- Email: [email protected]
WO2.72 heterostructures fabricated by vapor deposition
Physica E 53 (2013) 260–265
Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Structure and field-emission properties of W/WO2.72 heterostructures fabricated by vapor deposition Xinli Liu, Min Song n, Shiliang Wang, Yuehui He State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China
H I G H L I G H T S
G R A P H I C A L
W/WO2.72 heterostructures were synthesized by the chemical vapor deposition process. The long and straight central axial W whiskers grow along [1 1 0] direction. WO2.72 grows on side surface of the W whiskers, with [0 1 0] as the growth direction. The heterostuctures have enhanced field emission property over the W whiskers.
A SEM image shows that a large number of the nanowires grow on the side surface of the W whisker stems.
art ic l e i nf o
a b s t r a c t
Article history: Received 28 March 2013 Received in revised form 15 May 2013 Accepted 21 May 2013 Available online 29 May 2013
One dimensional W/WO2.72 heterostructures were successfully synthesized using WO3 as the raw material by a simple two-step chemical vapor deposition process. The morphology and microstructure of the W/WO2.72 heterostuctures were characterized using scanning electron microscopy and transmission electron microscopy. The results indicate that the long and straight central axial W whiskers grow along [1 1 0] direction, while the branched WO2.72 nanowires grow on the side surface of the W whiskers along the radial direction, with [0 1 0] as the growth direction. The as-synthesized heterostuctures exhibit enhanced field emission property over the single W whiskers, and could be used as a candidate for fieldemission devices and ultrahigh sensitivity sensors due to their unique composition and structure. & 2013 Elsevier B.V. All rights reserved.
Keywords: Heterostructure Tungsten Tungsten oxide Field emission
A B S T R A C T
1. Introduction One-dimensional (1-D) nanostructures attract extensive attentions and show potential applications as functional materials due to their fascinating properties [1]. Among 1-D nanostructures, 1-D heterostructures with modulated compositions and interfaces show unique characteristics and potential applications as nanoscaled building blocks for future optoelectronic devices and systems [2].
n
Corresponding author. Tel.: +86 73188877677; fax: +86 73188710855. E-mail addresses: [email protected], [email protected] (M. Song). 1386-9477/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2013.05.017
The well-designed and controlled heterostructures usually have special properties because the cores and branches are composed of various materials with different characteristics. For example, core/ shell structured CdS/ZnS heterojunctions have enhanced photoluminescence efficiencies and electrical response, compared to those of the single CdS nanorods [3]. Core/shell structured TiO2/CdS nanorods show potential applications for the reversible conversion between solar energy and electrical energy, due to their long-lived charge-separated state and large photo current density [4]. As one of the best electrode materials for high-performance super capacitors, core/shell structured Co3O4/NiO nanowire arrays exhibit excellent super capacitor performance with high specific capacitance and good cycling stability. The dual phase ZnS with tetrapod tree-like
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heterostructures and branched architectures have excellent fieldemission properties due to their specific crystallographic feature and cone shape patterned branch with nanometer-sized tips [5,6]. The hierarchical core-shell structures consisting of primary ZnS nanotubes, indium (In) core nanowires, and ZnS nanowire secondary branches present multifold enhanced field-emission property [7]. Till now, many methods have been developed to fabricate 1-D nanostructures, such as vapor phase methods, solution phase methods, lithography processes, electro spinning and template methods [2]. Various types of the 1-D heterostructures have been reported, including core/shell and core/multi-shell nanocable structures, coaxial nanowire heterojunctions, epitaxial nanorod heterostructures, and hierarchical heterostructures [8]. For example, Shen et al. successfully synthesized co-axial and C-coated ZnS core/shell nanocables by thermal evaporation of a mixture of ZnS and SnS powders in a graphite crucible [9]. Ren et al. reported the fabrication of a hierarchical ZnO/In2O3 nanostructure using a simple evaporation method [10]. It should be noted that most of the 1-D heterostructures are the combination of various semiconductors, and the reports on metal/metal oxide 1-D heterostructures are very rare. Till now, most