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Defect-induced ferromagnetism in undoped In2O3 nanowires
Materials Research Bulletin 60 (2014) 690–694
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Defect-induced ferromagnetism in undoped In2O3 nanowires Li-Chia Tien *, Yu-Yun Hsieh Department of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 June 2014 Received in revised form 16 September 2014 Accepted 19 September 2014 Available online 22 September 2014
We report the observation of intrinsic room-temperature ferromagnetism in undoped In2O3 nanowires synthesized by a thermal evaporation method. The as-grown sample was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL), Ultraviolet–visible spectroscopy (UV/vis), X-ray photoelectron spectroscopy (XPS), and superconducting quantum interference device (SQUID). Structural characterizations show that the as-grown sample is single crystal cubic bixbyite structure with 80–100 nm in diameter and 2–4 mm long. Both PL and UV/vis results reveal that the sample exhibits large amount of oxygen vacancies and indium vacancies (VO and VIn). XPS results indicate that the In2O3 nanowires are significantly oxygen deficient and non-stoichiometric at surface. Experimentally measured magnetic hysteresis curves for undoped In2O3 nanowires clearly display ferromagnetic behavior at room temperature. We believe the ferromagnetism of undoped In2O3 nanowires is ascribed to the large surface area and intrinsic defects mediating the ferromagnetic ordering. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Optical properties D. Defects
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted much attention because of its potential to provide new functionalities by combination of charge based semiconductors and spin-based magnetism for developing spintronics [1]. As particularly promising candidates, oxide materials provide a convenient platform to study the spin-based functionality in host semiconducting material [2]. In general, the ferromagnetism can be achieved by doping oxide semiconductors with transition metal elements to achieve ferromagnetism through the coupling of their magnetic moments. The ferromagnetism has been reported as an intrinsic property in undoped and non-magnetic oxides, such as CeO2, TiO2, ZnO, SnO2 [3–7]. The “d0 ferromagnetism” was suggested to describe the phenomena [8]. Although the origin of d0 ferromagnetism is still not clear, it can be concluded that the ferromagnetism in undoped oxides may be related to structural defects, which may confine the compensating charges in molecular orbitals, forming a local magnetic moment. Due to the large surface-to-volume ratios, the ferromagnetism has also been observed in various undoped oxide nanostructures, such as nanoparticles, nanoribbons, and nanowires [9–14]. It is believed
* Corresponding author. Tel.: +886 3 863 4208; fax: +886 3 863 4200. E-mail address: [email protected] (L.-C. Tien). http://dx.doi.org/10.1016/j.materresbull.2014.09.043 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
that both non-stoichiometry surface and defects of oxide nanostructures play a crucial role in inducing ferromagnetism since conventional ideas of magnetism are unable to account for the ferromagnetism. Indium oxide (In2O3) is a direct wide band gap semiconductor, has been widely used as a transparent conductor in window heaters, solar cells, and flat-panel displays, etc. The magnetic properties of In2O3 are still underexplored as compared to TiO2 and ZnO [15–17]. It has been reported that weak ferromagnetism exists in undoped In2O3 thin films. Hong et al. observed the room-temperature ferromagnetism in undoped In2O3 thin films, which is linked to defects or oxygen vacancy [3]. Panguluri et al. reported ferromagnetism in In2O3 thin films and attributed it to the spin-polarized charge carriers [18]. Most recently, Sun et al. also observed ferromagnetism in n- and p-type In2O3 thin films, where single ionized oxygen and indium vacancies are considered to mediate the ferromagnetism [19]. The large surface-to-volume ratio of In2O3 nanostructures should strongly affect the observed ferromagnetism. Therefore, a comprehensive study on the correlation between defects and ferromagnetism of In2O3 nanowires is of great interest to address further. In this work, we study on the structural, compositional, optical and magnetic properties of undoped In2O3 nanowires synthesized by using a thermal evaporation process. The sample has been examined comprehensively using a variety of characterization
