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Catalytic growth of Ga2O3 nanowires by physical evaporation and their photoluminescence properties
Chemical Physics 289 (2003) 243–249 www.elsevier.com/locate/chemphys
Catalytic growth of Ga2O3 nanowires by physical evaporation and their photoluminescence properties Jun Zhang *, Feihong Jiang Department of Physics, Yantai University, Yantai 264005, PR China Received 13 October 2002
Abstract Gallium oxide (Ga2 O3 ) nanowires were fabricated through simple physical evaporation in oxygen atmosphere using GaN powder as raw material and an Al2 O3 substrate coated with a thin indium layer. The morphology and structure of the nanowires were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectrum. These nanowires have diameters ranging from 20 to 60 nm and lengths ranging from several tens to several hundreds micrometers. Photoluminescence spectrum under excitation at 254 nm shows that these Ga2 O3 nanowires have a blue emission at 436 nm and an ultraviolet emission at 330 nm, which may be attributed to the defects such as the oxygen vacancies and gallium–oxygen vacancy pairs. Ó 2003 Published by Elsevier Science B.V.
1. Introduction One-dimension nanomaterials (nanotubes, nanowires and nanorods) have attracted intensive experimental and theoretical interests due to their novel physical properties and potential application for nanodevices [1,2]. Recently, much attention has been paid to the preparation of nanowires [3– 9] and nanobelts [10] of the family of oxides for their interesting optical and electrical properties. Several binary oxide nanowires such as MgO [3,4], SiO2 [5,6], In2 O3 [7], GeO2 [8], ZnO [9] and
*
Corresponding author. Tel.: +86-535-690-2146; fax: +86535-690-1947. E-mail address: [email protected] (J. Zhang).
nanobelts of semiconducting oxides (ZnO, SnO2 , In2 O3 and CdO) [10] have been successfully synthesized. As a wide bandgap (Eg 4:9 eV) compound [11], nanometer-scale gallium oxide (Ga2 O3 ) with large surface area/volume ratio has exhibited particular conduction and optical properties [12] and has great potential application in optoelectronic nanodevices and gas sensors [13, 14]. Ga2 O3 nanowires have been successfully synthesized by a simple physical evaporation via the vapor–solid (VS) process using Ga powder [15], an arc-discharge method based on the mechanism of catalytic step growth [16], and GaAs was used as Ga precursor and Au was the catalyst [17]. Here, through a simple physical evaporation method in oxygen atmosphere using GaN powder as raw material and an Al2 O3 substrate coated with a thin
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indium (In) layer as catalyst, we have successfully synthesized Ga2 O3 nanowires in a high yield. The In nanoparticles play a crucial role in the growth of Ga2 O3 nanowires based on the vapor–liquid– solid (VLS) mechanism. Photoluminescence (PL) spectrum of the Ga2 O3 nanowires at room temperature exhibits strong blue–green light emission and a blue shift compared to that of Ga2 O3 powder. Finally, the mechanism of PL will also be discussed.
2. Experimental procedure Our synthesis is based on simple thermal evaporation using GaN powder as raw material. An Al2 O3 membrane was used as a substrate. The substrate was rinsed with deionized water before use. A layer of In nanoparticles (about several tens of nanometers in thickness) was evaporated onto one side of the substrate under high vacuum (2 105 torr). The GaN powders were placed at the center of a ceramic boat. The In-coated substrate was typically placed at 0.5–5.0 cm from the center of the boat, and the ceramic boat were transferred into a quartz tube (30 mm inner diameter). This quartz tube was then placed inside a horizontal electronic resistance furnace (heated by silicon–carbon rods) with the center of the ceramic boat positioned at the center of the furnace and the substrate placed downstream of the gas flow. The temperature of crucible was rapidly increased to 960 °C (about 6 min) from room temperature and kept at 960 °C for 1 h under a constant flow of mixture gases of argon and oxygen (Ar/O2 is 4:1). Argon was introduced into the quartz tube through a mass-flow controller at the rate of 100 standard cubic centimeters per minute (sccm). After the furnace was cooled to room temperature, white wool-like products were found on the surface of the substrate. The morphology and structure of the synthesized product were characterized by scanning electron microscopy (SEM, JOEL JSM-5610LV), X-ray diffraction (XRD, Philips PW 1710 with Cu Ka radiation), transmission electron microscopy (TEM, JEM-200CX), high-resolution transmission electron microscopy (HRTEM, JEOL-2010) and
energy-dispersive X-ray spectroscopy (EDX). In addition, because Raman spectroscopy has been proven to be a powerful tool for the investigation of material properties such as doping concentration, defect identification and crystal orientation. Raman spectrum (Spex-1403) of the Ga2 O3 nanowires was also investigated.