reported metal/metal oxide heterostuctures are limited to Zn/ZnO [10]. Tungsten (W) is a refractory metal with a series of outstanding physical and chemical properties, such as the highest melting point of about 3420 1C and the lowest vapor pressure among all metals, the decent strength and rigidity at room and elevated temperatures, the excellent corrosion resistance against metal and oxide vapors and a near-ideal midgap work function [8,11]. W can be used in metal gates in the applications of complementary metal oxide semiconductor (CMOS) technology in nanodevices smaller than 100 nm [8], and can also be used as field emission (FE) gun [11]. Previous studies have revealed that individual tungsten nanowires possess an ultra-high field enhancement factor and a high stability of the FE current density [11,12]. Tungsten oxides are particularly applicable in flat panel displays, photoelectrochromic smart windows, optical modulation devices, writing–reading–erasing optical devices, lithium-ion batteries, dye-sensitized solar cells, catalysts and gas sensors [13–19]. When the size decreases to the nano-scale, tungsten oxide nanostructures exhibit some fascinating properties and applications superior to their bulk counterparts, such as ultra sensitive and highly selective gas sensors [20], and excellent field-emission properties [21,22]. Although 1-D nanostructures of W and WOx have been studied extensively, with a variety of nanometer-scaled structures of W and WOx being synthesized and their structures and properties being also investigated thoroughly, few studies on W/WOx heterostructures have been reported. Baek et al. reported the fabrication of a novel W/WO3 hierarchical heterostructures, with single-crystalline W nanothorns grown on WO3 nanowhiskers using Ni as the catalyst by a two-step evaporation process [8]. However, the growth process and morphology cannot be controlled effectively [8]. Our previous work showed that W whiskers can grow on Si, SiO2 or Al2O3 substrate, and WO2.72 nanowires can grow on W substrate using the vapor deposition method [11,23]. It is therefore reasonable to propose that WO2.72 nanowires can grow on W whiskers. In this paper, a novel hierarchical heteronanostructure with single-crystalline WO2.72 nanothorns growing on W whiskers (referred to W/WO2.72) was fabricated successfully. The morphology, microstructure, growth mechanism and field emission property of the heteronanostructure were investigated.
2. Experimental The heterostructures were fabricated using chemical vapor deposition in a system similar to that reported previously [23].
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The synthesis process took place in a conventional tube furnace without vacuum pump. Tungsten trioxide powders (99.9% in purity) were used as the raw material and Si wafers (6 mm 4 mm 1 mm) were used as the substrate. WO3 powder and Si substrates were placed on both sides of the small exit (3 mm 3 mm) of a quartz tube (20 mm in diameter), and the quartz tube was placed in the horizontal tube furnace (60 mm in diameter and 600 mm in length). High-purity Ar with a constant flow of 200 standard cubic centimeters per minute (sccm) was introduced into the quartz tube and H2 with a constant flow of 100 sccm was introduced into the furnace tube. The pressure in the reactor was 1 bar during the whole reaction process. Then the temperature of the furnace was increased from room temperature to 950 1C with a rate of 10 1C/min. When the furnace was heated to 600 1C, Ar gas contained water vapor was introduced into the system (the content was controlled by water bath's temperature). After maintained at 950 1C for 4 h, the furnace was naturally cooled down to 700 1C and the H2 flow was shut off. Then the furnace was holding at 700 1C for 30 min, 2 h and 4 h. After the furnace was naturally cooled down to room temperature, the samples were taken out for characterization. The final products were investigated using X-ray diffraction (XRD, D/max 2550VB with Cu Kα-radiation), scanning electron microscopy (SEM, Nova Nano SEM 230) and transmission electron microscopy (TEM, JEOL-2100F) operating at 200 kV. Field-emission property of the W/WO2.72 heterostructures was tested at room temperature, using a two-parallel-plate configuration in a lab-built high vacuum system with a pressure of 10−7 Pa. The Si substrate was fixed on a Mo holder that acts as cathode. A rod-shaped copper probe with an equivalent cross-sectional area of 0.8 mm2 was used as anode. The distance between the anode and cathode was kept at 320 μm. A high voltage source measurement system was used to sweep the applied voltage from 0 to 4000 V with an increment of 100 V per step. At the same time, the emission current was measured.
