Facile Synthesis and Characterization of Gallium Oxide (β-Ga2O3) 1D Nanostructures: Nanowires, Nanoribbons, and Nanosheets Guoxiu Wang,*,† Jinsoo Park,† Xiangyang Kong,‡ Paten R. Wilson,† Zhixin Chen,† and Jung-ho Ahn§
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School of Mechanical, Materials and Mechatronic Engineering, UniVersity of Wollongong, NSW 2522, Australia, School of Materials Science and Engineering, Shanghai Jiao Tong UniVersity, Shanghai, China, and Department of Materials Engineering, Andong National UniVersity, Andong, South Korea ReceiVed December 20, 2007; ReVised Manuscript ReceiVed February 21, 2008
ABSTRACT: β-Ga2O3 one-dimensional (1D) nanostructures including nanowires, nanoribbons and nanosheets were synthesized via a combined carbon thermal reduction and chemical vapor deposition (CVD) approach. All of these nanostructures consist of single-crystalline monoclinic structure, which has been confirmed via high-resolution TEM and electron diffraction analysis. We obtained β-Ga2O3 nanowires at high temperature zone, nanosheets, and nanoribbons at medium temperature zone at low-pressure environments. We found that β-Ga2O3 nanowires and nanoribbons have diversified different growth directions. The optical properties of β-Ga2O3 nanowires and nanoribbons from UV-vis spectra show that the bandgaps are red-shifted by 0.13 eV. Introduction One-dimensional (1D) semiconductor nanostructures including nanowires, nanotubes, nanorods and nanoribbons, have been under extensive investigations. They are ideal structures for studying dimensionality confined transport phenomena. Those 1D nanostructures are building blocks for fabricating functional nanoscale devices with high device density, having a wide range of applications such as nanoelectronics, photonics, nanocomputing, molecular chemical and biological sensing, etc.1–5 Because element and compound semiconductors provide a vast availability to select materials with desired electronic and optical properties, semiconductor nanostructures also play a key role not only in exploring new physic properties, but also in developing nanodevices. Gallium oxide is an important wide band gap (Eg ) 4.9 eV) semiconductor material. In particular, monoclinic gallium oxide (β-Ga2O3) has excellent chemical and thermal stability. Its n-type conduction could be achieved when synthesized under reducing conditions because of the oxygen vacancies that acts as shallow donors.6 β-Ga2O3 has various technological applications such as transparent conducting oxides, optical emitter for ultraviolet radiation, flat-pannel display, spin tunneling junctions, and high-temperature gas sensing.7–9 β-Ga2O3 1D nanostructures have been synthesized by a variety of techniques such as arc discharge of GaN,10 thermal oxidation of GaN,11 laser ablation of Ga2O3 target,12 and microwave plasma chemical vapor deposition (MPCVD).13 In general, two mechanisms have been proposed to govern the growth of Ga2O3 1D nanostructures. The first one is vapor-liquid-solid (VLS) mechanism, in which extrinsic or intrinsic catalytic metal nanoparticles generate energetically favorable site to confine and direct the growth along one direction. The second one relies on vapor-solid (VS) process involving the formation of several gallium suboxide species.14 Therefore, it is important to achieve the synthesis of gallium oxide 1D nanostructures in a controllable manner to obtain the deserved physical properties. In this paper, we report a facile synthesis of β-Ga2O3 1D nanostructures including nanowires, nanoribbons, and nanosheets * Corresponding author. E-mail:
[email protected]. † University of Wollongong. ‡ Shanghai Jiao Tong University. § Andong National University.