3. Results and discussion SEM image shown in Fig. 1(a) reveals that the products consist of a large quantity of wire-like nanostructures with typical lengths in the range of several tens to several hundreds of micrometers. Some of them even have lengths on the order of millimeter, which results in a very large aspect ratio. The typical diameters of the nanowires are in the range of 20–60 nm. Another typical SEM image of the specimen surface shown in Fig. 1(b) indicates that Ga2 O3 nanowires cross each other and are randomly distributed on the surface of the substrate. Each nanowires terminate in a nanoparticle at the tip. The image provides further details of VLS growth mechanism in Ga2 O3 nanowires. The crystal structure of these nanowires was characterized by XRD. Fig. 1(c) shows the structural characterization of the nanowires stripped from the substrate. Miller indices are indicated on each diffraction peaks. It can be seen that all peaks can be indexed to a monoclinic crystalline Ga2 O3 phase, in good agreement with the reported data (JCPDS 11-370). The lattice constants of the crystalline phase are a0 ¼ 0:58 nm, b0 ¼ 0:304 nm, c0 ¼ 1:223 nm and b ¼ 103:7°, and its space group is identified as C2/m. Further structural and elemental analyses of the individual Ga2 O3 nanowire were performed using TEM. For TEM observation, the above specimen was pulverized in ethyl alcohol, and then dispersed onto a copper grid-coated carbon film. A lowmagnified TEM image of a long individual Ga2 O3 nanowire is displayed in Fig. 2(a). The nanowire has a uniform diameter. The diameter of the nanowire is about 40 nm. The inset in the bottomleft-hand corner of Fig. 2(a) is a selected area electron diffraction pattern (SAED) of the nano-
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Fig. 2. (a) TEM image of an individual Ga2 O3 nanowire. The image shows the nanowire with a uniform diameter. The inset in the bottom-left-hand corner shows the SAED pattern of the nanowire. (b) HRTEM image of Ga2 O3 nanowire, showing a clean and structurally perfect surface. In this image, [1 3 5] direction is the growth direction of the nanowires. [1 3 5] direction is indicated by an arrow. The space of about 0.213 nm between arrow heads corresponds to the distance between (1 1 3) planes.
Fig. 1. (a) SEM image of the as-synthesized Ga2 O3 nanowires obtained from thermal evaporation of GaN at 960 °C. (b) A typical SEM image of the Ga2 O3 nanowires terminated with nanoparticles at the tips. (c) XRD pattern recorded from the Ga2 O3 nanowires.
wire. Electron diffraction analysis shows that the Ga2 O3 nanowire is single crystalline. HRTEM image in Fig. 2(b) of the Ga2 O3 nanowire shows a clean and structurally perfect surface. The clear lattice fringes in the image indicated a single crystal structure of the nanowire.