3. Results and discussion 3.1. Morphology and structure characterizations Fig. 1a is a typical SEM image of the 1-D heterostructures holding at 700 1C for 4 h, showing that a large number of nanowires grow on the side surface of the W whisker stem. The whisker stem is of a diameter of 200–500 nm and a length of 5–15 mm. The branched nanowires are straight and smooth with a typical length of several hundred nanometers and a diameter ranging from 20 to 50 nm. Fig. 1b is the SEM image of the heterostructures holding at 700 1C for 2 h, clearly showing that much less nanowires grow on the W whisker stem, with the length and diameter being much smaller than those of the heterostructures synthesized for 4 h. Fig. 1c is the SEM image of the 1-D heterostructures holding at 700 1C for only 30 min, from which a small number of nanowires with quite short lengths were observed, indicating that the growth of the nanowires on W whiskers has just started. Further observation on the heterostructure in Fig. 1c indicates that the nucleation and growth of nanowires on W whiskers are not homogenous, leading to the inhomogeneous length and diameter of nanowires (Fig. 1a) during the following growing process. One should be noted that the diameter of the W stems in Fig. 1a differs from that in Figs. 1b and c. In general, the diameter of the W whiskers are controlled by growth temperature and the distance between Si substrate and the exit of the quartz tube substrate, as shown by Liu et al. and Wang et al. [11,24]. In this study, the difference in diameter of the W stems is due to the distance between the Si substrate and the exit of the quartz tube substrate, since the growth temperature
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Fig. 2. (a) XRD patterns of the synthesized heterostuctures and (b) enlarged view of the peaks between 201 and 301.
Fig. 1. SEM images of the synthesized heterostuctures holding at 700 1C for (a) 4 h, (b) 2 h and (c) 30 min.
remains the same for each case. The XRD patterns of the product synthesized for 4 h are shown in Fig. 2a. It can be seen that the dominant peaks can be indexed to the body-centered cubic (bcc) tungsten with the lattice parameter of a ¼0.316 nm (JCPDS: 040806). Apart from the main peaks from the W stem, the peaks between 201 and 301 (Fig. 2b) can be well indexed by the monoclinic WO2.72 structure with the lattice parameters of a¼ 1.828 nm, b¼ 0.3775 nm, c ¼1.398 nm and β¼115.21 (JCPDS 5-392). It can thus be concluded that the heterostructures are composed of W and WO2.72. Detailed morphology and crystal structure characterizations were carried out using TEM. Fig. 3a is a bright field TEM image of the W/WO2.72 heterostuctures, showing that some nanowires grow on the whisker stem. High-resolution TEM (HRTEM) image (Fig. 3b) shows that a thin amorphous film exists on the W whisker stem and also on the branch surface. This amorphous coating is the result of the superfluous tungsten oxide provided during the growth process. Fig. 3c presents an HRTEM image of the W whisker stem. Two sets of the orthogonal parallel lattice fringes with the spacing of 0.22 and 0.22 nm were marked by white lines in the image, corresponding to the (1 1 0) and (1 1 0) atomic planes of bcc W, respectively. The W whisker grows along [1 1 0] direction, as indicated by the white arrow. The corresponding selected-area electron diffraction (SAED) (Fig. 3d) further proves
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Fig. 3. (a) A typical TEM image and (b) a HRTEM of the heterostuctures, (c) a HRTEM image and (d) the corresponding SAED pattern of the W whisker stem, (e) a HRTEM image and (f) the corresponding SAED pattern of the W/WO2.72 branch.