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tools. Our results show that the In2O3 nanowires are with oxygen deficient surface and accompany with large amount of defects. The In2O3 nanowires exhibit room temperature ferromagnetism with distinct hysteresis loops. Our studies suggest that the observed ferromagnetism is closely correlated with defects and oxygen deficient surface of In2O3 nanowires. Moreover, the large surface-to-volume ratio of In2O3 nanowires may also affect the ferromagnetism. 2. Materials and methods 2.1. Sample preparation The growth of In2O3 nanowires was carried out using a thermal evaporation process. A horizontal tube furnace (Lindberg Blue) with three independent heating zones was used for the sample growth. Indium powder (1.2 g, 99.99%, Alfa Aesar) was used as source material, put in an alumina boat and placed at center of the quartz tube. Prior to deposition, the silicon(0 0 1) substrates were ultrasonically cleaned with acetone, and methanol, followed by compressed N2 drying. Thin layers (5 nm) of Au were deposited on the silicon(0 0 1) substrates by DC sputtering. The substrates were put in another boat and placed at downstream. The quartz tube was evacuated by a mechanical pump and purge several times with flowing argon. The furnace was then heated to 750 C at a rate of 10 C/min. Once furnace reaches growth temperature, the oxygen and argon mixtures (5/ 95 SCCM) were fed into quartz tube. The growth temperature was estimated to be 450 C in a background pressure of 0.5 Torr during growth. The typical growth time was 1 h. 3. Characterization The as-grown samples were characterized using a X-ray diffraction (XRD, Rigaku D/Max 2500), a field emission scanning electron microscopy (FE-SEM, JEOL-7000F), and a 300-keV transmission electron microscopy (TEM, JEOL-3010). The XPS experiments were carried out in an X-ray photoelectron spectrometer system (XPS, Thermo Scientific) with a base pressure better than 3 109 mbar. XPS measurements were performed using an Al Ka (hn = 1486.6 eV) source equipped with micro-focused monochromator. The spectra are given in binding energy referred to the binding energy of carbon 1s (284.85 eV). The optical properties of the samples were examined using Ultraviolet-visible spectroscopy (UV/vis Hitachi U-3900) and photoluminescence (PL). For PL measurements, a spectrometer was used (Horiba J-Y, iHR 550) as the optical dispersion unit, while a CCD (Horiba J-Y, Synapse) was used for optical detection. A He–Cd (325 nm) laser were used as the excitation source. The magnetic properties of the samples were measured with a superconducting quantum interference device (SQUID, Quantum Design, MPMS-SQUID-VSM) magnetometer. Note that all the tools used for handing the samples were non-magnetic and plastic based. 4. Results and discussion 4.1. Structural properties The crystalline structures of the as-deposited sample were determined by XRD. Fig. 1 shows XRD patterns of as-deposited sample on Si(0 0 1). All patterns were indexed to the In2O3 cubic bixbyite structure with a lattice constant of a = 10.12 Å (JCPDS card no. 71-2194). The absence of any other peaks suggests that there is no secondary phase or impurity observed with the XRD detection limit. Inset of Fig. 1 shows top-view FE-SEM image
Fig. 1. XRD spectra of as-grown In2O3 nanowires on Si(0 0 1). Inset shows the FESEM image of In2O3 nanowires.
