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J. Phys. Chem. C 2009, 113, 1980–1983
Quasi-Aligned Ga2O3 Nanowires Grown on Brass Wire Meshes and Their Electrical and Field-Emission Properties Yang Huang,*,†,‡ Zongli Wang,§ Qiang Wang,§ Changzhi Gu,§ Chengchun Tang,‡ Yoshio Bando,‡ and Dmitri Golberg*,†,‡ Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tennodai 1, Tsukuba, 305-0005, Ibaraki, Japan, World Premier International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Ibaraki, Japan, and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed: NoVember 6, 2008; ReVised Manuscript ReceiVed: December 4, 2008
Quasi-aligned β-Ga2O3 nanowires were fabricated using the brass wire meshes as the substrates. The nanowire roots were tightly connected to the brass wire surfaces that resulted in a very good physical and electrical contact. Electrical measurements showed that the ohmic contacts were achieved and the resistivities of the nanowires were 300-500 Ω · cm. The Ga2O3 nanowire/brass wire hybrid structures exhibited an enhanced field-emission performance. A turn-on field of ∼6.2 V · µm-1 at an emission current density of 10 µA · cm-2 and an emission current fluctuation within (13% at a mean field-emission current density of ∼0.56 µA · cm-2 were measured. 1. Introduction One-dimensional (1D) nanoscale materials such as nanotubes, nanowires, and nanobelts have attracted prime attention due to their importance in understanding fundamental physical concepts and potential applications in building blocks of electronic and optical nanodevices.1,2 Over the past decades, numerous methods were employed to prepare different 1D nanomaterials. These methods can be grouped into two categories: syntheses on substrates and syntheses without substrates. The former methods, e.g., floating catalyst chemical vapor deposition, usually give a high yield, but the prepared nanowires are typically entangled together and could not be easily dispersed.3 Thus, they may be hardly integrated into devices. The second strategy allows one to control the growth positions of nanowires and make it possible to fabricate a macroscale nanowire film.4 Such films are promising due to a relative ease in their applications as anodes for batteries, field-emission devices, and so on.5,6 Especially, the films made of standing-up nanowire arrays hold the highest promise. So far, many methods and substrates have been utilized to grow the nanowires of various compositions; however, the problem of a loose substrate/nanowire contact remains and hinders the reliable array integration into devices.7 Monoclinic gallium oxide (β-Ga2O3) has a wide band gap of ∼4.9 eV. It has widely been used as an insulating oxide layer in gallium-based electric devices. Introduction of oxygen vacancies in β-Ga2O3 can result in n-type conduction. Such semiconductor can then be used in optoelectronic devices, spintunneling junctions, and high-temperature gas sensors.8-10 Ga2O3 nanowires or nanobelts have been synthesized for years. Some novel properties such as a photoinduced wettability conversion and gas sensitivity at room temperature have been discovered * To whom correspondence should be addressed. E-mail: huang.yang@ nims.go.jp (Y.H.);
[email protected] (D.G.). † University of Tsukuba. ‡ World Premier International Center for Materials Nanoarchitectonics. § Chinese Academy of Sciences.
in them.11,12 However, the reports on the reliable preparation and properties of Ga2O3 nanowire arrays have still been limited.13 Brass, a metallic alloy with main Cu and Zn constituents, possesses fairly good plasticity, superb wear, and corrosion resistance and thus has a frequent appearance in our daily life. As an example, a brass wire mesh is used in many filters and shields. Here, we report a novel method to prepare the quasi-aligned Ga2O3 nanowires on such brass wire meshes. The nanowires were found to be tightly physically and mechanically joined with the meshes providing desirably good electrical contacts. The electrical measurements on a single nanowire and fieldemission measurements of the nanowire arrays were then performed. 2. Experimental Section The commercially available brass wire meshes had a mesh pitch of 360 µm and a wire diameter of 115 µm. Energydispersive X-ray (EDX) analysis (see Supporting Information, Figure S1) showed that a brass mesh mainly contained Cu and Zn with the traces of Al (with a weight ratio of 60.50:21.68: 00.74). In order to grow Ga2O3 nanowires, 500 mg of Ga particles was put into an alumina crucible and covered with a piece of a brass wire mesh at a distance of 3 mm. Then the crucible was transferred into the center of a conventional tube furnace. The chamber of the furnace was first flushed with an intense flow of Ar (800 sccm) and then quickly heated to 900 °C in 20 min and kept at this temperature for 20 min. After the reaction, the crucible was cooled to room temperature naturally. During the reaction and cooling process, a protecting Ar flow of 200 sccm was used. The annealed mesh and the as-grown Ga2O3 nanowires were characterized using an X-ray diffractometer (XRD), a scanning electron microscope (SEM), and a transmission electron microscope (TEM, JEOL-3100F) equipped with an EDX system.
10.1021/jp809800n CCC: $40.75 2009 American Chemical Society Published on Web 01/12/2009
Quasi-Aligned Ga2O3 Nanowires
Figure 1. SEM images of a brass wire mesh and the nanowires on its surface after reaction for 20 min.