that the tungsten whisker has a single crystalline bcc structure, and grows along the [1 1 0] direction. Fig. 3e presents the HRTEM image of the branched nanowire. The parallel lattice fringes with spacing of 0.38 nm correspond to the (0 1 0) atomic planes of monoclinic WO2.72, similar to our previous study [23], revealing that the branched WO2.72 nanowires grow along [0 1 0] direction. Fig. 3f is the corresponding SEAD pattern of the branched nanowire, further confirming that the WO2.72 branched nanowires are single crystalline and grow along the [0 1 0] direction. 3.2. Growth mechanism Fig. 4. Schematic of the growth mechanism of the W/WO2.72 heterostructures.
Based on the experimental results, the formation of W/WO2.72 heterostructures can be described by a two-step chemical vapor deposition process. Fig. 4 shows the schematic of the growth mechanism for W/WO2.72 heterostructures. (1) Step 1: the growth of W whiskers on Si substrate In this work, neither catalyst nor template was used, so the formation of tungsten whiskers is through direct vapor–solid (VS) mechanism. The detailed process can be described as follows. First, after introducing water vapor into the quartz tube by Ar carrier gas, the water vapor reacts with WO3 powders at 950 1C [25], leading to the generation of gaseous WO2(OH)2 inside the quartz tube through the following reaction: WO3 ðsÞ þ H2 OðgÞ↔WO2 ðOHÞ2 ðgÞ
ð1Þ
It should be noted that gaseous WO2(OH)2 is the most volatile compound formed in the system of W–O–H [25]. Second, the resultant WO2(OH)2 is transported to the substrate surfaces by the Ar carrier gas. Then, WO2(OH)2 reacts with H2 and generates W atoms via the following reaction : WO2 ðOHÞ2 ðgÞ þ 3H2 ðgÞ↔WðsÞ þ 4H2 OðgÞ
ð2Þ
Third, the generated tungsten atoms deposit on the Si substrate to form crystal nucleus, which gradually grow to form tungsten whiskers during the subsequent VS process. (2) Step 2: the growth of branched WO2.72 nanowire on the surface of W whisker stem
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The resultant gaseous WO2(OH)2, carried by Ar gas, flows out from the quartz tube, and cannot react with H2 once the H2 flow was shut off, resulting in the following chemical reactions [25]: 20WO2 ðOHÞ2 ðgÞ↔20WO2:9 ðsÞ þ O2 ðgÞ þ 20H2 O
ð3Þ
100WO2:9 ðsÞ↔100WO2:72 ðsÞ þ 9O2 ðgÞ
ð4Þ
Then the generated WO2.72 atoms deposit on the W whiskers, and gradually nucleate, absorb incoming WO2(OH)2 vapor and grow to form WO2.72 nanowires via the subsequent VS process. It should be noted that the W whisker might also provide W source for the nucleation of WO2.72, resulting from the oxidation of W whisker at 700 1C [26]. However, the W source from the oxidation of W whiskers should be negligible in this work since our parallel experiments showed that almost no WO2.72 naowires can be found on the side surface of the W whiskers if the WO3 are not supplied during the growth of WO2.72. It should be noted that the close-packed planes {0 1 0} of monoclinic WO2.72 results in the nanowires growing along the [0 1 0] direction to decrease the energy [27].