In this image, the [1 3 5] direction is indicated by an arrow. The space of about 0.213 nm between arrowheads corresponds to the distance between (2 1 1) planes. The surfaces of the nanowires are clean and without any sheathed amorphous phase. Raman spectra can provide further structural information about these nanowires. Raman spectra were obtained by illuminating the sample with the 514 nm line of an Arþ laser and 200 mW output power of a laser Raman scattering spectrometer. Spectra of the Ga2 O3 nanowires are in the frequency range from 50 to 1000 cm1 , and spectral resolution is 1.0 cm1 . To improve the signal-to-noise, Raman spectra of several different
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spots of the sample surface were recorded. Each measurement was reproduced 2–3 times. Fig. 3 shows the Raman spectrum of the obtained Ga2 O3 nanowires. The peak positions for the nanowires are in excellent agreement with those reported [15,16]. The related peaks of GaN are not observed in either the XRD pattern or the Raman spectrum. It can be concluded from these structural analyses that the observed nanowires are Ga2 O3 . Several sharp peaks at 112, 146, 170 and 232 cm1 were observed in addition to the previously reported peaks at 253, 321, 345, 476, 630, 653 and 767 cm1 [16], and peaks at 201 and 417 cm1 [15]. For the origin of the extra sharp peaks in the Raman spectrum of Ga2 O3 nanowires (Fig. 3), Abello et al. [18] proposed that the relaxation of the k ¼ 0 selection rule is progressive when the rate of disorder increase or the size decreases and infrared (IR) modes can become weakly active when the structural changes induced by disorder and size effects take place. Therefore, the weak Raman bands at 112, 146, 170 and 232 cm1 seem to correspond to IR-active Eu (TO) (TO is the mode of the transverse optical (TO) phonons) and Eu (LO) (LO is the mode of the longitudinal optical (LO) phonons) [19]. It is reasonable to assign the two volume modes to IR modes whose Raman activities are induced the size effect, which are due to the smaller diameter of Ga2 O3 nanowires. These volume modes of IR modes in GaN epitaxial layers high quality [20] and GaN nanowires [21] observed the Raman spectra.
Fig. 3. Raman spectrum of the Ga2 O3 nanowires at room temperature.
Data from TEM and SEM analyses altogether reveal that the growth of the Ga2 O3 nanowires may be dominated by the VLS process [22] proposed for the nanowires grown by a catalytic-assisted technique [23–26]. In VLS growth process, a metal particle is located at the growth front of the wire and acts as the catalytic active site. In our observation, a typical small-scale SEM image of two nanowires in Fig. 4(a) may provide further details of VLS growth in Ga2 O3 nanowire. The image shows a long nanowire terminating in a nanoparticle at the tip. EDXS analyses shown in Fig. 4(b) indicated that the nanoparticle on the tip mainly consisted of In, Ga and O, but that the nanowire (stem) was only composed of Ga and O (Cu signals are generated from microgrid mesh that supports the nanowires). The molecular ratio of Ga/O of the nanowire calculated from the EDX data was closed to that of Ga2 O3 powder. It can be concluded from these analyses of the observed nanowires that the In nanoparticles play a crucial role in the growth of Ga2 O3 nanowires based on the VLS mechanism and the growth of nanowires is governed by the presence of eutectic metal alloys at the end of nanowires. There are two models for the growth of conventional crystal nanowires, the screw dislocation and VLS. It is well known that the presence of the VLS mechanism. The solidified spherical droplets at the tips of the nanowires are commonly considered to be the evidence for the operation of the VLS mechanism, which is in agreement with our experimental conditions and the observed results (shown in Figs. 1(b) and 4(a)). Therefore, the formed Ga2 O3 nanowires with nanoparticles attached at their ends would be carried away from the substrate. Without the catalytic nanoparticles under similar conditions, Wang and co-workers [27] have successfully prepared Ga2 O3 nanoribbons and nanosheets. Therefore, according to the former report [10], the growth mechanism of the Ga2 O3 nanoribbons or nanobelts may be ascribed to the VS growth. PL spectra of the Ga2 O3 nanowires and Ga2 O3 powders as shown in Fig. 5(a) were measured (Hitachi 850 fluorescence spectrophotometer) with a Xe lamp at room temperature under 254 nm (4.88 eV) ultraviolet light excitation and 310 nm
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(a)
(b)
Fig. 5. PL spectrum of the Ga2 O3 nanowires at room temperature. (a) Excited at 254 nm, filter at 310 nm. (b) Excited at 370 nm, filter at 430 nm.
