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Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires
TSF-34301; No of Pages 5 Thin Solid Films xxx (2015) xxx–xxx
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Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires Sunghoon Park a, Soohyun Kim a, Seungbok Choi b, Sangmin Lee c, Chongmu Lee a,⁎ a b c
Department of Materials Science and Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea Department of Mechanical Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea Department of Electrical Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea
a r t i c l e
i n f o
Available online xxxx Keywords: Zinc tin oxide Nanowires Poly-methyl methacrylate Coating Photoluminescence
a b s t r a c t Polymethyl methacrylate (PMMA)-coated Zn2SnO4 nanowires were synthesized by a two-step process: thermal evaporation of a mixture of Zn and Sn powders at 700 °C in an oxidizing atmosphere followed by spin coating of PMMA solution. The nanowires were 30–80 nm in diameter and up to a few hundred of micrometers in length. Photoluminescence showed that the near-band edge (NBE) emission-to-visible emission ratio of Zn2SnO4 was enhanced significantly by PMMA coating. The highest NBE emission-to-visible emission ratio was obtained at a PMMA concentration of 0.25 mM. INBE/IDL was increased almost 10 times by coating the nanowires with 0.25-mM PMMA. The enhanced NBE emission and suppressed visible emission might be due to enhanced excitonic emission efficiency. © 2015 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a promising candidate for applications to optoelectronic devices such as short-wavelength light emitting diodes (LEDs) and laser diodes (LDs) [1,2]. The optical properties of ZnO can be tailored by controlling dimension, morphology and doping with other elements. Recently, studies of the optical properties of ZnO nanostructures doped with elements such as Al, Ga, In, Sn, and Sb have been carried out to tailor the optical and gas sensing properties of ZnO [3–5]. In particular, the ternary semiconductor zinc stannate (Zn2SnO4 or ZnSnO3) formed by doping ZnO with Sn has attractive optical properties as well as high electron mobility, high electrical conductivity which make it suitable for a wide range of applications, such as transparent conducting oxides for photovoltaic devices, flat panel displays, smart windows, architectural windows, and polymer-based electronics and sensors for humidity and combustible gases [6,7]. Zn2SnO4 exhibits two characteristic PL emissions: near band edge (NBE) emission in the ultraviolet (UV) region and visible emission in the green region [8]. Of these two emissions, the former is generally dominant. According to recent study, the photoluminescence (PL) spectrum of Zn2SnO4 exhibits a weak ultraviolet (UV) emission peak at ~382 nm and a strong green emission peak at ~525 nm at room temperature [9,10]. Over the past few decades considerable effort has been made to enhance the NBE emission and suppress deep level (DL) emission of ZnO, because strong NBE emission is essential for realizing highperformance short-wavelength optoelectronic devices. A range of ⁎ Corresponding author. Tel.: +82 32 860 7536; fax: +82 32 862 5546. E-mail address: [email protected] (C. Lee).
techniques such as thermal annealing in a hydrogen atmosphere [11], hydrogen [12] or argon plasma treatment [13], hydrogen [14] or gallium-doping [15], and thin-film coating [16–30] of ZnO nanostructures have been studied to enhance the NBE emission and suppress the visible emission of ZnO nanostructures. In particular, in the case of ZnO one-dimensional nanostructures, encapsulation or capping techniques, using a variety of coating materials including metals [16–20], ceramics [21–31] and polymers [32,33], have been studied widely to enhance NBE emission. Poly-methyl methacrylate (PMMA) [32] and polyaniline have been evaluated as a polymer coating material [33]. This study examined the effects of PMMA coating on the PL of Zn2SnO4 nanowires. In particular, the effects of PMMA concentration on the NBE emission intensity of PMMA-coated Zn2SnO4 nanowires were compared and the low temperature PL properties were examined. 2. Experimental details PMMA-coated Zn2SnO4 nanowires were prepared using a two-step process: thermal evaporation of a mixture of Zn and Sn powders (Zn:Sn = 8:2) in an oxidizing atmosphere followed by spin coating of PMMA solution. A 3 nm-thick gold (Au)-coated p-type (100) Si substrate was placed on top of an alumina boat containing a mixture of Zn and Sn powders positioned at the center of a quartz tube furnace. The furnace was heated to 700 °C and maintained at that temperature at a constant total pressure of 133 Pa using a mixture of N2 and O2 gases for 1 h. The flow rates of N2 and O2 gases were 98 and 2 cm3/min, respectively. Subsequently, 1 mM/0.5 mM/0.25 mM/0.1 mM PMMA solution (solvent: Toluene) was spin-coated on the ZnSnO3 nanowires at a
