentally measured reflectance data of TiO2 nanowires have been compared with the spectrum predicted reflectance by the EMA model.
2. Effective media approximation (EMA) When the size of particles is much smaller than the wavelength of light, optical properties can be modeled by EMA [21,22]. In our EMA analysis, we used Bruggeman’s method which is given by [21] ep e em e f þ ð1f Þ ¼0 ð1Þ ep þ ke em þ ke where f is the particle’s volume fraction, e is the composite permittivity, em is the matrix permittivity, ep is the particle’s permittivity, and k is a geometric factor which is: (i) k¼1 for an
3384
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
array of cylinders with their axes collinear with the incident radiation, and (ii) k ¼2 for spherical nanoparticles. In Eq. (1), if we consider em for air to be equal to unity, and for nanowires assume k¼1, then we have ep e 1e f þð1f Þ ¼0 ð2Þ ep þ 1e 1þ 1e
where E is photon energy, _ is the Plank’s constant divided by 2p, and c is the speed of light. Using the real and imaginary parts of the refractive index the normal reflectance can be calculated as below
where ep is the permittivity of the bulk TiO2, and e is the TiO2/air composite’s permittivity. The real and imaginary parts of the permittivity, i.e. e1 and e2, respectively, can be obtained through using Eq. (2). The refractive index n, and extinction coefficient k, can be obtained in terms of e
Consequently, the optical parameters of a material can be determined from the EMA analysis.
e1 ¼ n2 k2 , e2 ¼ 2nk
With the use of a two-step thermal evaporation method TiO2 nanowires were successfully synthesized on a p-type silicon substrate with the orientation [1 0 0] and the resistivity of 2–5 O cm. Si wafers were ultrasonically cleaned using acetone and ethanol for 10 min. In the first step a Ti buffer layer with a thickness of 50 nm was deposited on each substrate by the e-beam evapora˚ under a pressure tion technique with the evaporation rate of 1.5 A/s of 10 5 mbar. In this step, voltage and current were held to be 4 kV and 100 mA, respectively. A gold thin film with the thickness of 3 nm was deposited as a catalyst on the Ti layer by a sputtering system. In the second step a mixture of Ti and graphite powder with the ratio of 1:1 was placed in a quartz boat as the source material. The boat was located at the center of a horizontal tube furnace in the high-temperature (HT) zone, and the Si substrate was placed over the quartz boat in the low-temperature (LT) zone (800–850 1C). The tube was heated with the heating rate of 20 1C/min up to a temperature 1050 1C, and then, held for 1 h at the same temperature under the atmospheric pressure. The vapor species were generated from the source material by reaction; consequently, they were transported to the substrate and finally they condensed on the surface of the substrate. As a result, this condensed vapor followed up the lower index planes of the seeds to form a 1-D product in order to decrease its surface energy. Then, the substrate was cooled down to room temperature. Thereafter, the sample was taken out for characterization. The structure and morphology of the sample were analyzed by X-ray diffraction (XRD: JEOL JDX-8030), by scanning electron microscopy (SEM: XL130, Philips) and a scanning probe microscopy (SPM: Dualscope/Rasterscope C26, DME, Denmark). The reflectance spectrum of the TiO2 nanowires was examined by spectrophotometer (Ocean Optics-HR4000 CG-UV-NIR). In Section 4.2 we will calculate optical constants by
ð3Þ
The absorption coefficient, a, is defined as
a¼
2E k _c
ð4Þ
Fig. 1. X-ray diffraction pattern shows TiO2 nanowires to be in rutile phase. Au diffraction peaks are the results of the diffraction from the catalysts.
R¼
1n2 þ k2 1þ n2 þ k2
ð5Þ
3. Experimental details
Fig. 2. SEM images of as synthesized TiO2 nanowires. Inset is a close-up view of nanowires. Scale bar for the inset is 500 nm.
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
3385
the EMA theoretical model, and then compare them with optical characteristics obtained from spectrophotometer.