Fig. 4. (a) A typical SEM image of Ga2 O3 nanowires terminating with nanoparticles. (b) EDX spectra of the nanoparticle at the tip and the wire stem.
filter wavelength. The two apparent PL emission peaks, centered at 330 nm (3.756 eV) and 436 nm (2.843 eV) are related to the Ga2 O3 nanowires, while two peaks at about 350 nm (3.542 eV) (with weak intensity), and 475 nm (2.610 eV) are related to the Ga2 O3 powders. The strong and broad emission in the blue light range, centered at 436 nm (2.843 eV), has a blue shift about 40 nm compared with the PL feature peak 475 nm (2.610 eV) of Ga2 O3 powder. The ultraviolet emission at 330 nm (3.756 eV) is much weaker than that of the blue-light emission in the Ga2 O3 nanowires, and has about 20 nm blue shift compared with the peak 350 (3.542 eV) nm of Ga2 O3 powder. When the specimens were excited by 370 nm (3.35 eV) ultraviolet light and 430 nm filter wavelength, the PL spectra are shown in Fig. 5(b). The blue-light emissions centered at 448 nm (2.767 eV) of Ga2 O3 nanowires increases largely in intensity than the
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PL peak centered at 454 nm (2.730 eV) of Ga2 O3 powder. For the PL mechanism of Ga2 O3 , several reports have suggested that the defect band emission of Ga2 O3 crystals may be attributed to Ga vacancies (VGa ), O vacancies (VO ) [28] and Ga–O vacancy pair (VO , VGa ) [29]. Harwing and Kellendouk [28] suggested that the PL of Ga2 O3 originates from the recombination of an electron on a donor formed by VO and a hole on an acceptor formed by VGa . VasilÕtasiv [29] proposed that the acceptor be formed by a (VO , VGa ). Binet and Gourier [12] also put forward a PL model of the Ga2 O3 crystal after investigation into the temperature dependence of the blue emission upon specific excitation of the acceptor defects. They thought that after excitation of the acceptor, a hole on the acceptor and an electron on the donor are created. Then an electron on donor is captured by a hole on acceptor to form a trapped exciton, which recombines to emit a blue photon. With increasing temperature, the electrons on the donors can be detrapped to the conduction band, and the holes on acceptors can be detrapped to form hole acceptors via the valence band. The electrons on the donors and hole on acceptors recombine via a self-trapped exciton to emit an ultraviolet photon. In our work, because Ga2 O3 nanowires are fabricated at high temperature, a quantity of O vacancies (VO ) or Ga–O vacancy pair (VO , VGa ) can also easily be produced. Therefore, the PL feature of the Ga2 O3 nanowires in ours is consistent with the model of the mentioned earlier.
4. Conclusions Ga2 O3 nanowires were fabricated through simple physical evaporation in oxygen atmosphere using GaN powder as raw material and In as catalyst. SEM images show that the nanowires exhibit high-yield products. The typical diameters of Ga2 O3 nanowires are in the range of 20–60 nm, and lengths in the range of several tens to several hundreds micrometers, respectively. Our observations further revealed the growth of the nanowires was dominated by VLS growth mechanism. PL
spectrum of Ga2 O3 nanowires consists of a strong blue-light emission peaks at 436 nm (2.843 eV) and an ultraviolet emission peak at 330 nm (3.756 eV) under excitation at 254 nm (4.88 eV) at room temperature. Moreover, because of the high bright blue light at 436 nm (2.843 eV) of Ga2 O3 nanowires, the optoelectronic nanodevices and nanosize sensors may be fabricated. Further studies on the luminescence of mechanism of nanowires are underway.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 60277023).