by a combined carbothermal reduction and chemical vapor deposition. The influence of the growth conditions to the morphologies of Ga2O3 1D nanostructures was explored systematically. We found that the morphologies of β-Ga2O3 1D nanostructures can be varied by controlling the synthetic parameters. Their crystal structures and optical properties were examined by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), highresolution TEM (HRTEM), UV-vis spectroscopy, and Raman spectroscopy. Experimental Section Materials Synthesis. Gallium oxide (Ga2O3, 99.99% Aldrich) and carbon black were mixed with a molar ratio of 1:1. The mixture powders were placed in an alumina boat located in a quartz tube reactor with 3 independent temperature zones. The Si wafer and quartz substrates were placed at different distances down stream from the source materials. During the chemical vapor deposition, the temperature of Ga2O3 source was set at 1000 °C, and the substrate at 800-900 °C. The reaction chamber was initially evacuated and then flowed with a mixture gas (90% Ar + 10% O2) at a rate of 100-200 SCCM (standard cubic centimeter per minute) for 1 h, during which the whole reactor chamber was maintained at the vacuum pressure of 300 Torr. After cooling down, a white-colored, wool-like fluffy products were deposited on the Si substrates at both the high-temperature zone (900 °C) and mediumtemperature zone (800 °C). Materials Characterization. The deposited Ga2O3 layers were peeled from the substrate via ultrasonic vibration. The phase, morphologies, and crystal structures of the Ga2O3 deposits were examined using X-ray diffraction (XRD, Philips 1730 X-ray diffractometer), fieldemission scanning electron microscope (FESEM, JEOL JSM-6700F), transmission electron microscope (TEM), and high-resolution TEM (HRTEM, JEOL 2011). The optical properties of Ga2O3 1D nanostructures were characterized by Raman spectroscopy (Jobin Yvon HR800 confocal Raman system with 632.8 nm diode laser excitation on a 300 lines/mm grating) and UV-vis spectroscopy (UV1700, Shimadzu).
Results and Discussion Morphologies of the as-deposited gallium oxide products were observed by FESEM and are shown in Figure 1. Figure 1a exhibited high-density nanowires with lengths extending to several tens to several hundreds of micrometers. Those gallium oxide nanowires were collected from the Si substrates positioned
10.1021/cg701251j CCC: $40.75 2008 American Chemical Society Published on Web 05/13/2008
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Figure 2. X-ray diffraction of β-Ga2O3 crystalline powders, nanosheets, and nanowires.
Figure 1. FESEM images of (a) β-Ga2O3 nanowires (b) β-Ga2O3 nanosheets.
at the temperature zone of 900 °C. On the contrary, gallium oxide nanosheets were formed on the bare quartz substrates positioned at the temperature zone of 800 °C (as shown in Figure 1(b)). We observed large size Ga2O3 nanosheets in the range of several tens square micrometers. Gallium oxide nanoribbons were harvested on Si substrates at 800 °C temperature zone. The chemical microanalysis was performed on both nanowires and nanosheets by using energy dispersive X-ray spectroscopy (EDX), indicating the composition of the as-deposited nanostructures being Ga2O3. The phase identities of the deposited products were determined by X-ray diffraction. Figure 2 shows the XRD patterns of β-Ga2O3 powders, nanowires and nanosheets. All diffraction lines can be indexed to a monoclinic (S.G.C2/m) β-Ga2O3 crystal structure without any other impurities. There is no shift of the diffraction line positions for all three samples with different morphologies. However, the intensities of individual diffraction peaks for different samples are significantly varied. The (002) and (111) diffraction peaks show high intensity for both β-Ga2O3 crystalline powders and nanowires. However, the (110) peaks in β-Ga2O3 nanosheets exhibited the highest intensity and the other diffraction peaks are weak and diminished. The (110) diffraction peaks originated from the [110] crystal planes. β-Ga2O3 nanosheets have an areas of a few tens micrometers and cover the surface like a thin film. Therefore, the XRD
Figure 3. (a) TEM image of a β-Ga2O3 nanowire with microtwins (b) selected area electron diffraction (SAED) pattern of the nanowire twin structure.
pattern of the β-Ga2O3 nanosheets is very much similar to thin films with the feature of textured diffraction pattern.15 The detailed crystal structures of β-Ga2O3 1D nanostructures were further analyzed by TEM, HRTEM, and selected area electron diffraction (SAED). Figure 3a shows a TEM image of a single gallium oxide nanowire with a diameter of 25 nm. The stripe textures are clearly visible, indicating microtwins in this Ga2O3 nanowire. Figure 3b shows the corresponding SAED pattern recorded along the [111] zone axis, through which the growth direction of the nanowire was identified as along the [1j1j2]* direction. The streaking in the diffraction pattern was due to the small thickness of the microtwins. The streaking direction is perpendicular to the twin boundaries and along the [31j2]* direction. Thus [31j2j] plane is the twin plane. The [111] zone axis of the half of the crystal plates was parallel to the electron beam direction and the [1j1j1j] zone axis of the twin plates was also parallel to the electron beam direction. The nanowire is composed of the alternating [111] and [1j1j1j] microplates. The split diffraction spots on the left and right sides in Figure 3b represent [1j1j2] [1j1j2] and [112j] [112j]t planes
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Figure 4. TEM image of a zigzag β-Ga2O3 nanowire (nanosaw) structure. The inset is the magnified view sawtooth structure.