http://dx.doi.org/10.1016/j.tsf.2015.04.064 0040-6090/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064
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spinning rate of 3000 rpm for 30 s. The PMMA-spin coated nanowires were then dried in vacuum at 100 °C for 10 min. The morphology and structure of the products were examined by scanning electron microscopy (SEM, Hitachi S-4200) at an operating voltage of 10 kV and transmission electron microscopy (TEM). Highresolution TEM (HRTEM, JEOL 2100F) and corresponding selected area electron diffraction (SAED) were performed at an accelerating voltage of 300 kV. For further examination of the crystal structure of the
nanowire samples glancing angle X-ray diffraction (XRD, Philips X'Pert MRD) was performed on using Cu-Kα radiation (λ = 0.1541 nm) at a scan rate of 4°/min and at a glancing angle of 0.5° with a rotating detector. The room-temperature PL measurements (SPEC-1403 PL spectrometer) were carried out using a He–Cd laser (325 nm, 55 MW) as the excitation source with an excitation density of ~ 100 mW/cm2 on the sample. Low-temperature PL measurements were also carried out in a helium cryostat between 10 and 300 K.
Fig. 1. SEM images of PMMA-coated Zn2SnO4 nanowires with different PMMA concentrations: (a) Zn2SnO4 nanowires without PMMA, (b) 0.1 mM-PMMA, (c) 0.25 mM-PMMA, (d) 0.5 mM-PMMA and (e) 1.0 mM-PMMA.
Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064
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3. Results and discussion Fig. 1(a) shows a SEM image of the Zn2SnO4 nanowires synthesized by thermal evaporation and Fig. 1(b)–(e) shows SEM images of the Zn2SnO4 nanowires coated with different concentrations of PMMA. The nanowires were 30–80 nm in diameter and up to a few hundred of micrometers in length. A particle was observed at the tip of each nanowire, suggesting vapor–liquid–solid growth of the nanowires. The PMMA-coated Zn2SnO4 nanowires with the highest PMMA concentration (Fig. 1(e)) showed significant agglomeration of nanowires with glue-like PMMA, whereas those with lower PMMA concentrations (Fig. 1(a)–(c)) did not. Fig. 2(a) presents an XRD pattern of the 0.25 mM PMMA-coated ZnSnO3 nanowires. Many sharp reflection peak characteristics of Zn2SnO4, ZnSnO3, ZnO and SnO2 were identified, indicating that the nanowires comprise those four phases. The larger number of reflection peaks from the Zn2SnO4 phase than those from the other phases suggests that the Zn2SnO4 phase is dominant among those four phases. A low-magnification TEM image of a typical Zn2SnO4 nanowire (Fig. 2(b)) shows that the diameter of the nanowire and the thickness of the PMMA layer on the Zn2SnO4 nanowires were ~ 45 nm and ~32 nm, respectively. Fig. 2(c) and (d) presents an HRTEM image and corresponding SAED pattern of a typical Zn2SnO4 nanowire, respectively. The spotty pattern was assigned to a face-centered cubic-structured Zn2SnO4 single crystal (JCPDS # 74-2184, a = 0.865 nm). Clear spots observed in the SAED pattern (Fig. 2(d)) showed that the nanowire was a Zn2SnO4 single crystal. Room-temperature PL measurements were carried out to examine dependence of the luminescence properties of PMMA-coated Zn2SnO4 and ZnO nanowires on the PMMA concentration. The pristine Zn2SnO4
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nanowires showed the typical PL spectrum of Zn2SnO4 with an NBE emission band centered at ~378 nm and a broad DL emission band centered ~555 nm (Fig. 3(a)). In contrast, the PMMA-coated Zn2SnO4 nanowires exhibited an NBE emission band centered at ~378 nm, a broad DL emission band centered ~569 nm and a shoulder at ~760 nm (Fig. 3(a)). In other words, PMMA coating resulted in a ~14 nm red-shift in the NBE emission and a new shoulder in the red region. A more important change in PL spectrum by PMMA coating was a significant increase in NBE emission intensity. The PL properties of PMMA-coated ZnO nanowires are basically similar to those of PMMA-coated Zn2SnO4 nanowires. The PMMAcoated ZnO nanowires also showed NBE emission peak at ~ 378 nm, but significantly different DL emission behavior. The DL emission peak of the PMMA-coated ZnO nanowire