4. Results and discussion 4.1. Structural properties Fig. 1 shows typical XRD patterns of TiO2 nanowires grown on the Si substrate. Our X-ray diffraction results show that the TiO2 nanowires were mainly composed of rutile phase (JCPDS: 73-1765) with a dominant reflectance of the (1 1 0) plane. This observation indicates that the (1 1 0) plan is the most thermodynamically stable plane in TiO2 rutile structure. The unit cell constants of ˚ The Au diffraction TiO2 rutile phase are a¼ b¼4.52 A˚ and c¼2.98 A. peaks (JCPDS: 04-0784) were caused by the diffraction from the catalyst. The scanning electron microscopy (SEM) images of the TiO2 nanowires have been represented in Fig. 2. The SEM image of the nanowires grown on the Au/Ti/Si substrate shows the growth of the nanowires with diameters of 50–90 nm and lengths of 500–1500 nm. Since Au was used as a catalyst the growth of the nanowires was mainly dominated by the vapor–liquid–solid (VLS) mechanism. The Ti buffer layer was partially oxidized into a TiO2 layer by the O2 gas of atmosphere at an elevated temperature. The Ti layer deposited on the silicon substrate diffused into the solid Au catalyst; then, a liquid alloy of Au–Ti was formed. The interfacial region between the TiO2 layer and the liquid Au catalyst formed by the buffer layer-assisted VLS mechanism was in a high-energy state; hence, it provided an active nucleation site for the TiO2 nanowires. The Ti buffer was introduced onto the surface of the Si substrate to prevent any undesirable diffusion reaction between the Ti vapor source and the Si substrate, and also to provide a good nucleation site for the growth of the nanowires. The morphology of TiO2 nanowires was characterized by AFM (Fig. 3). A three-dimensional view of the surface roughness and the height of the surface in the Z-direction are illustrated in the left and right columns of the figure, respectively. It can be inferred from the figure that the roughness is 83.96 nm. 4.2. Optical properties by the EMA method In the EMA method the complex dielectric constant of TiO2/air composite can be calculated from Eq. (2). In this equation it is necessary to know the real and imaginary parts of the dielectric constant of bulk TiO2 and the volume fraction (f) of its nanostructures. The latter is determined by using roughness and correlation
Fig. 4. Dielectric constants of TiO2 nanowires: (a) real part of dielectric constants; (b) imaginary part of dielectric constants.
Fig. 3. AFM images of TiO2 nanowires. Left column shows 3D image rough surface and the right column shows surface height in Z-direction. It shows roughness to be 83.96 nm.
3386
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
function. It was estimated that the nanowires volume fraction in our prepared sample was 70%. Thereafter, with the use of the bulk TiO2 optical constant [23] and f, optical parameters were determined from Eqs. (2)–(5). Fig. 4 shows the spectra e1 and e2 of TiO2/air composite. Both e1 and e2 constants show significant decrease with respect to the bulk TiO2, duo to the depression of the transparency coefficient of TiO2 nanowires and also from scattering effects. For the light incident in the range of E44.2 eV, we have e1 o0; that is, there is a high amount of reflection in this range. It can be seen that the TiO2 nanowires have less reflectance in comparison with the bulk TiO2. These results are in good agreement with the reflectance spectrum shown in Fig. 6. The appearance of the peak energy at 4 eV indicates an interband transition in the energy level (d) of Ti. The decrease of the peak energy at 4 eV confirms the nanostructural properties of the TiO2 nanowires, which is in accordance with the results of Ref. [24]. From Eq. (3), the refractive index was estimated to be around 2.5 at 3 eV (Fig. 5(a)). It can be inferred from Fig. 5(a) that the refractive index of the nanowires decreases from its corresponding value for the bulk material. This reduction can be attributed to
the variation of the packing density [25]. The packing density of TiO2 nanowires is less than the bulk TiO2. The decrease of the refractive index and packing density in the TiO2 nanowires can be ascribed to their porous structure as shown in the SEM images.
Fig. 6. Reflectance for TiO2 nanowires as the function of wavelength.
Table 1 Charecterization and volume fraction nanowires under different system pressure.
Fig. 5. (a) Refractive index and (b) absorption coefficient of TiO2 nanowires.
System pressure (mbar)
Nanowire diameters (nm)
Nanowire lengths (nm)
Volume fractions of Porosity nanowires (f) (1 f)
60 60 30
50–90 45–90 40–80
500–1500 300–1100 300–2500
0.71 0.65 0.63
0.29 0.35 0.37
Fig. 7. Reflectance for TiO2 nanowires as a function wavelength under different system pressure.
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
3387
the nanowires and also an increase in voids’ roughness. This result is in agreement with the results reported by Keis and Roos [27]; they studied optical characteristics of ZnO and TiO2 nanostructures, and observed the same reflectance spectrum for TiO2 nanostructures [27]. It can be seen from Fig. 6 that our measured experimental optical data are in excellent agreement with the predictions made by the EMA modeling, which indicates that our estimation of the volume fraction is correct. In the other experiments, with other parameters (i.e., reaction time, heating and cooling rate, substrate temperature, etc.) unchanged, system pressure was varied from 1000 to 30 mbar. Thereafter, characterization and volume fraction samples were analyzed that listed in Table 1. Fig. 7 shows the measured reflection spectra using spectrophotometer for samples with different volume fraction of TiO2 nanowires. As we can see in Fig. 7, with a decrease in the volume fraction of nanowires, reflectance spectrum also decreases duo to the increase in the porosity and decrease in the nanowire size. It can be seen from Table 1 that with decrease in the system pressure from 1000 to 30 mbar, nanowire diameter decreases, thus indicating the porosity increase. Fig. 8 shows the reflectance spectra (dashed lines) calculated by EMA and the measured reflection spectra (solid lines) by spectrophotometer for two samples. By comparing the results of theoretical and experimental reflection spectra, it seems that our estimation of the volume fraction of the nanowires is good.
5. Conclusion
Fig. 8. Comparing the experimental reflectance with the calculated reflectance for nanowires under different system pressures: (a) system pressure 60 mbar, (b) system pressure 30 mbar.
