Field emission properties and growth mechanism of In2O3 nanostructures
© Wang et al.; licensee Springer. 2014
Received: 27 November 2013
Accepted: 18 February 2014
Published: 10 March 2014
Four kinds of nanostructures, nanoneedles, nanohooks, nanorods, and nanotowers of In2O3, have been grown by the vapor transport process with Au catalysts or without any catalysts. The morphology and structure of the prepared nanostructures are determined on the basis of field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The growth direction of the In2O3 nanoneedles is along the , and those of the other three nanostructures are along the . The growth mechanism of the nanoneedles is the vapor-liquid–solid (VLS), and those of the other three nanostructures are the vapor-solid (VS) processes. The field emission properties of four kinds of In2O3 nanostructures have been investigated. Among them, the nanoneedles have the best field emission properties with the lowest turn-on field of 4.9 V/μm and the threshold field of 12 V/μm due to possessing the smallest emitter tip radius and the weakest screening effect.
Recent reports show that reducing the size of In2O3 to a nanoscale gives it various morphologies, such as wires/belts, cubes, octahedrons, and bamboos [3–7]. Recently, the nanostructures of In2O3 have also been paid considerable attention due to their esthetic morphologies , novel characteristics, and important potential applications in various nanodevices [8–13]. It is well known that the properties of nanostructures strongly depend on their morphologies. In previous reports, most of the efforts were focused on the synthesis and properties of single morphology nanostructures. Research on the complex nanostructure was limited, while investigation of the synthesis and properties of complex nanostructures represented developing directions of nanoscience and nanotechnology, which have important potential applications in realizing the multiple functions of nanodevices .
Field emission is one of the most fascinating properties of nanomaterials, such as carbon nanotube, ZnO nanoneedles, and SnO2 nanograss [15–19], and has been extensively studied due to its diverse technological applications in flat-panel displays, microwave-generation devices, and vacuum micro/nanoelectronic devices . In2O3 can be one of the most attractive conductive oxides for field emission because of its relatively low electron affinity, convenience of n-type doping, high chemical inertness, and sputter resistance .
In this paper, four kinds of In2O3 structures, nanoneedles, nanohooks, nanorods, and nanotowers have been grown by the vapor transport process. The morphology and structure of the prepared nanostructures are determined on the basis of field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The field emission properties of the four kinds of In2O3 nanostructures have been investigated, and the In2O3 nanoneedles have preferable characteristics among the four nanostructures due to possessing the smallest emitter tip radius and the weakest screening effect. The growth mechanism is discussed, and the analysis is helpful to understand the relationship between the kinetic factors and the complex structures. It is valuable to realize the controlled synthesis of complex nanostructures.
The synthesis of these In2O3 nanostructures is by the vapor transport process. The fabrication of the In2O3 nanoneedles is as follows: the Au layer (about 10 nm in thickness) is deposited on one single crystal silicon (001) substrate with area of 5 mm2 by sputtering. The active carbon and In2O3 powders (both 99.99%) are mixed in a 1:1 weight ratio and placed into a small quartz tube. One Si substrate covered by Au is put near the mixture of carbon and In2O3 inside the small quartz tube. Then the small quartz tube is pulled into a large quartz tube, and the large quartz tube is put in an electric furnace. The whole system is evacuated by a vacuum pump for 20 min, then the argon gas is guided into the system at 200 sccm, and the pressure is kept at 300 Torr. Afterwards, the system is rapidly heated up to 1,000°C from the room temperature and kept at the temperature for 1 h. Finally, the system is cooled down to the room temperature in several hours. When the substrate is taken out, we can see yellow products on the substrate.
The fabrication process of the In2O3 nanohooks, In2O3 nanorods, and In2O3 nanotowers is basically same with that of In2O3 nanoneedles besides the following contents: Three Si substrates without any catalysts are put far away from the mixture of carbon and In2O3 inside the small quartz tube, and the distance between every two Si substrates is about 2 cm. The argon gas is guided into the system at 250 sccm, the pressure is kept at 350 Torr, and the system is rapidly heated up to 1,050°C from the room temperature.
FESEM, XRD, and TEM are employed to identify the morphology and structure of the synthesized productions. Note that we can easily repeat the experimental results, suggesting that our method is flexible and reproducible.
