- Original paper
- Open Access
Field emission from in situ-grown vertically aligned SnO2 nanowire arrays
Nanoscale Research Letters volume 7, Article number: 117 (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
Figure 1a shows a top view SEM image of the in situ-grown SnO2 nanowire arrays. It can be seen that the nanowire was vertically aligned on the silicon substrate with diameters between 40 to 60 nm. Noticeably, on the top of each nanowire, there was a globule, which strongly indicates that the nanowire grew via a vapor-liquid-solid mechanism . The density of the SnO2 nanowire arrays, determined by counting nanowires in a representative area of a SEM image, was estimated to be 3 × 107/mm2. Figure 1b presents a high-resolution TEM image of a single SnO2 nanowire. It shows that the SnO2 nanowire is a single crystalline nanowire with an interplanar spacing of 0.34 nm corresponding to the (110) plane of a rutile crystalline SnO2.
XRD characterization was employed to investigate the crystal structure of the in situ-grown SnO2 nanowire arrays. Figure 2a shows a typical XRD pattern. The diffraction peaks can be well indexed to the standard values of bulk SnO2 (JCPDS card: 41-1445), Si (JCPDS card: 27-1402), and Pt (JCPDS card: 88-2343). The peaks attributed to SnO2 demonstrate that the in situ-grown sample crystallized with the tetragonal rutile structure with lattice constants of a = 4.738 Å and c = 3.187 Å.
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 .
To investigate FE properties of the in situ-grown SnO2 nanowire arrays, FE measurements were performed at various anode-cathode separations. Figure 3 presents the FE current density (J) of the in situ-grown SnO2 nanowire arrays as a function of the applied electric field (E) measured at anode-cathode separations of 100, 150, and 200 μm. The turn-on field is defined as the applied electric field which produces a current density distinguished from the background noise (here, defined as 0.01 mA cm-2) . It can be seen from the figure that the turn-on fields were dependent on the anode-cathode distance: their value decreased as the anode-cathode distance increased, and the turn-on fields were measured to be 2.8, 2.0, and 1.6 V μm-1, respectively. These values are lower than those reported by He et al.  (5.8 V μm-1) and Wang et al.  (3.77 V μm-1) for the SnO2 nanowire. The lower turn-on fields may be attributed to the good alignment of the SnO2 nanowire. Additionally, we believe that the in situ fabrication method, which made good electrical contacts between the SnO2 nanowire and the silicon substrate, contributed greatly to the lower turn-on fields. Moreover, the emitter radius of the SnO2 nanowire among the arrays was approximately 50 nm, which is small enough to make the FE performance excellent .
In order to understand the emission characteristics, FE properties were also analyzed by applying the classic Fowler-Nordheim [FN] law using the following equation :
where J is the FE current density, Φ is the barrier height of the emission tip surface, and Eloc is the local microscopic electric field at the emission sites. The a and b in the equation are constants with value of 1.54 × 10-10 (A V-2 eV) and 6.83 × 109 (V eV-3/2 μm-1), respectively. Eloc, which could be up to a hundred or thousand times of the macroscopic electric field between the cathode and anode, can be calculated using the following equation:
where β is the field enhancement factor, V is the applied voltage, and d is the anode-cathode separation. The value of β, which is related to the spatial distribution of emitting centers, the crystal structure, and the geometry morphology of emitters, reflects the ability of the emitters to enhance the applied local electric field around the probe compared to the macroscopic electric field. The FN emission behavior can be evaluated from the linearity of the curves plotting ln(J/E2) versus 1/E. Figure 4 shows the corresponding FN plots. It can be seen that, besides the noise districts, the three plots go near to a straight line. It indicates that the field emission from the in situ-grown SnO2 nanowire arrays follows the FN relationship well, and the field emission process is a barrier tunneling quantum mechanical process [30, 31].
In general, FE characteristics depend on the work function and field enhancement factor (β) of emitters . Both density and tip morphology influence the β value of emitters. Ordinarily, emitters with high aspect ratios exhibit a favorable FE performance due to their much higher β value. The value of β can be calculated from the slope of a FN plot (kFN) according to the following equation:
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.
The temporal FE current stability was measured over 120 min at an anode-cathode separation of 100 μm at a fixed voltage of 500 V. The current density fluctuation is lower than 5%, as shown in Figure 5. The stability test results confirm that the in situ-grown SnO2 nanowire arrays are competent for being high-performance 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.
