Sn-doped In2O3 nanowires: enhancement of electrical field emission by a selective area growth

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.


Background
Recently, the Sn-doped In 2 O 3 (indium tin oxide (ITO)) material as a transparent conducting oxides is widely used on many technological applications, such as solar cell [1] and flat panel display [2,3]. Especially in nanoscale region, the Sn-doped In 2 O 3 (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][5][6][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][9][10][11], electron beam evaporation [12], sputtering [13], and pulse laser deposition [14]. 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 [6].
For a good field emission performance from nanowires, it highly depends on the shape of the nanowire [15], circus radius of the nanowire at the tip region [16], work function [17], and packing density of the nanowire [15]. 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 [18]. 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 [19]. Several selective growth methods had been used, such as nanosphere lithography [20], electron-beam lithography [21,22], and conventional photolithography [19].
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 (Al 2 O 3 ). 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/H 2 gas (10% H 2 ) 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.

Characterization
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 Figure 1 shows the growth of ITO NWs catalyzed by a selected-area gold film. According to the vapor-liquidsolid (VLS) growth mechanism [25][26][27], the possible reaction routes can be assumed as follows: Sn g ð Þ þ In 2 In Sn The reaction of the VLS method is at a hightemperature 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 (In 2 O 3 , SnO 2 ) 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 H 2 O. Finally, the   arbitrary growth of ITO NWs (Figure 2b), and the inset shows ITO nanowires with catalytic Au nanoparticles, confirming the VLS method of Sn-doped ITO NWs. In addition, the dispersion of ITO nanowire diameter ranges from 40 to approximately 200 nm with an average diameter of 110 nm.
To illuminate the detailed structure and components of the ITO NWs, the as-prepared nanowires were characterized by XRD, TEM, and XPS. Figure 3a shows the X-ray spectra of ITO NWs. All the peaks are indexed being the In 2 O 3 cubic structure, while a small peak shows Au 9 In 4 phase, which comes from the catalytic gold nanoparticles on the top of ITO nanowires. Furthermore, the high-resolution TEM image and the corresponding selected area electron diffraction (SAED) pattern with zone axis of [001] are shown in Figure 3b and the inset, respectively. The symmetric spots in the SAED pattern exhibit a single crystalline phase with the growth direction of [100]. The lattice spacing of 0.506 nm corresponding to (200) plane was indexed, which is consistent with In 2 O 3 cubic phase. The XPS analysis is used to confirm the chemical compositions of ITO NWs. Figure 3c shows the XPS spectra of O 1s, In 3d, and Sn 3d core levels in the ITO NWs. The binding energy of Sn 3d 5/2 and Sn 3d 3/2 at 495.1 ± 0.1 eV and 486.6 ± 0.1 eV, correspond to the Sn 4+ ion, respectively, which are relative to the electrical conduction of the nanowires [28]. The O 1s peak is deconvoluted by a Gaussian function into three positions. The lower binding energy component at 530 ± 0.1 eV is due to the O 2− ions whose neighboring indium atoms are surrounded by the six nearest O 2− ions. The medium binding energy at 531.3 ± 0.1 eV corresponds to the oxygen deficiency regions, which are called oxygen vacancies [28,29]. The higher binding energy at 532.6 ± 0.1 eV is associated with the oxygen of free hydroxyl group, which is possibly due to the water molecules absorbed on the surface [30]. All XPS results show that Sn atoms are doped into the In 2 O 3 NWs with the existence of oxygen vacancies. Consequently, the oxygen vacancies and Sn ions contribute the electron concentration to the NWs, resulting in an n-type semiconducting behavior. Figure 4a shows field emission properties of the ITO NWs grown on Au film and patterned Au film with growth time of 3 and 10 h, respectively. The turn-on field (E on ) is defined as the electric field required for generating a current density of 0.01 mA/cm 2 , and 0.1 mA/cm 2 is sufficient for operating display panel devices. It is found that the turn-on field decreases from 9.3 to 6.6 V μm −1 after the selective area growth of ITO NWs at the growth time of 3 h. Insets in Figure 4b reveal a linear relationship, so-called ln(J/E 2 )-(1/E) plot, indicating that the field-emission behavior follows Fowler-Nordheim relationship, i.e., electrons tunneling through a potential barrier, which can be expressed as follows [31][32][33]: where J is the emission current density; E, the applied field; φ, the work function of emitter material; β, the enhancement factor; A, constant (1.56 × 10 −10 A V −2 eV); and B, constant (6.8 ×10 3 eV −3/2 V μm −1 ) The field  enhancement factor, β, reflects the degree of the field emission enhancement of the tip shape on a planar surface, which is also dependent on the geometry of the nanowire, the crystal structure, and the density at the emitting points. It can be determined by the slope of the ln(J/E 2 )-(1/E) plot with a work function value of 4.3 eV [6]. Consequently, the turn-on fields and the β values of the ITO NWs with and without selective area growth at different growth times are listed in Table 1. Obviously, the field enhancement factors (β) from 1,621 to 1,857 can be achieved 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. As a result, the turn-on fields increase from 6.6 to 13.6 V μm −1 , and β values decrease from 1,857 to 699 after 10-h growth. Compared to the β values of other materials, such as Si nanowires (β = 1,000) [34], NiSi 2 nanorods (β = 630) [35], NiSi 2 nanowires (β = 501) [36], SnO 2 (β = 1402.9) [37], AlN (β = 950) [38], and ZnO (β = 1,464) [39], the Sn-doped ITO NWs are promising emitters. The findings indicate that the less stacking density via the selective area growth and the reduction of the NW length could decrease the screen effect, resulting in the increase of the enhancement factor. The cross-sectional SEM images for the growth of Sndoped ITO NWs at 10 and 3 h are shown in Figure 5a,b to confirm the reduction of the screen effect, respectively. Obviously, ITO NWs are tangled together due to the longer length (10-h growth), while the quasi-vertical growth could be achieved at the shorter time (3-h growth). According to the screening effect, the electrical field around ITO NWs with longer length and random growth would interfere together to result in screen effect, thereby a poor field emission [40,41]. The corresponding potential distribution of the ITO NWs for Sn-doped ITO NWs grown at 10 and 3 h related to the electrical field are shown in Figure 5c,d, respectively. Notably, Figure 5c (10-h growth) reveals that the NWs significantly tangled together, resulting in lower current emission because of the lesser equipotential lines owing to the server screen effect. Therefore, only the higher NWs would emit current. On the contrary, Figure 5d (3h growth) reveals that the shorter NWs could decrease the screen effect due to the much larger dispersive equipotential lines around the NWs, triggering a higher current emission. This is why the shorter grown time of ITO NWs shows the much better FE property. The findings provide an effective way of improving the field emission properties for nanodevice application.

Conclusion
We present a selective area growth of single crystalline Sn-doped In 2 O 3 (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 turnon 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.