The electrical measurements on the single nanowires were taken in a double-probe SEM system described in the previous study.14,15 Briefly, a piece of nanowire-covered mesh was attached to a metallic sample stage; one of the tungsten probes was approached and connected to the tip of a given nanowire; a bias voltage was then applied between the probe and sample stage, and the I-V curves were recorded using a Keithley 6517A device. The field-emission measurements were performed in a vacuum chamber at a pressure of 4.2 × 10-6 Pa. A rodlike aluminum probe with a cross section of 1 mm2 was used as an anode, which was positioned by a linear-motion step controller.16 A piece of a nanowire-covered mesh was used as a cathode. A dc voltage sweeping from 100 to 1100 V was applied to the probe to collect the electrons emitted from the sample. 3. Results and Discussion 3.1. Morphology and Microstructure of a Product. Figure 1a shows the morphology of a brass wire mesh after reaction over 20 min. The wire diameters and the intercrossing texture are well preserved. In addition, a magnified SEM image (Figure 1, parts b and c) reveals that the entire wire surfaces are densely covered by abundant nanowires of 30-80 nm in diameter and several tens of micrometers long. In comparison to twisted and entangled nanowires previously prepared on Si substrates that usually lie flat,12 the present nanowires are perfectly straight, protrude outward from the mesh surface, and are quasi-aligned. By analyzing the cross-sectional samples it was found that nanowires are firmly rooted to the brass filaments. Figure 2 shows the high-resolution TEM images of a typical nanowire with a diameter of ∼60 nm. The framed area is enlarged and shown together with the corresponding defocused selected area electron diffraction (SAED) and a Fourier transform (FT) pattern in the inset. The lattice fringes reflect the [-110] crystal zone of β-Ga2O3. The long axis of the nanowire is parallel to the (11-1) crystal plane and has an ∼13° angle with respect to the [001] axis. EDX analyses confirm that the nanowires are composed of Ga and O at an atomic ratio of ∼2: 3. Although there are some byproduct cubic ZnGa2O4 phase nanowires, as shown in the Supporting Information (Figures S2 and S3), the β-Ga2O3 nanowires are the dominating phase (>vol 90%), as revealed by TEM and EDX analyses.
J. Phys. Chem. C, Vol. 113, No. 5, 2009 1981
Figure 2. (a) High-resolution TEM image of a typical nanowire and (b) the magnified image of the framed region marked in panel a. The insets are the corresponding defocused SAED (top-right inset) and FT pattern (bottom-right inset), respectively.
The fact that the Ga2O3 nanowires/nanobelts can grow on a Si substrate at a high temperature via reaction of Ga with the residual O atoms in the system has been known. During the growth vapor-solid (VS) or vapor-liquid-solid (VLS) processes may dominate.17,18 In both scenarios, the nucleation of Ga2O3 takes place on the surface of the Si substrates. However, without the lattice-match relationship, the following growth will be random in different orientations. And the final nanowires mainly lie flat on the substrate. When the brass wire meshes substitute for the Si wafers the situation is completely different. It is noted that without adding a Ga metal in the process of annealing at 900 °C a brass wire mesh becomes porous, as shown in Figure 3. This is caused by the well-known dezincification process, in which Zn is selectively removed in an aggressive environment, leaving a copperrich structure behind.19 Such high-temperature-induced dezincification should influence the nucleation and growth of the present Ga2O3 phase. As the temperature increases, the dezincification will be intensified due to enhanced surface and/or volume diffusion and clustering of zinc atoms.20 Zn migration within the copper mesh leads to the formation of numerous active Zn-rich domains. Due to the gradual evaporation of Zn the holes form at the wire surface. At the same time, the Ga and O atoms reach the surface of a wire mesh and preferentially nucleate in the holes, in which some residual active Zn domains remain.21 The growth of Ga2O3 nanowires on these sites thus resembles planting of trees in the ground holes. These holes can direct the growth of nanowires and promote their quasialignment. 3.2. Contact Properties and Electrical Measurements. Since the nanowires are directly rooted to the brass wires, the contact between the nanowires and copper should be good. This assumption was indeed confirmed by the in situ manipulation (Figure S4 in the Supporting Information) and electrical measurements (Figure 4) of the nanowires in an SEM system equipped with a movable probe. The nanowires were hardly folded but still stood on the meshes. This indicates the superb stability of the achieved physical contact. The linear I-V curves were obtained indicative of ohmic contacts.22 The nanowire resistivities were measured to be in the range of 300-500 Ω · cm.12 According to the previous studies, the resistivity of β-Ga2O3 single crystals was strongly related to the growth conditions
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Huang et al.
Figure 3. SEM images of a brass wire mesh annealed at 900 °C without adding Ga metal in a crucible.
Figure 4. (a) SEM setup for the electrical measurement of the individual nanowires with a moveable probe. (b) A typical I-V curve taken on a nanowire.
and post-treatments and could be modified from