of as-grown sample. The obtained FE-SEM image indicates that the as-synthesized products consist of a large quantity of one-dimensional nanostructures with relatively uniform diameter and high aspect ratio. The In2O3 nanowires were approximately 80–100 nm in diameter and approximately 2–4 mm long. Further structural characterizations were carried out on a single In2O3 nanowire using TEM. Fig. 2(a) shows a TEM image of a single In2O3 nanowire with uniform diameter approximately 90 nm. It clearly shows the arrow-like tip of nanowire with a gold nanoparticle embedded. The growth of In2O3 nanowires is believed to be proceeded via vapor–liquid–solid (VLS) mechanism where the gold serves as nucleation sites and facilitates the adsorption of indium vapor onto the interface. Fig. 2(b) shows the selected-area electron diffraction (SAED) patterns of In2O3 nanowire, indicating that the nanowire has a single crystal cubic structure with a growth direction of [1 0 0]. A high resolution TEM image is shown in Fig. 2(c) reveling that the nanowire is structurally uniform and with few observable structural defects such as stacking faults or dislocations. 5. Optical properties Temperature-dependent PL spectra of the as-grown In2O3 nanowires were taken at temperature ranging between 10 and 300 K, using a He-Cd laser as the excitation source are shown in Fig. 3(a). The spectra do not exhibit any emission related to free or bound excitons in the region where the energy gap lies. Note the near band-edge emission does not appear, even as the temperature decreases from room temperature to 10 K. Mazzera et al. reported that the presence of surface states or defects in In2O3 nanostructures may hinder the free-exciton radiative recombination [20]. We employed XPS to confirm the large amount of surface oxygen vacancies are formed during growth, which will be discussed later. In contrast, a broad and intense visible emission ranging from 500 to 800 nm was observed. With decreasing temperature, the emission intensity is simultaneously increased. Moreover, the energy position of visible emission showing a red shift with decreasing temperature. Although bulk In2O3 does not emit visible light at room temperature, the observations of deep level photoluminescence in nanostructures have been reported [21–23]. PL emissions in the visible range due to high-density oxygen vacancies have been suggested in In2O3 nanostructures. The observed visible emissions originates from the recombination
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Fig. 2. (a) TEM image of a single In2O3 nanowire (b) SAED patterns clearly shows the single crystal cubic structure with preferred growth direction along [1 0 0] (c) high-resolution TEM image showing the gold catalyst embed in tip of In2O3 nanowire.
of donor from oxygen vacancy band (VO) to the acceptor band consists of indium vacancy (VIn). Fig. 3(b and c) report on the Gaussian deconvolution of the PL spectra taken at 10 and 300 K, respectively. The spectra were fitted into three Gaussians with wavelengths of 680 (E1), 600 (E2), and 740 nm (E3). The E1 and E2 emissions were interpreted as recombination of oxygen vacancy band to the indium vacancy band, as shown in the inset in Fig. 3(a). The thermal ionization effect cause upper acceptor level (VIn) partially occupied at room temperature (RT), resulting a higher intensity of E2 emissions. The upper acceptor levels are nearly empty at low temperature (LT), leading to a higher intensity of E1 emissions. The E3 emission band originates from the interstitial nitrogen doping in In2O3 nanostructures based on the first principles density functional theory computations [24]. As a consequence, the decrease of temperature leads to a red shift of emission peak. Fig. 4 shows the UV/vis absorption spectrum of In2O3 nanowires. The absorption spectrum was cut off at 340 nm and accompany with a high absorption ranging from 450 to 800 nm. The presence of defects contributes to the visible light absorption, which significantly extends the absorption to lower energy. The optical band gap for the absorption edge can be obtained by extrapolating the linear portion of the plot (ahn)2 versus hn. The inset of Fig. 4 shows the plot of (ahn)2 versus hn calculated from the absorption spectrum. The direct band gap is determined to be 3.43 eV for the In2O3 nanowires, which is in consistent with reported values [25]. The observed results in the UV/vis spectrum were in good agreement with the PL results, showing that the In2O3 nanowires are with large amount of defects such as VO and VIn. 6. XPS investigation The XPS was used to analyze and determine the surface electronic states of In2O3 nanowires. The In 3d and O 1s core-level X-ray photoemission spectra of In2O3 nanowires are given in Fig. 5. The binding energies of spectra were calibrated using the C
1s peak at 284.5 eV. As for the emission from In 3d core-level spectrum, the sample shows two peaks at 451.5 eV and 443.9 eV for 3d3/2 and 3d5/2, respectively, with a spin–orbital splitting of 7.6 eV. The observed binding energies are corresponding to the binding energy of In3+ in In2O3. The O 1s core-level spectrum can be deconvoluted into two components, located at 529.5 and 531.8 eV, respectively. The lower binding energy peak is attributed to In2O3 lattice oxygen, while the higher energy peak is assigned to the oxygen ions in the oxygen vacancies (VO). The stoichiometric ratio (SO,In) was calculated from the XPS spectra using the following equation: SO;In ¼
CO IO =ASFO ¼ C In IIn =ASFIn
where CO and CIn are the concentrations, IO and IIn are the background corrected intensities of the photoelectron emission lines, and ASFO (0.711) and ASFIn (3.777) are the atomic sensitivity factors for oxygen and indium, respectively [26]. Hence, the atomic concentration ratio of indium to oxygen is about 1:1.24, suggesting the sample surface is non-stoichiometry and does exhibit large amount of oxygen vacancies. The VLS growth mechanism could be applied to explain the origin of large amount of defects on nanowires surface. In the growth process, the Au catalysts serve as nucleation sites, where AuIn alloy formed and further result in one-dimensional growth. Under ambient oxygen, indium vapor pressure was reduced significantly as gaseous species (In2O(g)) formed. Here, an insufficient supply of indium vapor during the growth may generate additional defect such as oxygen vacancies (VO) and indium vacancies (VIn) in the samples. Therefore, the large numbers of defects (VO and VIn) were introduced during growth in In2O3 nanowires.