sample tended to red-shift as the PMMA concentration increases (Fig. 3(b)). Fig. 3(c) and (d) compares the NBE emission intensity and NBE-toDL emission intensity ratio, INBE/IDL, respectively, of the PMMA-coated Zn2SnO4 and ZnO nanowire samples. Overall, the PMMA-coated Zn2SnO4 nanowire sample showed higher NBE intensity and higher INBE/IDL ratio than the PMMA-coated ZnO nanowire sample regardless of the PMMA concentration. The Zn2SnO4 nanowire sample showed a maximum INBE/IDL ratio at a PMMA concentration of 0.25 mM. The INBE/IDL ratios of the Zn2SnO4 and ZnO nanowires were increased ~ 10 and ~ 2 fold, respectively, by coating the nanowires with 0.25-mM PMMA. In addition, the INBE/IDL ratio of the PMMA-coated Zn2SnO4 nanowire sample was almost three times larger than that of the PMMA-coated ZnO nanowire sample at 0.25 mM of PMMA despite the INBE/IDL ratio of pristine Zn2SnO4 nanowires is lower than that of pristine ZnO nanowires. The enhanced NBE emission of the 0.25 mM of PMMAcoated ZnO nanowire sample might be due to the significantly improved excitonic emission efficiency [34]. The excitonic emission
Fig. 2. (a) XRD pattern of low magnification-TEM image of PMMA-coated Zn2SnO4 nanowires. (b) Low magnification-TEM image of PMMA-coated Zn2SnO4 nanowires. (c) HRTEM image of PMMA-coated Zn2SnO4 nanowires. (d) SAED pattern corresponding to (c).
Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064
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Fig. 3. (a) Room temperature-PL spectra of PMMA-coated Zn2SnO4 nanowires with different PMMA concentrations. (b) Room temperature-PL spectra of PMMA-coated ZnO nanowires with different PMMA concentrations. (c) NBE emission intensities of PMMA-coated Zn2SnO4 and ZnO nanowires as a function of the PMMA concentration. (d) Intensity ratios of NBE emission to DL PMMA-coated Zn2SnO4 and ZnO nanowires as a function of the PMMA concentration.
efficiency appeared to be much higer in the Zn2SnO4 nanowires than in the ZnO nanowires due to PMMA-functionalization. Low-temperature PL measurements were carried out to determine the origin of the enhanced PL properties of the PMMA-coated Zn2SnO4 nanowires. Fig. 4(a) and (b) presents the temperature-dependent NBE emission spectra of the Zn2SnO4 nanowires with and without PMMA. Basically, the two samples showed similar changes with increasing temperature, but some differences in the PL intensities and wavelengths. The PL curves of the two samples showed two characteristic peaks at ~374 nm and ~377 nm, which were assigned to free-to-bound recombination (FB) and excitons bound to donor (DX), respectively [35–38]. The PMMA-coated Zn2SnO4 nanowires showed a stronger FB band than the DX band in intensity, whereas the pristine Zn2SnO4 nanowires showed a stronger DX band than the FB band. A close examination revealed the wavelength of the DX peak of the PMMA-coated Zn2SnO4 nanowires to be slightly smaller than that of the pristine Zn2SnO4 nanowires at the same temperature. For both samples, as the temperature increased, the DX peak intensity decreased and the peak shifted slightly towards larger wavelength. A comparison of Fig. 4(a) and (b) showed that the two peaks shifted slightly towards shorter wavelength after coating the Zn2SnO4 nanowires with PMMA. Fig. 4(c) presents the NBE emission spectra of Zn2SnO4 nanowires with and without PMMA at T = 10 K. A broad asymmetric peak centered at ~377 nm dominated the near band-edge spectrum of the PMMA-coated Zn2SnO4 nanowires. Richter et al. assigned the main peak in the low-temperature NBE emission spectra of PMMA-coated ZnO nanowires to a surface exciton band, based on the excitation density dependent time resolved measurement results and reported significantly enhanced UV excitonic emission and suppressed green luminescence in the PL spectra [34]. Liu et al. obtained similar results for PMMA-coated vertically aligned ZnO nanowire arrays [35]. Further PL measurements including the excitation density dependent on time resolved measurements of the PMMA-coated Zn2SnO4 nanowires will be needed to identify the tall asymmetric peak at