Results and discussion
The growth mechanism of the In2O3 nanoneedles can be explained on the basis of the 1-D growth along the  crystalline direction controlled by vapor-liquid–solid (VLS) initiated due to the existence of Au catalysts [22–24]. In addition, the formation mechanism of the layered nanohooks, layered nanorods, and nanotowers is mainly led by the bottom growth of vapor-solid (VS) without a catalyst droplet [25–27]. The formation mechanism of the layered nanorods with octahedral tops is explained by the periodical 1-D growth along the  direction and the continuous 0-D growth along the  direction [14, 28, 29]. Beside the formation of the hook-shaped top rather than the octahedral top, the formation mechanism of the layered nanohooks is the same with the stages of the layered nanorods [14, 28, 29]. The formation mechanism of the nanotowers is due to a periodical 1-D growth along the  direction and 0-D growth along the  direction .
where A = 1.54 × 10-6 A eV V-2, B = 6.83 × 103 eV-3/2 V μm-1, β is the field enhancement factor, and Φ is the work function of an emitting material. The nonlinearity of the FN plots of the samples in Figure 4b may attribute to the space charge effects, which results from collision and ionization of residual gas molecules by the emitted electrons . In addition, it has demonstrated that the different crystal facets of the emitter tip possess the different work functions . According to the TEM results above, the crystal facets in the emitter tip of four kinds of In2O3 nanostructures are (001) or (100) planes, which indicates that the values of their work function are same. Assuming the work function of the In2O3 is 5.0 eV , β values of the In2O3 nanoneedles, nanohooks, nanorods, and nanotowers are estimated to be 3,695, 1,770, 1,374, and 458, respectively. Comparing with the other three kinds of In2O3 nanostructures, the In2O3 nanoneedles have the threshold field, the lowest turn-on field, and highest β, which demonstrates the In2O3 nanoneedles have the best field emission properties among all of the samples. The corresponding reasons can be described as follows.
It is known that the field enhancement factor β is a key parameter, which reflects the enhanced electron emission due to the localized electronic states by the geometrical configuration of the emitters. In theoretical case, β can be expressed as h/r, where h is the height of emitter and r is the average radius of the emitter tips . In this paper, the In2O3 nanostructures in Figure 1 are in random alignment so that the height of emitter is difficult to measure. Based on the length of the four kinds of In2O3 nanostructures in Figure 1 being all close to 2 μm, their height of emitter can be regarded as being approximately equal. In this case, the field enhancement factor β is mainly depending on 1/r. According to the FE mechanism, the field emission current is mainly produced from the tip of the materials so as to deduce that the field emission current is mainly produced from the tip of the nanostructures. Among the four kinds of In2O3 nanostructures in this paper, the In2O3 nanoneedles had the sharpest tip with the size of 50 nm so as to possess the highest β value. Therefore, the emitter tip radius and the emitter height are two factors that can affect the field emission properties of the In2O3 nanostructures.
Field emission parameters and morphological sizes of the synthesized In 2 O 3 nanostructures
In addition, different electrical properties, i.e., work function (different facet) and substrate-nanostructure electrical contact can affect the field emission properties of the In2O3 nanostructures too. According to the TEM results in Figure 3, the four kinds of In2O3 nanostructures possess the same work function due to the crystal facets in their emitter tip being (001) or (100) planes, which has been discussed above. In addition, nanostructures grown on different substrates can result in different conductivity . In this paper, all of the substrates are single crystal silicon (001) substrates, so the effects of substrate-nanostructure electrical contact for the four kinds of In2O3 nanostructures are same, which may not cause the difference to their field emission properties.
From the TEM results shown in Figure 3, it is observed that the Au nanoparticles are only present at the tip of In2O3 nanoneedles. The presence of these Au nanoparticles at the tip of the nanoneedles could influence the field emission results. As the work function of Au is 5.1 eV, which is quite similar to that of In2O3. Therefore, the effect of the catalyst in the field emission properties is negligible .
In summary, four kinds of In2O3 nanostructures, nanoneedles, layered nanohooks, layered nanorods, and nanotowers, have been grown on single silicon substrates with Au catalysts- or without any catalysts-assisted carbothermal evaporation of In2O3 and active carbon powders. The growth direction of the In2O3 nanoneedles is along the , and those of the other three nanostructures are along the . The growth mechanism of the nanoneedles is the VLS, and those of the other three nanostructures are the VS processes. The field emission measurements demonstrated that the In2O3 nanoneedles have relatively excellent performance among the four kinds of In2O3 nanostructures mainly due to possessing the smallest emitter tip radius and the weakest screening effect.
This work was supported by National Natural Science Foundation of China (50902097), Three Industry Basic Research Emphasis Project of Shenzhen (JC201104210013A), Guangdong Natural Science Foundation of China (9451806001002303), Project of Department of Education of Guangdong Province (2013KJCX0165), Outstanding Young Teacher Training Project in the institutions of higher learning of Guangdong Province (Yq2013145), and open project of Shenzhen Key Laboratory of Micro-Nano Photonic Information Technology (MN201107).
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