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Fig. 4. UV/vis absorption spectrum of In2O3 nanowires. The inset shows calculated band gap energy approximately 3.43 eV.
In our study, the observed intrinsic ferromagnetism in In2O3 is attributed to intrinsic defects such as oxygen vacancies and indium vacancies which were formed during synthesis. In addition, the high surface to volume ratio of In2O3 nanowires may also introduce notable surface states and surface defects. Both experimental and theoretical results show that the size of the lower dimensional systems, such as film thickness or diameter of nanostructures, has an effect on the vacancy concentration as well as their magnetic behavior [27–29]. This implies that the small diameter of In2O3 nanowires should also contribute to the observed ferromagnetism. Our results are consistent with previous studies which room temperature ferromagnetism is
Fig. 3. (a) Temperature-dependent PL spectra of In2O3 nanowires between 10 and 300 K excited by a 325 nm He-Cd laser. Inset shows the luminescence mechanism for donor-acceptor pair (DAP) transitions between oxygen vacancies and indium vacancies at room temperature (RT) and low temperature (LT). Deconvoluted PL spectra of In2O3 nanowires measured at (b) 10 K and (b) 300 K, respectively.
7. Magnetic properties The magnetization versus magnetic field (M–H) curves for In2O3 nanowires are displayed in Fig. 6(a) which were measured at 300 K and 10 K under the maximum applied magnetic field of 5000 Oe. The same measurement procedures were done for the Si(0 0 1) substrate before growth, which shows that it is diamagnetic. The diamagnetic signal of the Si(0 0 1) was subtracted from the measured magnetic signal of the sample. (Fig. 6(b)) It can be seen that the Ms decreases with the increasing temperature, which is a typical ferromagnetic behavior. The hysteresis loops suggest that In2O3 nanowires exhibit clearly ferromagnetism. Since no magnetic impurities was detected in the sample, the ferromagnetism may mainly attributed to intrinsic defects such as oxygen vacancies or indium vacancies, establishing a long range ferromagnetic interaction.
Fig. 5. (a) In 3d and (b) O 1s core-level X-ray photoemission spectra of In2O3 nanowires.
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References
Fig. 6. (a) Magnetic hysteresis loops of as-grown In2O3 nanowires measured at 10 K and 300 K. (b) Magnetic hysteresis loops of the as-grown In2O3 nanowires after subtracting the background diamagnetic signal.
found only in low-dimensional nanostructures with high surface-to-volume ratio [30,31]. 8. Conclusions In conclusion, the structural, compositional, optical, and magnetic properties of undoped In2O3 nanowires were examined. Characterization results reveal that the nanowire surface is oxygen deficient and exhibit large amount of defects. The M–H curves indicate that the undoped In2O3 nanowires are ferromagnetic at room temperature. The results are most consistent with the ferromagnetism in In2O3 thin films where surface defects mediate the ferromagnetic ordering. Thus, the correlation between defects and ferromagnetism of In2O3 nanowires has been verified. The results suggest that introduction of large surface area and defects may offer an effective way to introduce magnetic moments in various non-magnetic materials. Acknowledgment The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 99-2112-M-259-006-MY3.