~ 377 nm, but we believe that the main peak centered at ~ 377 nm in Fig. 4(c) was assigned to surface exciton. Therefore, based on the room-temperature PL data (Fig. 3(a)) and the low-temperature PL data (Fig. 4(a)–(c)), PMMA coating enhances UV excitonic emission and suppresses green luminescence of Zn2SnO4 nanowires significantly. However, the origin of the enhanced UV excitonic emission of the PMMA-functionalized semiconductor nanostructures is not completely understood at present, but it originates from the surface modification of the radiation-induced polymerization [39]. The oxygen on Zn2SnO4 surfaces might initiate graft polymerization during laser irradiation for PL measurement. The radiation produced σ anion free radicals on the ZnO nanoparticle surface, and σ anion free radicals are highly active. Brailsford and Morton [40] suggested that the σ anion free radical species originate from decomposition or are produced by trapped electrons reacting with oxygen atoms. 4. Conclusions The NBE emission and DL emission of Zn2SnO4 nanowires are enhanced and suppressed, respectively, by coating them with PMMA. The highest INBE/IDL was obtained for a PMMA concentration of 0.25 mM. The INBE/IDL was increased almost 10 fold by coating the nanowires with 0.25-mM PMMA. This enhanced NBE emission might be due to improved excitonic emission efficiency. This suggests that Zn2SnO4 nanowire is a promising candidate for applications in fabricating short-wavelength optical or optoelectronic devices. Acknowledgment This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2010-0020163).
Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064
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Fig. 4. Temperature-dependent low-temperature PL spectra of Zn2SnO4 nanowires (a) with and (b) without PMMA. (c) Low-temperature PL spectra of PMMA-coated Zn2SnO4 nanowires with and without PMMA at 10 K.
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Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires Sunghoon Park a, Soohyun Kim a, Seungbok Choi b, Sangmin Lee c, Chongmu Lee a,⁎ a b c
Department of Materials Science and Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea Department of Mechanical Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea Department of Electrical Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea
a r t i c l e
i n f o
Available online xxxx Keywords: Zinc tin oxide Nanowires Poly-methyl methacrylate Coating Photoluminescence
a b s t r a c t Polymethyl methacrylate (PMMA)-coated Zn2SnO4 nanowires were synthesized by a two-step process: thermal evaporation of a mixture of Zn and Sn powders at 700 °C in an oxidizing atmosphere followed by spin coating of PMMA solution. The nanowires were 30–80 nm in diameter and up to a few hundred of micrometers in length. Photoluminescence showed that the near-band edge (NBE) emission-to-visible emission ratio of Zn2SnO4 was enhanced significantly by PMMA coating. The highest NBE emission-to-visible emission ratio was obtained at a PMMA concentration of 0.25 mM. INBE/IDL was increased almost 10 times by coating the nanowires with 0.25-mM PMMA. The enhanced NBE emission and suppressed visible emission might be due to enhanced excitonic emission efficiency. © 2015 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a promising candidate for applications to optoelectronic devices such as short-wavelength light emitting diodes (LEDs) and laser diodes (LDs) [1,2]. The optical properties of ZnO can be tailored by controlling dimension, morphology and doping with other elements. Recently, studies of the optical properties of ZnO nanostructures doped with elements such as Al, Ga, In, Sn, and Sb have been carried out to tailor the optical and gas sensing properties of ZnO [3–5]. In particular, the ternary semiconductor zinc stannate (Zn2SnO4 or ZnSnO3) formed by doping ZnO with Sn has attractive optical properties as well as high electron mobility, high electrical conductivity which make it suitable for a wide range of applications, such as transparent conducting oxides for photovoltaic devices, flat panel displays, smart windows, architectural windows, and polymer-based electronics and sensors for humidity and combustible gases [6,7]. Zn2SnO4 exhibits two characteristic PL emissions: near band edge (NBE) emission in the ultraviolet (UV) region and visible emission in the green region [8]. Of these two emissions, the former is generally dominant. According to recent study, the photoluminescence (PL) spectrum of Zn2SnO4 exhibits a weak ultraviolet (UV) emission peak at ~382 nm and a strong green emission peak at ~525 nm at room temperature [9,10]. Over the past few decades considerable effort has been made to enhance the NBE emission and suppress deep level (DL) emission of ZnO, because strong NBE emission is essential for realizing highperformance short-wavelength optoelectronic devices. A range of ⁎ Corresponding author. Tel.: +82 32 860 7536; fax: +82 32 862 5546. E-mail address: [email protected] (C. Lee).