Figure 5. (a) TEM image of β-Ga2O3 nanoribbons under low magnification. (b) HRTEM image of nanoribbon 1 marked in (a). (c) Electron diffraction pattern of nanoribbon 1. (d) HRTEM image and electron diffraction pattern of nanoribbon 2 marked in (a).
respectively. The other diffraction spots of crystal planes and their twin planes are overlapped, which have been indexed in Figure 3b. We also observed a zigzag nanosaw structure among the gallium oxide nanowires collected from this high temperature zone (as shown in Figure 4). A magnified view of the sawtooth structure is shown as the inset in Figure 4. The zigzag nanosaw structure is different from that previously reported as gallium oxide zigzag nanoropes, which was prepared by subliming a mixture of Ga2O3 and GaN at 1200 °C.16 Our gallium oxide zigzag nanostructure is more like a nanosize saw with homogeneously distributed saw teeth on both sides. The formation of such zigzag gallium oxide nanosaw structures could have originated from the defect-driven crystal growth mechanism of Ga2O3 nanowires. As indicated in ref 16, oxygen deficiency in Ga2O3 may cause the disordered connection of octahedral chains, inducing faults along the a and c axes, eventually forming the zigzag structure. The crystal structures of β-Ga2O3 nanoribbons were systematically examined by TEM and HRTEM analysis. Figure 5a shows a general view of β-Ga2O3 nanoribbons harvested from the medium temperature zone of 800 °C. We noted that the majority of the nanostructures are nanoribbons. Gallium oxide nanoribbons were well dispersed during the TEM preparation process. Among them, a few triangle-shaped nanosheetlike structures are also present. We randomly chose two nanoribbons marked by circles as 1 and 2 for HRTEM and SAED analysis.
Wang et al.
Figure 6. (a) Low-magnification TEM view of the mixture of β-Ga2O3 nanoribbons from the second batch. (b) TEM image of a β-Ga2O3 nanoribbon with a growth direction along the 11j0* direction. (c) Electron diffraction pattern of nanoribbon shown in (b). (d) HRTEM image of β-Ga2O3 nanoribbon in (b).
The corresponding HRTEM image and SAED pattern for nanoribbon 1 are shown in images b and c in Figure 5. Atomicresolved lattice images of β-Ga2O3 nanoribbons are presented. In Figure 5b, the [202j] planes have an interplanar spacing of 0.234 nm parallel to the [01j0]* nanoribbon growth direction. The SAED pattern was recorded along the [1j01] zone axis. The diffraction spots with strong intensity are believed to be from the [1j01] crystal zone of the β-Ga2O3 nanoribbon, representing zero-order Laue zone reflection. Extra weak diffraction spots between the bright diffraction spots are due to the first-order Laue zone reflections of the thin β-Ga2O3 nanoribbon.11 Figure 5d shows the HRTEM image of nanoribbon 2, in which the SAED pattern was recorded along the [101] zone axis. The growth direction of nanoribbon 2 is along the [010]* direction, which is parallel to the [202j] lattice planes. Nanoribbon 2 is enclosed by [1j01] and [101j] side surfaces. Figure 6a shows a TEM image of β-Ga2O3 nanoribbons from another preparation batch. The Ga2O3 nanoribbon marked in Figure 6a with a square was rotated, and its magnified TEM image is shown in Figure 6b, which has a width of about 80 nm. Its SAED pattern was recorded along the [110] zone axis (Figure 6c), from which the interplane angle between the (11j0) crystal planes and (001j) planes was determined to be 86.6°. The growth direction of this nanoribbon was determined to be along [11j0]* direction. The lattice-resolved HRTEM image of this nanoribbon is presented in Figure 6(d). The (001) crystal planes have a d-spacing of 0.56 nm and 3.4° with the growth direction [11j0]*. Figure 7 shows an atomically resolved HRTEM image of a Ga2O3 nanosheet and an electron diffraction pattern recorded along the [111] zone axis. The as-deposited Ga2O3 nanosheets are single crystalline, without defects and dislocations. We measured the distances between the (11j0) planes to be 0.29 nm and between the (2j02) planes to be 0.28 nm, which match well with the values from the ICSD database. We also determined the angle between the (11j0) and (202j) crystal planes to be 93.3°, as shown in the SAED pattern in Figure 7. For the controlled growth, the synthesis of Ga2O3 1D nanostructures could be subjected to the following reaction procedures.17,18 First, the Ga2O3 source react with carbon black powders to form G2O or Ga vapor at a high processing
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Figure 7. HRTEM image of a β-Ga2O3 nanosheet. The inset is the corresponding electron diffraction pattern recorded along [111] zone axis.