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Defect-induced ferromagnetism in undoped In2O3 nanowires Li-Chia Tien *, Yu-Yun Hsieh Department of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 June 2014 Received in revised form 16 September 2014 Accepted 19 September 2014 Available online 22 September 2014
We report the observation of intrinsic room-temperature ferromagnetism in undoped In2O3 nanowires synthesized by a thermal evaporation method. The as-grown sample was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL), Ultraviolet–visible spectroscopy (UV/vis), X-ray photoelectron spectroscopy (XPS), and superconducting quantum interference device (SQUID). Structural characterizations show that the as-grown sample is single crystal cubic bixbyite structure with 80–100 nm in diameter and 2–4 mm long. Both PL and UV/vis results reveal that the sample exhibits large amount of oxygen vacancies and indium vacancies (VO and VIn). XPS results indicate that the In2O3 nanowires are significantly oxygen deficient and non-stoichiometric at surface. Experimentally measured magnetic hysteresis curves for undoped In2O3 nanowires clearly display ferromagnetic behavior at room temperature. We believe the ferromagnetism of undoped In2O3 nanowires is ascribed to the large surface area and intrinsic defects mediating the ferromagnetic ordering. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Optical properties D. Defects
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted much attention because of its potential to provide new functionalities by combination of charge based semiconductors and spin-based magnetism for developing spintronics [1]. As particularly promising candidates, oxide materials provide a convenient platform to study the spin-based functionality in host semiconducting material [2]. In general, the ferromagnetism can be achieved by doping oxide semiconductors with transition metal elements to achieve ferromagnetism through the coupling of their magnetic moments. The ferromagnetism has been reported as an intrinsic property in undoped and non-magnetic oxides, such as CeO2, TiO2, ZnO, SnO2 [3–7]. The “d0 ferromagnetism” was suggested to describe the phenomena [8]. Although the origin of d0 ferromagnetism is still not clear, it can be concluded that the ferromagnetism in undoped oxides may be related to structural defects, which may confine the compensating charges in molecular orbitals, forming a local magnetic moment. Due to the large surface-to-volume ratios, the ferromagnetism has also been observed in various undoped oxide nanostructures, such as nanoparticles, nanoribbons, and nanowires [9–14]. It is believed
* Corresponding author. Tel.: +886 3 863 4208; fax: +886 3 863 4200. E-mail address: [email protected] (L.-C. Tien). http://dx.doi.org/10.1016/j.materresbull.2014.09.043 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
that both non-stoichiometry surface and defects of oxide nanostructures play a crucial role in inducing ferromagnetism since conventional ideas of magnetism are unable to account for the ferromagnetism. Indium oxide (In2O3) is a direct wide band gap semiconductor, has been widely used as a transparent conductor in window heaters, solar cells, and flat-panel displays, etc. The magnetic properties of In2O3 are still underexplored as compared to TiO2 and ZnO [15–17]. It has been reported that weak ferromagnetism exists in undoped In2O3 thin films. Hong et al. observed the room-temperature ferromagnetism in undoped In2O3 thin films, which is linked to defects or oxygen vacancy [3]. Panguluri et al. reported ferromagnetism in In2O3 thin films and attributed it to the spin-polarized charge carriers [18]. Most recently, Sun et al. also observed ferromagnetism in n- and p-type In2O3 thin films, where single ionized oxygen and indium vacancies are considered to mediate the ferromagnetism [19]. The large surface-to-volume ratio of In2O3 nanostructures should strongly affect the observed ferromagnetism. Therefore, a comprehensive study on the correlation between defects and ferromagnetism of In2O3 nanowires is of great interest to address further. In this work, we study on the structural, compositional, optical and magnetic properties of undoped In2O3 nanowires synthesized by using a thermal evaporation process. The sample has been examined comprehensively using a variety of characterization