techniques such as thermal annealing in a hydrogen atmosphere [11], hydrogen [12] or argon plasma treatment [13], hydrogen [14] or gallium-doping [15], and thin-film coating [16–30] of ZnO nanostructures have been studied to enhance the NBE emission and suppress the visible emission of ZnO nanostructures. In particular, in the case of ZnO one-dimensional nanostructures, encapsulation or capping techniques, using a variety of coating materials including metals [16–20], ceramics [21–31] and polymers [32,33], have been studied widely to enhance NBE emission. Poly-methyl methacrylate (PMMA) [32] and polyaniline have been evaluated as a polymer coating material [33]. This study examined the effects of PMMA coating on the PL of Zn2SnO4 nanowires. In particular, the effects of PMMA concentration on the NBE emission intensity of PMMA-coated Zn2SnO4 nanowires were compared and the low temperature PL properties were examined. 2. Experimental details PMMA-coated Zn2SnO4 nanowires were prepared using a two-step process: thermal evaporation of a mixture of Zn and Sn powders (Zn:Sn = 8:2) in an oxidizing atmosphere followed by spin coating of PMMA solution. A 3 nm-thick gold (Au)-coated p-type (100) Si substrate was placed on top of an alumina boat containing a mixture of Zn and Sn powders positioned at the center of a quartz tube furnace. The furnace was heated to 700 °C and maintained at that temperature at a constant total pressure of 133 Pa using a mixture of N2 and O2 gases for 1 h. The flow rates of N2 and O2 gases were 98 and 2 cm3/min, respectively. Subsequently, 1 mM/0.5 mM/0.25 mM/0.1 mM PMMA solution (solvent: Toluene) was spin-coated on the ZnSnO3 nanowires at a
http://dx.doi.org/10.1016/j.tsf.2015.04.064 0040-6090/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064
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spinning rate of 3000 rpm for 30 s. The PMMA-spin coated nanowires were then dried in vacuum at 100 °C for 10 min. The morphology and structure of the products were examined by scanning electron microscopy (SEM, Hitachi S-4200) at an operating voltage of 10 kV and transmission electron microscopy (TEM). Highresolution TEM (HRTEM, JEOL 2100F) and corresponding selected area electron diffraction (SAED) were performed at an accelerating voltage of 300 kV. For further examination of the crystal structure of the
nanowire samples glancing angle X-ray diffraction (XRD, Philips X'Pert MRD) was performed on using Cu-Kα radiation (λ = 0.1541 nm) at a scan rate of 4°/min and at a glancing angle of 0.5° with a rotating detector. The room-temperature PL measurements (SPEC-1403 PL spectrometer) were carried out using a He–Cd laser (325 nm, 55 MW) as the excitation source with an excitation density of ~ 100 mW/cm2 on the sample. Low-temperature PL measurements were also carried out in a helium cryostat between 10 and 300 K.
Fig. 1. SEM images of PMMA-coated Zn2SnO4 nanowires with different PMMA concentrations: (a) Zn2SnO4 nanowires without PMMA, (b) 0.1 mM-PMMA, (c) 0.25 mM-PMMA, (d) 0.5 mM-PMMA and (e) 1.0 mM-PMMA.
Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064
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3. Results and discussion Fig. 1(a) shows a SEM image of the Zn2SnO4 nanowires synthesized by thermal evaporation and Fig. 1(b)–(e) shows SEM images of the Zn2SnO4 nanowires coated with different concentrations of PMMA. The nanowires were 30–80 nm in diameter and up to a few hundred of micrometers in length. A particle was observed at the tip of each nanowire, suggesting vapor–liquid–solid growth of the nanowires. The PMMA-coated Zn2SnO4 nanowires with the highest PMMA concentration (Fig. 1(e)) showed significant agglomeration of nanowires with glue-like PMMA, whereas those with lower PMMA concentrations (Fig. 1(a)–(c)) did not. Fig. 2(a) presents an XRD pattern of the 0.25 mM PMMA-coated ZnSnO3 nanowires. Many sharp reflection peak characteristics of Zn2SnO4, ZnSnO3, ZnO and SnO2 were identified, indicating that the nanowires comprise those four phases. The larger number of reflection peaks from the Zn2SnO4 phase than those from the other phases suggests that the Zn2SnO4 phase is dominant among those four phases. A low-magnification TEM image of a typical Zn2SnO4 nanowire (Fig. 2(b)) shows that the diameter of the nanowire and the thickness of the PMMA layer on the Zn2SnO4 nanowires were ~ 45 nm and ~32 nm, respectively. Fig. 2(c) and (d) presents an HRTEM image and corresponding SAED pattern of a typical Zn2SnO4 nanowire, respectively. The spotty pattern was assigned to a face-centered cubic-structured Zn2SnO4 single crystal (JCPDS # 74-2184, a = 0.865 nm). Clear spots observed in the SAED pattern (Fig. 2(d)) showed that the nanowire was a Zn2SnO4 single crystal. Room-temperature PL measurements were carried out to examine dependence of the luminescence properties of PMMA-coated Zn2SnO4 and ZnO nanowires on the PMMA concentration. The pristine Zn2SnO4
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nanowires showed the typical PL spectrum of Zn2SnO4 with an NBE emission band centered at ~378 nm and a broad DL emission band centered ~555 nm (Fig. 3(a)). In contrast, the PMMA-coated Zn2SnO4 nanowires exhibited an NBE emission band centered at ~378 nm, a broad DL emission band centered ~569 nm and a shoulder at ~760 nm (Fig. 3(a)). In other words, PMMA coating resulted in a ~14 nm red-shift in the NBE emission and a new shoulder in the red region. A more important change in PL spectrum by PMMA coating was a significant increase in NBE emission intensity. The PL properties of PMMA-coated ZnO nanowires are basically similar to those of PMMA-coated Zn2SnO4 nanowires. The PMMAcoated ZnO nanowires also showed NBE emission peak at ~ 378 nm, but significantly different DL emission behavior. The DL emission peak of the PMMA-coated ZnO nanowire sample tended to red-shift as the PMMA concentration increases (Fig. 3(b)). Fig. 3(c) and (d) compares the NBE emission intensity and NBE-toDL emission intensity ratio, INBE/IDL, respectively, of the PMMA-coated Zn2SnO4 and ZnO nanowire samples. Overall, the PMMA-coated Zn2SnO4 nanowire sample showed higher NBE intensity and higher INBE/IDL ratio than the PMMA-coated ZnO nanowire sample regardless of the PMMA concentration. The Zn2SnO4 nanowire sample showed a maximum INBE/IDL ratio at a PMMA concentration of 0.25 mM. The INBE/IDL ratios of the Zn2SnO4 and ZnO nanowires were increased ~ 10 and ~ 2 fold, respectively, by coating the nanowires with 0.25-mM PMMA. In addition, the INBE/IDL ratio of the PMMA-coated Zn2SnO4 nanowire sample was almost three times larger than that of the PMMA-coated ZnO nanowire sample at 0.25 mM of PMMA despite the INBE/IDL ratio of pristine Zn2SnO4 nanowires is lower than that of pristine ZnO nanowires. The enhanced NBE emission of the 0.25 mM of PMMAcoated ZnO nanowire sample might be due to the significantly improved excitonic emission efficiency [34]. The excitonic emission
Fig. 2. (a) XRD pattern of low magnification-TEM image of PMMA-coated Zn2SnO4 nanowires. (b) Low magnification-TEM image of PMMA-coated Zn2SnO4 nanowires. (c) HRTEM image of PMMA-coated Zn2SnO4 nanowires. (d) SAED pattern corresponding to (c).
Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064
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Fig. 3. (a) Room temperature-PL spectra of PMMA-coated Zn2SnO4 nanowires with different PMMA concentrations. (b) Room temperature-PL spectra of PMMA-coated ZnO nanowires with different PMMA concentrations. (c) NBE emission intensities of PMMA-coated Zn2SnO4 and ZnO nanowires as a function of the PMMA concentration. (d) Intensity ratios of NBE emission to DL PMMA-coated Zn2SnO4 and ZnO nanowires as a function of the PMMA concentration.
efficiency appeared to be much higer in the Zn2SnO4 nanowires than in the ZnO nanowires due to PMMA-functionalization. Low-temperature PL measurements were carried out to determine the origin of the enhanced PL properties of the PMMA-coated Zn2SnO4 nanowires. Fig. 4(a) and (b) presents the temperature-dependent NBE emission spectra of the Zn2SnO4 nanowires with and without PMMA. Basically, the two samples showed similar changes with increasing temperature, but some differences in the PL intensities and wavelengths. The PL curves of the two samples showed two characteristic peaks at ~374 nm and ~377 nm, which were assigned to free-to-bound recombination (FB) and excitons bound to donor (DX), respectively [35–38]. The PMMA-coated Zn2SnO4 nanowires showed a stronger FB band than the DX band in intensity, whereas the pristine Zn2SnO4 nanowires showed a stronger DX band than the FB band. A close examination revealed the wavelength of the DX peak of the PMMA-coated Zn2SnO4 nanowires to be slightly smaller than that of the pristine Zn2SnO4 nanowires at the same temperature. For both samples, as the temperature increased, the DX peak intensity decreased and the peak shifted slightly towards larger wavelength. A comparison of Fig. 4(a) and (b) showed that the two peaks shifted slightly towards shorter wavelength after coating the Zn2SnO4 nanowires with PMMA. Fig. 4(c) presents the NBE emission spectra of Zn2SnO4 nanowires with and without PMMA at T = 10 K. A broad asymmetric peak centered at ~377 nm dominated the near band-edge spectrum of the PMMA-coated Zn2SnO4 nanowires. Richter et al. assigned the main peak in the low-temperature NBE emission spectra of PMMA-coated ZnO nanowires to a surface exciton band, based on the excitation density dependent time resolved measurement results and reported significantly enhanced UV excitonic emission and suppressed green luminescence in the PL spectra [34]. Liu et al. obtained similar results for PMMA-coated vertically aligned ZnO nanowire arrays [35]. Further PL measurements including the excitation density dependent on time resolved measurements of the PMMA-coated Zn2SnO4 nanowires will be needed to identify the tall asymmetric peak at
~ 377 nm, but we believe that the main peak centered at ~ 377 nm in Fig. 4(c) was assigned to surface exciton. Therefore, based on the room-temperature PL data (Fig. 3(a)) and the low-temperature PL data (Fig. 4(a)–(c)), PMMA coating enhances UV excitonic emission and suppresses green luminescence of Zn2SnO4 nanowires significantly. However, the origin of the enhanced UV excitonic emission of the PMMA-functionalized semiconductor nanostructures is not completely understood at present, but it originates from the surface modification of the radiation-induced polymerization [39]. The oxygen on Zn2SnO4 surfaces might initiate graft polymerization during laser irradiation for PL measurement. The radiation produced σ anion free radicals on the ZnO nanoparticle surface, and σ anion free radicals are highly active. Brailsford and Morton [40] suggested that the σ anion free radical species originate from decomposition or are produced by trapped electrons reacting with oxygen atoms. 4. Conclusions The NBE emission and DL emission of Zn2SnO4 nanowires are enhanced and suppressed, respectively, by coating them with PMMA. The highest INBE/IDL was obtained for a PMMA concentration of 0.25 mM. The INBE/IDL was increased almost 10 fold by coating the nanowires with 0.25-mM PMMA. This enhanced NBE emission might be due to improved excitonic emission efficiency. This suggests that Zn2SnO4 nanowire is a promising candidate for applications in fabricating short-wavelength optical or optoelectronic devices. Acknowledgment This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2010-0020163).
Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064
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Fig. 4. Temperature-dependent low-temperature PL spectra of Zn2SnO4 nanowires (a) with and (b) without PMMA. (c) Low-temperature PL spectra of PMMA-coated Zn2SnO4 nanowires with and without PMMA at 10 K.
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Please cite this article as: S. Park, et al., Photoluminescence properties of polymethyl methacrylate-coated Zn2SnO4 nanowires, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.04.064