temperature of 1000 °C. The vapor phases then cool down and are deposited on substrates as Ga2O3 nanostructures. Evaporation:
Ga2O3(s) + 2C(s) f Ga2O(g) + 2CO(g)
(1)
Ga2O3(s) + 3C(s) f 2Ga(g) + 3CO(g)
(2)
Figure 8. UV-vis spectra from suspensions of β-Ga2O3 nanoribbons and nanowires; arrowheads mark the steepest absorption edge, at which the bandgaps were determined.
or
Deposition:
Ga2O(g) + O2(g) f Ga2O3(s)
(3)
4Ga(g) + 3O2(g) f Ga2O3(s)
(4)
or
Compared to the direct thermal evaporation,11,16 the carbon thermal reduction approach can lower the heating temperature by about 200 °C. This technique also allows us to avoid the use of expensive organometallic precursors.19 In addition, we obtained Ga2O3 nanowires, nanoribbons and nanosheets simultaneously at different deposition temperature zones, which could be influenced by different growth mechanisms. We found the existing of micro-twin structures along the growth direction in Ga2O3 nanowires. The bandgaps of as-prepared β-Ga2O3 nanowires and nanoribbons were determined from UV-vis absorption spectroscopy. Ga2O3 nanowires and nanoribbons were diluted by factors of 20 in absolute ethanol forming a homogeneous suspension and then transferred to a Teflon-stopped quartz cuvettes for absorption spectroscopy measurement. Figure 8 shows the UV-vis absorption spectra of β-Ga2O3 nanoribbons and nanowires. The bandgaps were calculated based on the steepest absorption edge in the UV-vis spectra. We found that both β-Ga2O3 nanowires and nanoribbons exhibit the same bandgaps of 4.77 eV (260 nm), which is slightly smaller than that of perfect single crystals (4.9 eV, 253 nm). This represents a redshift of 7 nm (0.13 eV) for nanowires and nanoribbons, possibly caused by defects in the crystal structures.20 The optical properties of β-Ga2O3 nanowires were further characterized by Raman spectroscopy. Figure 9 shows Raman spectra of β-Ga2O3 nanowires and crystalline powders. The main Raman vibration bands at 201, 417, 631, 653 and 767 cm-1 are clearly visible, which are characteristic for bulk β-Ga2O3. The positions of Raman shift peaks for nanowires match well with those for crystalline powders, and also are consistent with the previous report.21 We noticed that the Raman shift at 477 cm-1 for
Figure 9. Raman spectra of β-Ga2O3 nanowires and crystalline powders.
crystalline powders disappeared for nanowires. Instead, a new Raman shift at 527 cm-1 appeared for nanowires. This could be induced by the surface stress or point defects in the nanowires.22 Conclusions Gallium oxide (β-Ga2O3) 1D nanostructures such as nanowires, nanoribbons, and nanosheets can be conventiently prepared by carbon thermal reduction CVD technique. All of those 1D nanostructures are single-crystalline in nature, which have been confirmed via HRTEM and SAED analysis. We obtained β-Ga2O3 nanowires at the 900 °C temperature zone on Si substrates governed by VLS mechanism, nanoribbons at the 800 °C temperature zone on Si substrates, and nanosheets at the 800 °C temperature zone on quartz substrates. We also observed the zigzag nanosaw structure among nanowires. The results from UV-vis spectra show that the bandgaps of β-Ga2O3 nanowires and nanoribbons are red-shifted by 0.13 eV. Acknowledgment. This work was financially supported by the Australian Research Council (ARC) through an ARC Discovery project (DP0559891).
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