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Catalytic growth of Ga2O3 nanowires by physical evaporation and their photoluminescence properties Jun Zhang *, Feihong Jiang Department of Physics, Yantai University, Yantai 264005, PR China Received 13 October 2002
Abstract Gallium oxide (Ga2 O3 ) nanowires were fabricated through simple physical evaporation in oxygen atmosphere using GaN powder as raw material and an Al2 O3 substrate coated with a thin indium layer. The morphology and structure of the nanowires were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectrum. These nanowires have diameters ranging from 20 to 60 nm and lengths ranging from several tens to several hundreds micrometers. Photoluminescence spectrum under excitation at 254 nm shows that these Ga2 O3 nanowires have a blue emission at 436 nm and an ultraviolet emission at 330 nm, which may be attributed to the defects such as the oxygen vacancies and gallium–oxygen vacancy pairs. Ó 2003 Published by Elsevier Science B.V.
1. Introduction One-dimension nanomaterials (nanotubes, nanowires and nanorods) have attracted intensive experimental and theoretical interests due to their novel physical properties and potential application for nanodevices [1,2]. Recently, much attention has been paid to the preparation of nanowires [3– 9] and nanobelts [10] of the family of oxides for their interesting optical and electrical properties. Several binary oxide nanowires such as MgO [3,4], SiO2 [5,6], In2 O3 [7], GeO2 [8], ZnO [9] and
*
Corresponding author. Tel.: +86-535-690-2146; fax: +86535-690-1947. E-mail address: [email protected] (J. Zhang).
nanobelts of semiconducting oxides (ZnO, SnO2 , In2 O3 and CdO) [10] have been successfully synthesized. As a wide bandgap (Eg 4:9 eV) compound [11], nanometer-scale gallium oxide (Ga2 O3 ) with large surface area/volume ratio has exhibited particular conduction and optical properties [12] and has great potential application in optoelectronic nanodevices and gas sensors [13, 14]. Ga2 O3 nanowires have been successfully synthesized by a simple physical evaporation via the vapor–solid (VS) process using Ga powder [15], an arc-discharge method based on the mechanism of catalytic step growth [16], and GaAs was used as Ga precursor and Au was the catalyst [17]. Here, through a simple physical evaporation method in oxygen atmosphere using GaN powder as raw material and an Al2 O3 substrate coated with a thin
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indium (In) layer as catalyst, we have successfully synthesized Ga2 O3 nanowires in a high yield. The In nanoparticles play a crucial role in the growth of Ga2 O3 nanowires based on the vapor–liquid– solid (VLS) mechanism. Photoluminescence (PL) spectrum of the Ga2 O3 nanowires at room temperature exhibits strong blue–green light emission and a blue shift compared to that of Ga2 O3 powder. Finally, the mechanism of PL will also be discussed.
2. Experimental procedure Our synthesis is based on simple thermal evaporation using GaN powder as raw material. An Al2 O3 membrane was used as a substrate. The substrate was rinsed with deionized water before use. A layer of In nanoparticles (about several tens of nanometers in thickness) was evaporated onto one side of the substrate under high vacuum (2 105 torr). The GaN powders were placed at the center of a ceramic boat. The In-coated substrate was typically placed at 0.5–5.0 cm from the center of the boat, and the ceramic boat were transferred into a quartz tube (30 mm inner diameter). This quartz tube was then placed inside a horizontal electronic resistance furnace (heated by silicon–carbon rods) with the center of the ceramic boat positioned at the center of the furnace and the substrate placed downstream of the gas flow. The temperature of crucible was rapidly increased to 960 °C (about 6 min) from room temperature and kept at 960 °C for 1 h under a constant flow of mixture gases of argon and oxygen (Ar/O2 is 4:1). Argon was introduced into the quartz tube through a mass-flow controller at the rate of 100 standard cubic centimeters per minute (sccm). After the furnace was cooled to room temperature, white wool-like products were found on the surface of the substrate. The morphology and structure of the synthesized product were characterized by scanning electron microscopy (SEM, JOEL JSM-5610LV), X-ray diffraction (XRD, Philips PW 1710 with Cu Ka radiation), transmission electron microscopy (TEM, JEM-200CX), high-resolution transmission electron microscopy (HRTEM, JEOL-2010) and
energy-dispersive X-ray spectroscopy (EDX). In addition, because Raman spectroscopy has been proven to be a powerful tool for the investigation of material properties such as doping concentration, defect identification and crystal orientation. Raman spectrum (Spex-1403) of the Ga2 O3 nanowires was also investigated.
