Sn-doped In2O3 nanowires: enhancement of electrical field emission by a selective area growth
© Chang et al.; licensee Springer. 2012
Received: 24 September 2012
Accepted: 8 November 2012
Published: 21 December 2012
Selective area growth of single crystalline Sn-doped In2O3 (ITO) nanowires synthesized via vapor–liquid–solid (VLS) method at 600°C was applied to improve the field emission behavior owing to the reduction of screen effect. The enhanced field emission performance reveals the reduction of turn-on fields from 9.3 to 6.6 V μm−1 with increase of field enhancement factors (β) from 1,621 to 1,857 after the selective area growth at 3 h. Moreover, we find that the screen effect also highly depends on the length of nanowires on the field emission performance. Consequently, the turn-on fields increase from 6.6 to 13.6 V μm−1 with decreasing β values from 1,857 to 699 after the 10-h growth. The detailed screen effect in terms of electrical potential and NW density are investigated in details. The findings provide an effective way of improving the field emission properties for nanodevice application.
Recently, the Sn-doped In2O3 (indium tin oxide (ITO)) material as a transparent conducting oxides is widely used on many technological applications, such as solar cell  and flat panel display [2, 3]. Especially in nanoscale region, the Sn-doped In2O3 (ITO) nanowires have exhibited some superior properties such as good thermal stability, higher metallic conductivity, and excellent oxidation resistance, which make ITO nanowires (NWs) being suitable as a promising candidate not only as a transparent electrode but also as an emitter [4–7]. Up to now, several research groups have reported the growth of ITO nanowires, nanorods, and nanowhisker with different synthetic methods, such as thermal evaporation [8–11], electron beam evaporation , sputtering , and pulse laser deposition . These nanostructures were found to exhibit a good performance at field emission as an electron emitter due to their high aspect ratio at the nanoscale region and unique extrinsic properties. In the previous report, Wan et al. has reported the epitaxial growth of vertically aligned ITO NWs on the (100) yttrium-stabilized zirconia substrate and showed a superior field emission property .
For a good field emission performance from nanowires, it highly depends on the shape of the nanowire , circus radius of the nanowire at the tip region , work function , and packing density of the nanowire . Thus, to obtain the high-density emission sites, one of the most important factors, the screen effect, due to the disturbance of electric field resulting from the interference of emission at different spacings between nanowires must be minimized . Therefore, the selective area growth of nanowires was required. However, how electrical field emission properties of ITO NWs influenced by the screen effect in the differently grown situations is still interesting . Several selective growth methods had been used, such as nanosphere lithography , electron-beam lithography [21, 22], and conventional photolithography .
In this regard, we present a selective area growth of single crystalline Sn-doped ITO NWs to improve the field emission properties owing to the reduction of the screen effect. In our previous study, the conductive properties of ITO NWs have been investigated, which is compatible with that of the high quality ITO thin films [23, 24]. A periodically arrayed Au film prepared via a copper grid mask is used to control the growth area of ITO NWs in order to investigate the screen effect. Importantly, the length of ITO NWs was found to significantly influence the field emission properties. As a result, the reduced turn-on fields from 9.3 to 6.6 V μm−1 and improved β values from 1,621 to 1,857 could be found after the selective area growth of Sn-doped ITO NWs at 3 h.
Growth of Sn-doped ITO nanowires
The ITO nanowires were grown by the hydrogen thermal reduction vapor transport method. Indium (99.9%) and tin (99.9%) were mixed as source powders with the weight ratio of 9:1 and placed in an alumina boat (Al2O3). The 5-nm-thick Au film as the catalyst was deposited on the silicon substrate by a sputter process and patterned by a copper grid mask. The alumina boat was placed in the center of the alumina tube and then the substrates were put into the low region (several center meters) next to the source powder. The system was heated up to 600°C with a heating rate of 5°C/min. Consequently, the ITO NWs were grown at 600°C for 10 and 3 h with a constant flow of mixed Ar/H2 gas (10% H2) at 90 sccm. Another oxygen gas was flowed into the furnace with 0.5 sccm as a source of oxygen to form ITO NWs. After the furnace had been cooled down to room temperature, gray products were found on the surface of the silicon substrate.
Structures of products were analyzed by X-ray diffractometer (XRD, Shimadzu XRD 6000, Nakagyo-ku, Kyoto, Japan) and transmission electron microscope (TEM, JEOL-2010, JEOL Ltd., Akishima, Tokyo, Japan). The morphology was analyzed by field emission scanning electron microscope (SEM, JEOL-6500). The X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, PHI Quantera SXM, Chanhassen, MN, USA) was used to examine the chemical composition of nanowires. Field emission measurement of ITO NW arrays was performed with a parallel plate as the cathode and a circular steeliness tip as the anode (1-mm diameter). A high voltage–current instrument, Keithley 237 (Cleveland, OH, USA), was operated to perform the field emission characteristics. All emission measurements were carried out in a vacuum chamber with a pressure kept under 10−6 Torr The applied voltage between the electrodes was increased to a maximum of 1,000 V by 20-V step.
Results and discussion
The reaction of the VLS method is at a high-temperature environment. As the temperature increases to 600°C, the Au drops could be formed, and the low melting point of the source powder (In and Sn) is evaporated to combine with oxygen gas to form metal oxide gases (In2O3, SnO2) through the chemical reactions of Equations 1 and 2. Subsequently, the metal oxide gases could be reduced by hydrogen to form the metal atoms and then enter to the liquid gold drops to form eutectic alloy through Equations 3 and 4. Furthermore, hydrogen and oxygen could combine to form H2O. Finally, the eutectic alloy drops would be oxidized to form the Sn-doped In2O3 NWs by H2O, namely, Equations 5 and 6. When the temperature increased to 600°C, the oxygen would be introduced into the alumina tube, resulting in the oxidization of In and Sn vapors, with which the growth time would be conducted at 600°C for 3 and 10 h.
Turn-on fields and field enhancement factors for the growth of the ITO NWs at different conditions
Eon (V μm−1) at J= 0.01 mA cm−2
Flat 10-h growth
Patterned 10-h growth
Flat 3-h growth
Patterned 3-h growth
We present a selective area growth of single crystalline Sn-doped In2O3 (ITO) nanowires synthesized via VLS method at 600°C in order to improve the field emission behavior by the reduction of screen effect. The enhanced field emission performance reveals the reduction of turn-on fields from 9.3 to 6.6 V μm−1 with increase of field enhancement factors (β) from 1,621 to 1857 after the selective area growth at 3 h. Moreover, we find that the screen effect also highly depends on the length of nanowires on the field emission performance. The turn-on fields increase from 6.6 to 13.6 V μm−1, and β values decrease from 1,857 to 699 after the 10-h growth. The screen effect is predominated after the length of nanowires increases, namely the longer growth time, thereby degrading the field emission performance. Consequently, the turn-on fields and β values change from 13.6 V μm−1 and 699 to 6.6 V μm−1 and 1,857, respectively, as the growth time of Sn-doped ITO NWs decreases into 3 h. The detailed screen effect in terms of electrical potential and NW density was investigated in details. The findings provide an effective way of improving the field emission properties for nano-emitter application.
This work was supported by the National Science Council, Taiwan, under grant number NSC-99-2221-E-007-069-MY3.
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