Gubbala S, Chakrapani V, Kumar V, Sunkara MK: Band-edge engineered hybrid structures for dye-sensitized solar cells based on SnO2nanowires. Adv Funct Mater 2008, 18: 2411–2418. 10.1002/adfm.200800099
Wan Q, Huang J, Xie Z, Wang TH, Dattoli EN, Lu W: Branched SnO2nanowires on metallic nanowire backbones for ethanol sensors application. Appl Phys Lett 2008, 92: 102101. 10.1063/1.2890735
Sysoev VV, Goschnick J, Schneider T, Strelcov E, Kolmakov A: A gradient microarray electronic nose based on percolating SnO2nanowire sensing elements. Nano Lett 2007, 7: 3182–3188. 10.1021/nl071815+
Kim H, Cho J: Hard templating synthesis of mesoporous and nanowire SnO2lithium battery anode materials. J Mater Chem 2008, 18: 771–775. 10.1039/b714904b
Wang S, Cheng G, Cheng K, Jiang X, Du Z: The current image of single SnO2nanobelt nanodevice studied by conductive atomic force microscopy. Nanoscale Res Lett 2011, 6: 541. 10.1186/1556-276X-6-541
Ying Z, Wan Q, Song ZT, Feng SL: SnO2nanowhiskers and their ethanol sensing characteristics. Nanotechnology 2004, 15: 1682–1684. 10.1088/0957-4484/15/11/053
Zhu SM, Zhang D, Gu JJ, Xu JQ, Dong JP, Li JL: Biotemplate fabrication of SnO2nanotubular materials by a sonochemical method for gas sensors. J Nanopart Res 2010, 12: 1389–1400. 10.1007/s11051-009-9684-0
Wang GX, Park JS, Park MS: Growth, characterization and technological applications of semiconductor SnO2nanotubes and In2O3nanowires. J Nanosci Nanotechnol 2009, 9: 1144–1147. 10.1166/jnn.2009.C106
Wang YL, Guo M, Zhang M, Wang XD: Hydrothermal synthesis of SnO2nanoflower arrays and their optical properties. Scripta Mater 2009, 61: 234–236. 10.1016/j.scriptamat.2009.03.040
El-Etre AY, Reda SM: Characterization of nanocrystalline SnO2thin film fabricated by electrodeposition method for dye-sensitized solar cell application. Appl Surf Sci 2010, 256: 6601–6606. 10.1016/j.apsusc.2010.04.055
Zhang YS, Yu K, Li GD, Peng DY, Zhang QX, Xu F, Bai W, Ouyang SX, Zhu ZQ: Synthesis and field emission of patterned SnO2nanoflowers. Mater Lett 2006, 60: 3109–3112. 10.1016/j.matlet.2006.02.053
Li LJ, Zong FJ, Cui XD, Ma HL, Wu XH, Zhang QD, Wang YL, Yang F, Zhao JZ: Structure and field emission properties of SnO2nanowires. Mater Lett 2007, 61: 4152–4155. 10.1016/j.matlet.2007.01.044
Jang HS, Kang SO, Kim YI: Enhancement of field emission of SnO2nanowires film by exposure of hydrogen gas. Solid State Commun 2006, 140: 495–499. 10.1016/j.ssc.2006.09.024
Wu J, Yu K, Li LJ, Xu JW, Shang DJ, Xu YE, Zhu ZQ: Controllable synthesis and field emission properties of SnO2zigzag nanobelts. J Phys D Appl Phys 2008, 41: 185302. 10.1088/0022-3727/41/18/185302
Bhise AB, Late DJ, Ramgir NS, More MA, Mulla IS, Pillai VK, Joag DS: Field emission investigations of RuO2-doped SnO2wires. Appl Surf Sci 2007, 253: 9159–9163. 10.1016/j.apsusc.2007.05.058
Fang XS, Bando Y, Gautam UK, Ye C, Golberg D: Inorganic semiconductor nanostructures and their field-emission applications. J Mater Chem 2008, 18: 509–522. 10.1039/b712874f
Pan W, He XW, Chen Y: Preparation and characterization of polyacrylonitrile/antimony doped tin oxide composite nanofibers by electrospinning method. Optoelectron Adv Mat 2010, 4: 390–394.