3. Results and discussion SEM image shown in Fig. 1(a) reveals that the products consist of a large quantity of wire-like nanostructures with typical lengths in the range of several tens to several hundreds of micrometers. Some of them even have lengths on the order of millimeter, which results in a very large aspect ratio. The typical diameters of the nanowires are in the range of 20–60 nm. Another typical SEM image of the specimen surface shown in Fig. 1(b) indicates that Ga2 O3 nanowires cross each other and are randomly distributed on the surface of the substrate. Each nanowires terminate in a nanoparticle at the tip. The image provides further details of VLS growth mechanism in Ga2 O3 nanowires. The crystal structure of these nanowires was characterized by XRD. Fig. 1(c) shows the structural characterization of the nanowires stripped from the substrate. Miller indices are indicated on each diffraction peaks. It can be seen that all peaks can be indexed to a monoclinic crystalline Ga2 O3 phase, in good agreement with the reported data (JCPDS 11-370). The lattice constants of the crystalline phase are a0 ¼ 0:58 nm, b0 ¼ 0:304 nm, c0 ¼ 1:223 nm and b ¼ 103:7°, and its space group is identified as C2/m. Further structural and elemental analyses of the individual Ga2 O3 nanowire were performed using TEM. For TEM observation, the above specimen was pulverized in ethyl alcohol, and then dispersed onto a copper grid-coated carbon film. A lowmagnified TEM image of a long individual Ga2 O3 nanowire is displayed in Fig. 2(a). The nanowire has a uniform diameter. The diameter of the nanowire is about 40 nm. The inset in the bottomleft-hand corner of Fig. 2(a) is a selected area electron diffraction pattern (SAED) of the nano-
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Fig. 2. (a) TEM image of an individual Ga2 O3 nanowire. The image shows the nanowire with a uniform diameter. The inset in the bottom-left-hand corner shows the SAED pattern of the nanowire. (b) HRTEM image of Ga2 O3 nanowire, showing a clean and structurally perfect surface. In this image, [1 3 5] direction is the growth direction of the nanowires. [1 3 5] direction is indicated by an arrow. The space of about 0.213 nm between arrow heads corresponds to the distance between (1 1 3) planes.
Fig. 1. (a) SEM image of the as-synthesized Ga2 O3 nanowires obtained from thermal evaporation of GaN at 960 °C. (b) A typical SEM image of the Ga2 O3 nanowires terminated with nanoparticles at the tips. (c) XRD pattern recorded from the Ga2 O3 nanowires.
wire. Electron diffraction analysis shows that the Ga2 O3 nanowire is single crystalline. HRTEM image in Fig. 2(b) of the Ga2 O3 nanowire shows a clean and structurally perfect surface. The clear lattice fringes in the image indicated a single crystal structure of the nanowire.