L.-C. Tien, Y.-Y. Hsieh / Materials Research Bulletin 60 (2014) 690–694
691
tools. Our results show that the In2O3 nanowires are with oxygen deficient surface and accompany with large amount of defects. The In2O3 nanowires exhibit room temperature ferromagnetism with distinct hysteresis loops. Our studies suggest that the observed ferromagnetism is closely correlated with defects and oxygen deficient surface of In2O3 nanowires. Moreover, the large surface-to-volume ratio of In2O3 nanowires may also affect the ferromagnetism. 2. Materials and methods 2.1. Sample preparation The growth of In2O3 nanowires was carried out using a thermal evaporation process. A horizontal tube furnace (Lindberg Blue) with three independent heating zones was used for the sample growth. Indium powder (1.2 g, 99.99%, Alfa Aesar) was used as source material, put in an alumina boat and placed at center of the quartz tube. Prior to deposition, the silicon(0 0 1) substrates were ultrasonically cleaned with acetone, and methanol, followed by compressed N2 drying. Thin layers (5 nm) of Au were deposited on the silicon(0 0 1) substrates by DC sputtering. The substrates were put in another boat and placed at downstream. The quartz tube was evacuated by a mechanical pump and purge several times with flowing argon. The furnace was then heated to 750 C at a rate of 10 C/min. Once furnace reaches growth temperature, the oxygen and argon mixtures (5/ 95 SCCM) were fed into quartz tube. The growth temperature was estimated to be 450 C in a background pressure of 0.5 Torr during growth. The typical growth time was 1 h. 3. Characterization The as-grown samples were characterized using a X-ray diffraction (XRD, Rigaku D/Max 2500), a field emission scanning electron microscopy (FE-SEM, JEOL-7000F), and a 300-keV transmission electron microscopy (TEM, JEOL-3010). The XPS experiments were carried out in an X-ray photoelectron spectrometer system (XPS, Thermo Scientific) with a base pressure better than 3 109 mbar. XPS measurements were performed using an Al Ka (hn = 1486.6 eV) source equipped with micro-focused monochromator. The spectra are given in binding energy referred to the binding energy of carbon 1s (284.85 eV). The optical properties of the samples were examined using Ultraviolet-visible spectroscopy (UV/vis Hitachi U-3900) and photoluminescence (PL). For PL measurements, a spectrometer was used (Horiba J-Y, iHR 550) as the optical dispersion unit, while a CCD (Horiba J-Y, Synapse) was used for optical detection. A He–Cd (325 nm) laser were used as the excitation source. The magnetic properties of the samples were measured with a superconducting quantum interference device (SQUID, Quantum Design, MPMS-SQUID-VSM) magnetometer. Note that all the tools used for handing the samples were non-magnetic and plastic based. 4. Results and discussion 4.1. Structural properties The crystalline structures of the as-deposited sample were determined by XRD. Fig. 1 shows XRD patterns of as-deposited sample on Si(0 0 1). All patterns were indexed to the In2O3 cubic bixbyite structure with a lattice constant of a = 10.12 Å (JCPDS card no. 71-2194). The absence of any other peaks suggests that there is no secondary phase or impurity observed with the XRD detection limit. Inset of Fig. 1 shows top-view FE-SEM image
Fig. 1. XRD spectra of as-grown In2O3 nanowires on Si(0 0 1). Inset shows the FESEM image of In2O3 nanowires.