Liu J, Li Y, Huang X, Zhu Z: Tin oxide nanorod array-based electrochemical hydrogen peroxide biosensor. Nanoscale Res Lett 2010, 5: 1177–1181. 10.1007/s11671-010-9622-1
Barth S, Hernandez-Ramirez F, Holmes JD, Romano-Rodriguez A: Synthesis and applications of one-dimensional semiconductors. Prog Mater Sci 2010, 55: 563–627. 10.1016/j.pmatsci.2010.02.001
Gu F, Wang SF, Cao HM, Li CZ: Synthesis and optical properties of SnO2nanorods. Nanotechnology 2008, 19: 095708. 10.1088/0957-4484/19/9/095708
Zheng MJ, Ma L, Xu WL, Ding GQ, Shen WZ: Preparation and structural characterization of nanocrystalline SnO2powders. Appl Phys A-mater 2005, 81: 721–723. 10.1007/s00339-005-3223-3
Nakashima S, Oima S, Mitsuishi A, Nishimura T, Fukumoto T, Akasaka Y: Raman scattering study of ion implanted and C.W.-Laser annealed polycrystalline silicon. Solid State Commun 1981, 40: 765–768. 10.1016/0038-1098(81)90825-5
Li LJ, Yu K, Mao HB, Zhu ZQ: Photoluminescence and field-emission properties of Cu-doped SnO2nanobelts. Appl Phys A-mater 2010, 99: 865–869. 10.1007/s00339-010-5620-5
Sun S: Raman scattering study of rutile SnO2nanobelts synthesized by thermal evaporation of Sn powders. Chem Phys Lett 2003, 376: 103–107. 10.1016/S0009-2614(03)00965-5
Li M-K, Wang D-Z, Ding Y-W, Guo X-Y, Ding S, Jin H: Morphology and field emission from ZnO nanowire arrays synthesized at different temperature. Mat Sci Eng A-struct 2007, 452–453: 417–421.
He JH, Wu TH, Hsin CL, Li KM, Chen LJ, Chueh YL, Chou LJ, Wang ZL: Beaklike SnO2nanorods with strong photoluminescent and field-emission properties. Small 2006, 2: 116–120. 10.1002/smll.200500210
Wang JB, Li K, Zhong XL, Zhou YC, Fang XS, Tang CC, Bando Y: Considerable enhancement of field emission of SnO2nanowires by post-annealing process in oxygen at high temperature. Nanoscale Res Lett 2009, 4: 1135–1140. 10.1007/s11671-009-9367-x
Zheng X, Chen G, Li Z, Deng S, Xu N: Quantum-mechanical investigation of field-emission mechanism of a micrometer-long single-walled carbon nanotube. Phys Rev Lett 2004, 92: 106803.
Zhang YA, Lin JY, Guo TL: Fabrication and properties of film-under-gate field emission arrays with SnO2emitters for flat lamp. Appl Surf Sci 2010, 257: 306–311. 10.1016/j.apsusc.2010.06.093
de Heer WA, Châtelain A, Ugarte D: A carbon nanotube field-emission electron source. Science 1995, 270: 1179–1180. 10.1126/science.270.5239.1179
Li LA, Cheng SH, Li HD, Yu Q, Liu JW, Lv XY: Effect of nitrogen on deposition and field emission properties of boron-doped micro- and nano-crystalline diamond films. Nano-Micro Lett 2010, 2: 154–159.
Sheini FJ, Singh J, Srivasatva ON, Joag DS, More MA: Electrochemical synthesis of Cu/ZnO nanocomposite films and their efficient field emission behaviour. Appl Surf Sci 2010, 256: 2110–2114. 10.1016/j.apsusc.2009.09.056
Luo S, Chu PK, Di Z, Zhang M, Liu W, Lin C, Fan J, Wu X: Vacuum electron field emission from SnO2nanowhiskers annealed in N2and O2atmospheres. Appl Phys Lett 2006, 88: 013109. 10.1063/1.2161573
Xue XY, Li LM, Yu HC, Chen YJ, Wang YG, Wang TH: Extremely stable field emission from AlZnO nanowire arrays. Appl Phys Lett 2006, 89: 043118. 10.1063/1.2236288
This work was primarily supported by the Scientific Research Starting Foundation for Outstanding talent, University of Electronic Science and Technology of China.
The authors declare that they have no competing interests.
ZHZ conducted all the experiments and drafted the manuscript. JW provided helpful guidance and suggestions. HDL helped in drafting the manuscript. ZMW supervised all of the study. All authors read and approved the final manuscript.
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Zhou, Z., Wu, J., Li, H. et al. Field emission from in situ-grown vertically aligned SnO2 nanowire arrays. Nanoscale Res Lett 7, 117 (2012) doi:10.1186/1556-276X-7-117
- Field Enhancement Factor
- SnO2 Nanowire
- Macroscopic Electric Field
- Current Density Fluctuation