In this image, the [1 3 5] direction is indicated by an arrow. The space of about 0.213 nm between arrowheads corresponds to the distance between (2 1 1) planes. The surfaces of the nanowires are clean and without any sheathed amorphous phase. Raman spectra can provide further structural information about these nanowires. Raman spectra were obtained by illuminating the sample with the 514 nm line of an Arþ laser and 200 mW output power of a laser Raman scattering spectrometer. Spectra of the Ga2 O3 nanowires are in the frequency range from 50 to 1000 cm1 , and spectral resolution is 1.0 cm1 . To improve the signal-to-noise, Raman spectra of several different
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spots of the sample surface were recorded. Each measurement was reproduced 2–3 times. Fig. 3 shows the Raman spectrum of the obtained Ga2 O3 nanowires. The peak positions for the nanowires are in excellent agreement with those reported [15,16]. The related peaks of GaN are not observed in either the XRD pattern or the Raman spectrum. It can be concluded from these structural analyses that the observed nanowires are Ga2 O3 . Several sharp peaks at 112, 146, 170 and 232 cm1 were observed in addition to the previously reported peaks at 253, 321, 345, 476, 630, 653 and 767 cm1 [16], and peaks at 201 and 417 cm1 [15]. For the origin of the extra sharp peaks in the Raman spectrum of Ga2 O3 nanowires (Fig. 3), Abello et al. [18] proposed that the relaxation of the k ¼ 0 selection rule is progressive when the rate of disorder increase or the size decreases and infrared (IR) modes can become weakly active when the structural changes induced by disorder and size effects take place. Therefore, the weak Raman bands at 112, 146, 170 and 232 cm1 seem to correspond to IR-active Eu (TO) (TO is the mode of the transverse optical (TO) phonons) and Eu (LO) (LO is the mode of the longitudinal optical (LO) phonons) [19]. It is reasonable to assign the two volume modes to IR modes whose Raman activities are induced the size effect, which are due to the smaller diameter of Ga2 O3 nanowires. These volume modes of IR modes in GaN epitaxial layers high quality [20] and GaN nanowires [21] observed the Raman spectra.
Fig. 3. Raman spectrum of the Ga2 O3 nanowires at room temperature.
Data from TEM and SEM analyses altogether reveal that the growth of the Ga2 O3 nanowires may be dominated by the VLS process [22] proposed for the nanowires grown by a catalytic-assisted technique [23–26]. In VLS growth process, a metal particle is located at the growth front of the wire and acts as the catalytic active site. In our observation, a typical small-scale SEM image of two nanowires in Fig. 4(a) may provide further details of VLS growth in Ga2 O3 nanowire. The image shows a long nanowire terminating in a nanoparticle at the tip. EDXS analyses shown in Fig. 4(b) indicated that the nanoparticle on the tip mainly consisted of In, Ga and O, but that the nanowire (stem) was only composed of Ga and O (Cu signals are generated from microgrid mesh that supports the nanowires). The molecular ratio of Ga/O of the nanowire calculated from the EDX data was closed to that of Ga2 O3 powder. It can be concluded from these analyses of the observed nanowires that the In nanoparticles play a crucial role in the growth of Ga2 O3 nanowires based on the VLS mechanism and the growth of nanowires is governed by the presence of eutectic metal alloys at the end of nanowires. There are two models for the growth of conventional crystal nanowires, the screw dislocation and VLS. It is well known that the presence of the VLS mechanism. The solidified spherical droplets at the tips of the nanowires are commonly considered to be the evidence for the operation of the VLS mechanism, which is in agreement with our experimental conditions and the observed results (shown in Figs. 1(b) and 4(a)). Therefore, the formed Ga2 O3 nanowires with nanoparticles attached at their ends would be carried away from the substrate. Without the catalytic nanoparticles under similar conditions, Wang and co-workers [27] have successfully prepared Ga2 O3 nanoribbons and nanosheets. Therefore, according to the former report [10], the growth mechanism of the Ga2 O3 nanoribbons or nanobelts may be ascribed to the VS growth. PL spectra of the Ga2 O3 nanowires and Ga2 O3 powders as shown in Fig. 5(a) were measured (Hitachi 850 fluorescence spectrophotometer) with a Xe lamp at room temperature under 254 nm (4.88 eV) ultraviolet light excitation and 310 nm
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(a)
(b)
Fig. 5. PL spectrum of the Ga2 O3 nanowires at room temperature. (a) Excited at 254 nm, filter at 310 nm. (b) Excited at 370 nm, filter at 430 nm.