of as-grown sample. The obtained FE-SEM image indicates that the as-synthesized products consist of a large quantity of one-dimensional nanostructures with relatively uniform diameter and high aspect ratio. The In2O3 nanowires were approximately 80–100 nm in diameter and approximately 2–4 mm long. Further structural characterizations were carried out on a single In2O3 nanowire using TEM. Fig. 2(a) shows a TEM image of a single In2O3 nanowire with uniform diameter approximately 90 nm. It clearly shows the arrow-like tip of nanowire with a gold nanoparticle embedded. The growth of In2O3 nanowires is believed to be proceeded via vapor–liquid–solid (VLS) mechanism where the gold serves as nucleation sites and facilitates the adsorption of indium vapor onto the interface. Fig. 2(b) shows the selected-area electron diffraction (SAED) patterns of In2O3 nanowire, indicating that the nanowire has a single crystal cubic structure with a growth direction of [1 0 0]. A high resolution TEM image is shown in Fig. 2(c) reveling that the nanowire is structurally uniform and with few observable structural defects such as stacking faults or dislocations. 5. Optical properties Temperature-dependent PL spectra of the as-grown In2O3 nanowires were taken at temperature ranging between 10 and 300 K, using a He-Cd laser as the excitation source are shown in Fig. 3(a). The spectra do not exhibit any emission related to free or bound excitons in the region where the energy gap lies. Note the near band-edge emission does not appear, even as the temperature decreases from room temperature to 10 K. Mazzera et al. reported that the presence of surface states or defects in In2O3 nanostructures may hinder the free-exciton radiative recombination [20]. We employed XPS to confirm the large amount of surface oxygen vacancies are formed during growth, which will be discussed later. In contrast, a broad and intense visible emission ranging from 500 to 800 nm was observed. With decreasing temperature, the emission intensity is simultaneously increased. Moreover, the energy position of visible emission showing a red shift with decreasing temperature. Although bulk In2O3 does not emit visible light at room temperature, the observations of deep level photoluminescence in nanostructures have been reported [21–23]. PL emissions in the visible range due to high-density oxygen vacancies have been suggested in In2O3 nanostructures. The observed visible emissions originates from the recombination
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Fig. 2. (a) TEM image of a single In2O3 nanowire (b) SAED patterns clearly shows the single crystal cubic structure with preferred growth direction along [1 0 0] (c) high-resolution TEM image showing the gold catalyst embed in tip of In2O3 nanowire.
of donor from oxygen vacancy band (VO) to the acceptor band consists of indium vacancy (VIn). Fig. 3(b and c) report on the Gaussian deconvolution of the PL spectra taken at 10 and 300 K, respectively. The spectra were fitted into three Gaussians with wavelengths of 680 (E1), 600 (E2), and 740 nm (E3). The E1 and E2 emissions were interpreted as recombination of oxygen vacancy band to the indium vacancy band, as shown in the inset in Fig. 3(a). The thermal ionization effect cause upper acceptor level (VIn) partially occupied at room temperature (RT), resulting a higher intensity of E2 emissions. The upper acceptor levels are nearly empty at low temperature (LT), leading to a higher intensity of E1 emissions. The E3 emission band originates from the interstitial nitrogen doping in In2O3 nanostructures based on the first principles density functional theory computations [24]. As a consequence, the decrease of temperature leads to a red shift of emission peak. Fig. 4 shows the UV/vis absorption spectrum of In2O3 nanowires. The absorption spectrum was cut off at 340 nm and accompany with a high absorption ranging from 450 to 800 nm. The presence of defects contributes to the visible light absorption, which significantly extends the absorption to lower energy. The optical band gap for the absorption edge can be obtained by extrapolating the linear portion of the plot (ahn)2 versus hn. The inset of Fig. 4 shows the plot of (ahn)2 versus hn calculated from the absorption spectrum. The direct band gap is determined to be 3.43 eV for the In2O3 nanowires, which is in consistent with reported values [25]. The observed results in the UV/vis spectrum were in good agreement with the PL results, showing that the In2O3 nanowires are with large amount of defects such as VO and VIn. 6. XPS investigation The XPS was used to analyze and determine the surface electronic states of In2O3 nanowires. The In 3d and O 1s core-level X-ray photoemission spectra of In2O3 nanowires are given in Fig. 5. The binding energies of spectra were calibrated using the C
1s peak at 284.5 eV. As for the emission from In 3d core-level spectrum, the sample shows two peaks at 451.5 eV and 443.9 eV for 3d3/2 and 3d5/2, respectively, with a spin–orbital splitting of 7.6 eV. The observed binding energies are corresponding to the binding energy of In3+ in In2O3. The O 1s core-level spectrum can be deconvoluted into two components, located at 529.5 and 531.8 eV, respectively. The lower binding energy peak is attributed to In2O3 lattice oxygen, while the higher energy peak is assigned to the oxygen ions in the oxygen vacancies (VO). The stoichiometric ratio (SO,In) was calculated from the XPS spectra using the following equation: SO;In ¼
CO IO =ASFO ¼ C In IIn =ASFIn
where CO and CIn are the concentrations, IO and IIn are the background corrected intensities of the photoelectron emission lines, and ASFO (0.711) and ASFIn (3.777) are the atomic sensitivity factors for oxygen and indium, respectively [26]. Hence, the atomic concentration ratio of indium to oxygen is about 1:1.24, suggesting the sample surface is non-stoichiometry and does exhibit large amount of oxygen vacancies. The VLS growth mechanism could be applied to explain the origin of large amount of defects on nanowires surface. In the growth process, the Au catalysts serve as nucleation sites, where AuIn alloy formed and further result in one-dimensional growth. Under ambient oxygen, indium vapor pressure was reduced significantly as gaseous species (In2O(g)) formed. Here, an insufficient supply of indium vapor during the growth may generate additional defect such as oxygen vacancies (VO) and indium vacancies (VIn) in the samples. Therefore, the large numbers of defects (VO and VIn) were introduced during growth in In2O3 nanowires.
