Considerable Enhancement of Field Emission of SnO2Nanowires by Post-Annealing Process in Oxygen at High Temperature
© to the authors 2009
Received: 13 February 2009
Accepted: 28 May 2009
Published: 24 June 2009
The field emission properties of SnO2nanowires fabricated by chemical vapor deposition with metallic catalyst-assistance were investigated. For the as-fabricated SnO2nanowires, the turn-on and threshold field were 4.03 and 5.4 V/μm, respectively. Considerable enhancement of field emission of SnO2nanowires was obtained by a post-annealing process in oxygen at high temperature. When the SnO2nanowires were post-annealed at 1,000 °C in oxygen, the turn-on and threshold field were decreased to 3.77 and 4.4 V/μm, respectively, and the current density was increased to 6.58 from 0.3 mA/cm2at the same applied electric field of 5.0 V/μm.
SnO2 is an important n type wide-band gap (E.g. = 3.6 eV, at 300 K) semiconducting material, which exhibits extensive applications in the fields of gas sensors, transparent conducting electrodes, transistors, electrode materials, catalysis, solar cells, and optoelectronic devices [1–5]. Recently, various SnO2 nanostructures [6–9] have been proved to be promising candidates for field emission-based flat panel displays due to their characteristic properties of high physical and chemical stability and causticity resistance, compared with ZnO nanostructures and carbon nanotubes. These excellent demonstrations of electron emission from SnO2 nanostructures have opened the door to a new area of applications of these materials for the production of efficient cold cathodes. In order to develop efficient and controllable field emission devices based on SnO2 nanostructures at a lowest energy expense, effective methods to optimize their field emission properties are highly desirable to achieve the highest current density at the lowest threshold electric field. However, the researches specialized on how to enhance the field emission properties of SnO2 nanostructures have been limited to only a few reports [10, 11]. In this paper, SnO2 nanowires are fabricated by a chemical vapor deposition method with metallic catalyst-assistance. It is found that the post-annealing process in oxygen is an effective method for enhancing the field emission of SnO2 nanowires.
A chemical vapor deposition method with metallic catalyst-assistance was employed for the preparation of SnO2 nanowires [7, 10]. A layer of Au (about 7 nm in the thickness) as a catalyst was first deposited on Si substrates with the area of ~8 mm2 by DC sputtering. Commercial SnO2 powders (99.9%) and graphite powders (99.99%) were mixed in a 3:1 molar ratio, transferred to a carnelian mortar and skived for 30 min to make the starting materials well mixed. The mixture was placed into a small quartz tube, and then the substrate was put near the mixture at a distance about 1.5 cm. The small quartz tube was pulled into the center of the large quartz tube furnace. During the whole reaction process, Ar (99.999%) was used as a carrier gas to create the inert atmosphere inside the furnace. When the tube furnace was pumped to 8 Torr by a mechanical pump, the starting materials were heated to 950 °C from room temperature in 30 min and kept at 950 °C for 15 min. After this process, the furnace was cooled down to room temperature in several hours. Finally, the substrate with a gray film-like production was taken out from the small quartz tube and used for analysis. The as-grown SnO2 nanowires were post-annealed in oxygen for 1 h at 700, 850, 900, 950, and 1,000 °C, respectively.
The general morphologies of the as-fabricated SnO2nanowires were characterized by a scanning electron microscope (SEM, LEO-1525). Phase identification and degree of crystallinity of the samples were studied by a D/max -rA X-ray diffractometer with Cu–Kα radiation using normal θ–2θ scanning method. The microstructure and composition of the samples were studied with a JEOL JEM-2100 transmission electron microscope (TEM), a high resolution-transmission electron microscope (HR-TEM) and an energy-dispersive X-ray spectroscope (EDX) attached to the TEM instrument. Field emission measurements for SnO2nanowires were carried out with diode structure in a vacuum chamber at a pressure of 5 × 10−6 Torr at room temperature. The sample (as a cathode) was separated from a Cu probe anode. The voltage with an increasing step was continuously applied from a Cu probe (anode) to the sample (cathode) until a short-current was detected.
Results and Discussion
Field Emission Performance
The inset of Fig. 4 is the corresponding Fowler–Nordheim (F–N) plot. The plot goes near to a straight line, which indicates that the field emission from SnO2 nanowires follows the F–N relationship and reveals that the field emission process is a barrier tunneling quantum mechanical process [13–15].
where V is the applied voltage, d is the distance between the anode and the cathode. β is the field enhancement factor, which reflects the ability of the emitters to enhance the applied local electric field around the probe compared to the macroscopic electric field. The value of β is related to the emitter geometry morphology, the crystal structure, and the spatial distribution of emitting centers.
whereI is the field emission current. And then, the field enhancement factorβ can be estimated by dint of the slopesS of the fitted straight lines of the F–N plots, viz. . Consequently, the field enhancement factorβ of the as-fabricated SnO2nanowires is 1,008.
FE properties of different morphologies of SnO2nanostructures
Turn-on field (Vμm−1)
Threshold field (Vμm−1)
Current density (at 5 Vμm−1) mA/cm2
Nanowires post-annealed in O2at 1,000 °C
The main reason for the low the turn-on and threshold fields, large current density of high temperature treated SnO2nanowires in oxygen can be probably attributed to the following factors: the high crystalline structure and the reduction of the oxygen vacancy concentration. Generally, the high temperature annealing can greatly improve the perfect of crystal, which can be seen from Fig. 1. Thus the improvement of crystalline structure of the treated nanowires is benefit to the field emission performance of nanowires. For nanostructures of semiconductor, it is well known that surface states play an important role in influencing the carrier concentration, carrier mobility, and the effective barrier height. Therefore, these surface states can act as electron traps, forming a high potential barrier on the nanowires surface to reduce the conductivity. Thermal annealing with high temperature in oxygen could not only promote the crystalline quality of nanostructures by removing structural defects such as point defects, but also passivate the surface defects such as oxygen vacancies of SnO2nanowires. Importantly, our XRD data show that the crystalline quality of SnO2nanowires is highly promoted by high temperature annealing in Fig. 1. Meanwhile, our EDS spectrums clearly show that the increase of O atomic content with the annealing temperature increased in Fig. 3.
In conclusion, the homogeneous compact tangly SnO2nanowires were synthesised by chemical vapor deposition with metallic catalyst-assistance. The considerable enhancement of field emission from SnO2nanowires is observed by post-annealing the nanowires in oxygen at high temperatures. The turn-on and threshold fields of SnO2nanowires decreases and the current density increases with the increment of annealing temperature. The enhancement of field emission from SnO2nanowires is considered to be related with the reduction of the oxygen vacancy concentration in nanowires by post-annealing in oxygen at high temperature.
This work was financially supported by the National Natural Science Foundation of China (Nos. 50702048 and 10525211), the Project supported by Hunan Provincial Natural Science Foundation of China (Nos. 09JJ1006), and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20070530010).
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