Field emission from in situ-grown vertically aligned SnO2 nanowire arrays
© Zhou et al; licensee Springer. 2012
Received: 11 December 2011
Accepted: 13 February 2012
Published: 13 February 2012
Vertically aligned SnO2 nanowire arrays have been in situ fabricated on a silicon substrate via thermal evaporation method in the presence of a Pt catalyst. The field emission properties of the SnO2 nanowire arrays have been investigated. Low turn-on fields of 1.6 to 2.8 V/μm were obtained at anode-cathode separations of 100 to 200 μm. The current density fluctuation was lower than 5% during a 120-min stability test measured at a fixed applied electric field of 5 V/μm. The favorable field-emission performance indicates that the fabricated SnO2 nanowire arrays are promising candidates as field emitters.
SnO2 is a wide bandgap semiconductor (Eg = 3.6 eV, 300 K) which has been widely applied in gas sensors, solar cells, lithium-ion batteries, and nanoelectronic devices [1–4] due to its outstanding optical and electrical properties . Nanoscaled SnO2 presenting peculiar properties superior to its bulk counterpart because of the quantum effects has attracted much interest in recent years. Various methods have been developed to fabricate a SnO2 nanostructure, including thermal evaporation of the metal tin (Sn) , sonochemical method , carbothermal reduction , hydrothermal method , electrodeposition method , and so on. The field-emission [FE] properties of SnO2 nanobelts, nanoflowers,  and nanowires  were also studied considering the potential applications of FE flat displays, X-ray sources, and microwave devices. It was found that the nanowires and long nanobelts of SnO2 exhibited outstanding FE properties [13–15]. In general, vertically aligned nanowire arrays are the best candidates for FE sources because the efficiency of the field emitters is based on the extremely small radii of the tips, and the diameter of the nanowire, the precise position, and the alignment can be well controlled [16–18]. Therefore, it is necessary to develop a method to fabricate well-aligned SnO2 nanowire arrays to further improve the FE performance.
In this work, an in situ catalytic thermal evaporation method was developed to fabricate vertically aligned SnO2 nanowire arrays on a silicon substrate. The FE characteristics of the SnO2 nanowire arrays were studied. The in situ-grown SnO2 nanowire arrays, benefiting from the well-aligned structure and the in situ-grown fabrication method, demonstrated favorable low turn-on electric fields and a relatively stable emission behavior.
For preparing the SnO2 nanowire arrays, 2 g of tin powder (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was put in a ceramic boat. A silicon (100) substrate sputtered with a 5-nm-thick Pt film was placed on the top of the ceramic boat. The distance between the tin powder and the substrate was about 0.5 cm. The ceramic boat was placed in the middle of an electric resistance tube furnace. The electric resistance tube furnace was purged with a continuous 100-sccm high-purity nitrogen gas for 15 min beforehand. Then, it was heated up to 850°C at a rate of approximately 30°C/min and kept at 850°C for 10 min. Lastly, it was cooled down to room temperature naturally, and a white layer of product was found on the silicon substrate.
The surface morphology and crystal structure of the in situ-grown SnO2 nanowire arrays were investigated by scanning electron microscopy [SEM] (JEOL JEM-6320F, JEOL Ltd., Akishima, Tokyo, Japan), high-resolution transmission electron microscopy [HRTEM] (JEOL JEM-2100, JEOL Ltd., Akishima, Tokyo, Japan), and X-ray diffraction [XRD] (Bruker-AXS D8, Bruker Optik Gmbh, Ettlingen, Germany). The optical properties of the in situ-grown nanowire arrays were studied by Raman spectroscopy (French Labrum-HR cofocal laser micro-Raman spectrometer (Dilor S.A, Villeneuve d'Ascq, France) using an argon-ion laser at 514.5 nm).
The FE measurements were performed in a vacuum chamber at a pressure of 3 × 10-5 Pa at room temperature. The silicon substrate with the in situ-grown SnO2 nanowire arrays served as the cathode, and a fluorine-tin-oxide coated glass served as the anode. The cathode and anode were separated with mica spacers. The applied electric field (E) was determined by dividing the applied voltage (V) by the anode-cathode separation (d). The emission density (J) was evaluated from the quotient of the obtained emission current divided by the cathode surface area (0.25 cm2).
Results and discussion
A typical room temperature Raman spectrum of the in situ-grown SnO2 nanowire arrays is shown in Figure 2b. It can be seen that there were three fundamental Raman peaks located at 475, 633, and 774 cm-1, which correspond to E g , A1g, and B2gvibration modes, respectively. The results are in good agreement with those of the rutile single crystal SnO2 nanowire reported previously . Besides the fundamental Raman peaks, the other two Raman peaks located at about 320 and 695 cm- 1were also observed, which correspond to IR-active Eu 3TO and A2uLO (TO is the mode of transverse optical phonons; LO is the mode of longitudinal optical phonons) modes, respectively . The strong and sharp peak located at about 522 cm-1 corresponds to the characteristic peak of the silicon substrate [22, 23]. It is believed that the broadening of the peaks in the Raman scattering results is attributed to the quantum confinement effect of the sample .
where φ is the work function of SnO2 (4.3 eV) . By analyzing the data in Figure 4, the values of β are estimated to be 1,082, 1,378, and 1,638 as d is 100, 150, and 200 μm, respectively. It can be seen that the values of β are high enough for practical application as field emitters.
To fabricate the vertically aligned SnO2 nanowire arrays, the thickness of a Pt catalyst is of vital importance. From many experiment results, it was found that the suitable thickness of the Pt catalyst was about 2 to 10 nm. The average length of a single SnO2 nanowire in the arrays can be controlled by adjusting the reaction time. It should be noted that the SnO2 nanowire will bend if their lengths exceeded ca. 100 μm due to the force of their own gravity. Additionally, it was found that no nanowire grew on a blank silicon substrate (without Pt catalyst). This feature makes the in situ-grown method meaningful because the emitter patterns can be well controlled and designed by selective sputtering of the Pt catalyst using traditional lithography mask technology.
In summary, vertically aligned SnO2 nanowire arrays were deposited on a silicon substrate by an in situ-grown method. The FE properties of the SnO2 nanowire arrays were systematically studied. The FE measurement results showed that the SnO2 nanowire arrays had a low turn-on field of 1.6 to 2.8 V μm-1 at anode-cathode separations of 100 to 200 μm. The low turn-on fields can be attributed to the vertically aligned structure and the high aspect ratio of the SnO2 nanowire. Moreover, the in situ-grown method, which makes good electrical contacts between the SnO2 nanowire and the silicon substrate, improved the FE performance greatly.
This work was primarily supported by the Scientific Research Starting Foundation for Outstanding talent, University of Electronic Science and Technology of China.
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