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Fig. 4. UV/vis absorption spectrum of In2O3 nanowires. The inset shows calculated band gap energy approximately 3.43 eV.
In our study, the observed intrinsic ferromagnetism in In2O3 is attributed to intrinsic defects such as oxygen vacancies and indium vacancies which were formed during synthesis. In addition, the high surface to volume ratio of In2O3 nanowires may also introduce notable surface states and surface defects. Both experimental and theoretical results show that the size of the lower dimensional systems, such as film thickness or diameter of nanostructures, has an effect on the vacancy concentration as well as their magnetic behavior [27–29]. This implies that the small diameter of In2O3 nanowires should also contribute to the observed ferromagnetism. Our results are consistent with previous studies which room temperature ferromagnetism is
Fig. 3. (a) Temperature-dependent PL spectra of In2O3 nanowires between 10 and 300 K excited by a 325 nm He-Cd laser. Inset shows the luminescence mechanism for donor-acceptor pair (DAP) transitions between oxygen vacancies and indium vacancies at room temperature (RT) and low temperature (LT). Deconvoluted PL spectra of In2O3 nanowires measured at (b) 10 K and (b) 300 K, respectively.
7. Magnetic properties The magnetization versus magnetic field (M–H) curves for In2O3 nanowires are displayed in Fig. 6(a) which were measured at 300 K and 10 K under the maximum applied magnetic field of 5000 Oe. The same measurement procedures were done for the Si(0 0 1) substrate before growth, which shows that it is diamagnetic. The diamagnetic signal of the Si(0 0 1) was subtracted from the measured magnetic signal of the sample. (Fig. 6(b)) It can be seen that the Ms decreases with the increasing temperature, which is a typical ferromagnetic behavior. The hysteresis loops suggest that In2O3 nanowires exhibit clearly ferromagnetism. Since no magnetic impurities was detected in the sample, the ferromagnetism may mainly attributed to intrinsic defects such as oxygen vacancies or indium vacancies, establishing a long range ferromagnetic interaction.
Fig. 5. (a) In 3d and (b) O 1s core-level X-ray photoemission spectra of In2O3 nanowires.
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References
Fig. 6. (a) Magnetic hysteresis loops of as-grown In2O3 nanowires measured at 10 K and 300 K. (b) Magnetic hysteresis loops of the as-grown In2O3 nanowires after subtracting the background diamagnetic signal.
found only in low-dimensional nanostructures with high surface-to-volume ratio [30,31]. 8. Conclusions In conclusion, the structural, compositional, optical, and magnetic properties of undoped In2O3 nanowires were examined. Characterization results reveal that the nanowire surface is oxygen deficient and exhibit large amount of defects. The M–H curves indicate that the undoped In2O3 nanowires are ferromagnetic at room temperature. The results are most consistent with the ferromagnetism in In2O3 thin films where surface defects mediate the ferromagnetic ordering. Thus, the correlation between defects and ferromagnetism of In2O3 nanowires has been verified. The results suggest that introduction of large surface area and defects may offer an effective way to introduce magnetic moments in various non-magnetic materials. Acknowledgment The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 99-2112-M-259-006-MY3.
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