Fig. 4. (a) A typical SEM image of Ga2 O3 nanowires terminating with nanoparticles. (b) EDX spectra of the nanoparticle at the tip and the wire stem.
filter wavelength. The two apparent PL emission peaks, centered at 330 nm (3.756 eV) and 436 nm (2.843 eV) are related to the Ga2 O3 nanowires, while two peaks at about 350 nm (3.542 eV) (with weak intensity), and 475 nm (2.610 eV) are related to the Ga2 O3 powders. The strong and broad emission in the blue light range, centered at 436 nm (2.843 eV), has a blue shift about 40 nm compared with the PL feature peak 475 nm (2.610 eV) of Ga2 O3 powder. The ultraviolet emission at 330 nm (3.756 eV) is much weaker than that of the blue-light emission in the Ga2 O3 nanowires, and has about 20 nm blue shift compared with the peak 350 (3.542 eV) nm of Ga2 O3 powder. When the specimens were excited by 370 nm (3.35 eV) ultraviolet light and 430 nm filter wavelength, the PL spectra are shown in Fig. 5(b). The blue-light emissions centered at 448 nm (2.767 eV) of Ga2 O3 nanowires increases largely in intensity than the
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PL peak centered at 454 nm (2.730 eV) of Ga2 O3 powder. For the PL mechanism of Ga2 O3 , several reports have suggested that the defect band emission of Ga2 O3 crystals may be attributed to Ga vacancies (VGa ), O vacancies (VO ) [28] and Ga–O vacancy pair (VO , VGa ) [29]. Harwing and Kellendouk [28] suggested that the PL of Ga2 O3 originates from the recombination of an electron on a donor formed by VO and a hole on an acceptor formed by VGa . VasilÕtasiv [29] proposed that the acceptor be formed by a (VO , VGa ). Binet and Gourier [12] also put forward a PL model of the Ga2 O3 crystal after investigation into the temperature dependence of the blue emission upon specific excitation of the acceptor defects. They thought that after excitation of the acceptor, a hole on the acceptor and an electron on the donor are created. Then an electron on donor is captured by a hole on acceptor to form a trapped exciton, which recombines to emit a blue photon. With increasing temperature, the electrons on the donors can be detrapped to the conduction band, and the holes on acceptors can be detrapped to form hole acceptors via the valence band. The electrons on the donors and hole on acceptors recombine via a self-trapped exciton to emit an ultraviolet photon. In our work, because Ga2 O3 nanowires are fabricated at high temperature, a quantity of O vacancies (VO ) or Ga–O vacancy pair (VO , VGa ) can also easily be produced. Therefore, the PL feature of the Ga2 O3 nanowires in ours is consistent with the model of the mentioned earlier.
4. Conclusions Ga2 O3 nanowires were fabricated through simple physical evaporation in oxygen atmosphere using GaN powder as raw material and In as catalyst. SEM images show that the nanowires exhibit high-yield products. The typical diameters of Ga2 O3 nanowires are in the range of 20–60 nm, and lengths in the range of several tens to several hundreds micrometers, respectively. Our observations further revealed the growth of the nanowires was dominated by VLS growth mechanism. PL
spectrum of Ga2 O3 nanowires consists of a strong blue-light emission peaks at 436 nm (2.843 eV) and an ultraviolet emission peak at 330 nm (3.756 eV) under excitation at 254 nm (4.88 eV) at room temperature. Moreover, because of the high bright blue light at 436 nm (2.843 eV) of Ga2 O3 nanowires, the optoelectronic nanodevices and nanosize sensors may be fabricated. Further studies on the luminescence of mechanism of nanowires are underway.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 60277023).
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