Figure shows the growth of ITO NWs catalyzed by a selected-area gold film. According to the vapor–liquid–solid (VLS) growth mechanism [25
], the possible reaction routes can be assumed as follows:
Schematics for the selective area growth of ITO nanowire growth.
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.
To decrease the screening effect on the arbitrarily grown ITO NWs, the Sn-doped ITO NWs were alternatively grown on the Au film with the selective area of patterned 50-μm square with a distance of 10 μm for each square pattern. Figure reveals a SEM image of Sn-doped ITO nanowires after the selective area growth. Clearly, the center of the patterned area shows the arbitrary growth of ITO NWs (Figure ), 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.
SEM images. (a) A SEM image of the selective area growth of ITO nanowires. (b) Enlarged SEM image taken from the center of the patterned area. The inset shows an ITO nanowire with catalytic gold nanoparticle.
To illuminate the detailed structure and components of the ITO NWs, the as-prepared nanowires were characterized by XRD, TEM, and XPS. Figure shows the X-ray spectra of ITO NWs. All the peaks are indexed being the In2
cubic structure, while a small peak shows Au9
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  are shown in Figure and the inset, respectively. The symmetric spots in the SAED pattern exhibit a single crystalline phase with the growth direction of . The lattice spacing of 0.506 nm corresponding to (200) plane was indexed, which is consistent with In2
cubic phase. The XPS analysis is used to confirm the chemical compositions of ITO NWs. Figure shows the XPS spectra of O 1s
, In 3d
, and Sn 3d
core levels in the ITO NWs. The binding energy of Sn 3d5/2
and Sn 3d3/2
at 495.1 ± 0.1 eV and 486.6 ± 0.1 eV, correspond to the Sn4+
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 O2−
ions whose neighboring indium atoms are surrounded by the six nearest O2−
ions. The medium binding energy at 531.3 ± 0.1 eV corresponds to the oxygen deficiency regions, which are called oxygen vacancies [28
]. 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 In2
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 3 XRD spectra and high-resolution TEM image. (a) XRD spectra of ITO NWs. (b) A high-resolution TEM image of ITO nanowire. The inset shows a corresponding selective area diffraction pattern, revealing that  is a preferred growth direction. (c) Chemical (more ...)
Figure 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 (Eon
) is defined as the electric field required for generating a current density of 0.01 mA/cm2
, and 0.1 mA/cm2
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 reveal a linear relationship, so-called ln(J
) 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
J-E field emission curves and Fowler-Nordheim plots. (a) J-E field emission curves for flat and selectively patterned growth at 3 and 10 h, respectively. (b) The corresponding Fowler-Nordheim plots from (a) for four samples.
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
eV); and B
, constant (6.8 ×103
) 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
) 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 . 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
= 630) [35
= 501) [36
= 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.
Turn-on fields and field enhancement factors for the growth of the ITO NWs at different conditions
The cross-sectional SEM images for the growth of Sn-doped ITO NWs at 10 and 3 h are shown in Figure 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
]. 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 , respectively. Notably, Figure (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 (3-h 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.
Cross-sectional SEM images for ITO NWs. NWs grown at (a) 10 and (b) 3 h, respectively. (c) and (d) The corresponding distribution of emission current and electric potential for ITO NWs grown at10 and 3 h, respectively.