TiO2 nanowires were grown by thermal evaporation on a silicon substrate coated with a catalytic Au-coated Ti layer. The nanowires analyzed with XRD, SEM and SPM were found to be in rutile phase with the major reflection along the (1 1 0) direction. Diameters and lengths of TiO2 nanowires were in the ranges of 50–90 nm and 500–1500 nm, respectively. The growth of the nanowires was dominantly governed by the vapor–liquid–solid (VLS) mechanism because Au was used as a catalyst. Optical properties of TiO2 nanowires were studied experimentally and theoretically. In the theoretical method optical constants were deduced by using the effective medium approximation (EMA). The reflectance spectrum of the TiO2 nanowire is lesser than that of the corresponding bulk material, as the roughness and scattering of surface of a nanowire are more than those of the corresponding bulk material. An excellent agreement has been found between our experimentally obtained optical data and the theoretical results of the EMA modeling. References
The absorption spectrum of the TiO2 nanowires calculated from Eq. (4) has been shown in Fig. 5(b). It can be found that in comparison with the bulk TiO2 the absorption edge in the TiO2 nanowires shifted to higher energies (blue shift). The blue shift in the absorption coefficient of TiO2 nanowires has been claimed to be a consequence of exciton confinement with the decrease in nanowires’ size [26]. Fig. 6 shows the optical reflectance curves of the TiO2 nanowires in the wavelength range of 250–800 nm. The dashed line shows the reflectance spectrum calculated by Eq. (5), and the solid line is the reflectance curve measured by spectrophotometer. The amount of the reflection in the visible region is about 16–24% for the TiO2 nanowires, which is less than the reflectance spectrum of the bulk TiO2 (30–34% in the dotted line) in the same region. The smaller amount of the reflection of the nanowires than that of the bulk is due to the scattering from the surface of
[1] A. Rothschild, A. Evakov, Y. Shapira, N. Ashkenasy, Y. Komen, Surf. Sci. 419 (2003) 532. [2] E.G.J. Wijnhoven, W.L. Vos, Science 281 (1998) 802. [3] A. Richel, N.P. Johnson, D. McComb, Appl. Phys. Lett. 76 (2000) 1816. [4] S.P. Albu, A. Ghicov, J.M. Macak, R. Hahn, P. Schmuki, Nano Lett. 7 (2007) 862. [5] A.R. Armstrong, G. Armstrong, J. Canales, R. Garcia, P.G. Bruce, Adv. Mater. 17 (2005) 862. [6] G. Armstrong, A.R. Armstrong, J. Canales, R. Garcia, P.G. Bruce, Electrochem. Solid Lett. 9 A (2006) 139. [7] N.G. Park, J. Van de Lagemaat, A.J. Frank, J. Phys. Chem. B. 104 (2000) 8989. [8] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 69. [9] K.Y. Dong, Y.K. Dong, Nano Lett. 3 (2003) 207. [10] G.H. Du, Q. Chen, R.C. Che, Z.Y. Yuan, L.M. Peng, Appl. Phys. Lett. 79 (2001) 3702. [11] L. Miao, S. Tanemura, S. Toh, K. Kaneko, M. Tanemura, J. Cryst. Growth 264 (2004) 246. [12] J. Wu, H. Shin, W. Wu, Y. Tseng, I. Chen, J. Cryst. Growth 281 (2005) 384. [13] J. Wu, W. Wu, H. Shin, J. Electrochem. Soc. 152 (2005) G613. [14] J. Wu, H. Shin, W. Wu, J. Vac. Sci. Technol. B 23 (2005) 2122. [15] H. Zhang, X. Luo, J. Xu, B. Xiang, D. Yu, J. Phys. Chem. B 108 (2004) 14866. [16] J. Wu, H. Shin, W. Wu, Nanotechnology 17 (2006) 105.
3388
[17] [18] [19] [20] [21] [22]
S. Ramezani Sani et al. / Physica B 406 (2011) 3383–3388
E. Aspnes, E. Kinsbron, D.D. Bacon, Phys. Rev. B 21 (1980) 3290. G. Granqvist, O. Hunderi, Phys. Rev. B 16 (1997) 3513. H. Savaloni, A.R. Khakpour, Eur. Phys. J. Appl. Phys. 31 (2005) 101. J.B. Baxter, C.A. Schmuttenmaer, J. Phys. Chem. B 110 (2006) 25229. G.A. Nikasson, C.G. Granqvist, J. Appl. Phys. 55 (1984) 3382. G.A. Niklasson, Mater. Sci. Sol. Energy Convers. Syst. (1991) 7–43.
[23] [24] [25] [26] [27]
H.J. Cho, H. Boek, Appl. Opt. 35 (1996) 5552. M.M. Abdel-Aziz, et al., J. Appl. Surf. Sci. 252 (2006) 8163. W. Heitman, Thin Solid Films 5 (1970) 61. L.Q. Jing, Z.L. Xu, X.J. Sun, et al., Appl. Surf. Sci. 180 (2001) 308. K. Keis, A. Roos, J. Opt. Mater. 20 (2002) 35.