3.3. Field emission properties Field emission properties of the sample holding at 700 1C for 4 h were tested in a vacuum system and a plate configuration was used as the anode to absorb electrons. Fig. 5 presents the relationship between the emission current density and the electric field (J–E) of the heterostructures and the single W whisker array. Usually the turn-on field was defined as the electric field being required to produce an emission current density of 10 μA cm−2. It can be seen from Fig. 5a that the turn-on fields of the heterostructures and the single W whisker are 7.1 and 9.1 V/mm, respectively. The better field emission property of W/WO2.72 than that of single W whisker is suggested to be essentially due to the special branched architectures. The applied field in the first structure is enhanced by a W whisker stem that acts as a substrate for the secondary WO2.72 branches. A stronger field at the bottom of the branches is equivalent to an effectively higher applied bias, which then, in turn, is further enhanced by the 1D WO2.72 branches [7]. It is known that the field emission property depends significantly on the diameter and the density of the emitters [28]. In present work, the diameter of the W whisker is about several hundred nanometers, while the diameter of the WO2.72 is less than 50 nm. Therefore, the current density of the heterostructures is larger than that of the sample with single W whiskers and the current density will reach 10 μA cm−2 at a lower voltage, resulting in the turn on field being smaller for the W/WO2.72 heterostructures. Meanwhile, the WO2.72 nanowires grown on the W whisker can provide more emitters, which substantially increases the emission current, as shown in Fig. 5a. Moreover, previous studies also showed that the turn on field of tungsten oxide is smaller than that of the W nanowires [11,21,28]. This may be another reason for the difference in the turn on field of the single W whisker and W/WO2.72 heterostructures. The turn on field of the W/WO2.72 is closed to that reported for WO2.72 nanowire with diameter of 30 nm (6.2 V/μm) [29], but it is lower than that of W nanorods (8.0 V/μm) [28]. The field-emission behavior of the W/WO2.72 heterostructure was further analyzed by Fowler–Nordheim (FN) theory [30]. The corresponding FN plot (Fig. 5b) showed the relation between emitted current density J and the applied field E (which is defined as V/d, where d is the anode-sample distance) by following equation [29] 2 2
3=2
J ¼ ðAβ E =ϕÞexpð−Bϕ
=βEÞ
ð5Þ
Fig. 5. FE performance of the nanostructure arrays. (a) Current density versus electric field plot and (b) the corresponding FN plots of the nanostructures.
where ϕ is the work-function, β is the field enhancement factor, A and B are two constants (A¼1.54 10−6 A eV V−2 and B¼ 6.83 103 eV−3/2 V μm−1). By plotting ln(J/E2) as a function of 1/E, a linear relationship with the slope of −Bϕ3/2/β was obtained, as shown in Fig. 5b. The approximate linear relationship of the FN plots implies that the electron emissions of the heterostructures follow FN behavior. The calculated field enhancement factors of the heterostructures and W whisker are 684 and 457, respectively, using the work-function of bulk tungsten oxide (5.7 eV) [31] and bulk tungsten (4.5 eV) [32]. The field enhancement factor of W/WO2.72 heterostructures is larger than that of W whiskers, due to the smaller diameter of the WO2.72 nanowires compared to the W stems and the intrinsic property of the material. Based on those results, the as-synthesized W/WO2.72 heterostructures have the potential applications in field emission microelectronic devices, such as FE displays and microwave amplifiers. In addition, the growth process permits us to predetermine the field emission property, which is important to FE applications.
4. Conclusion In this paper, one dimensional W/WO2.72 hierarchical heterostuctures were successfully synthesized by a simple two-step vapor deposition process. The W whisker stem has a diameter of
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200–500 nm and a length of 5–15 mm, and grows along [1 1 0] direction. The branched WO2.72 nanowires grow on the side surface of the W whisker along the radial direction, with [0 1 0] as the growth direction. The WO2.72 nanowires have a length of several hundred nanometers and a diameter ranging from 20 to 50 nm. The W/WO2.72 hierarchical heterostuctures exhibit promising field-emission performance and could be used as a candidate for field-emission device and ultrahigh sensitivity sensors due to the unique composition and structure.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant nos. 51174235 and 50721003) and the Program for New Century Excellent Talents in Universities (NCET-10–0842). References [1] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Advanced Materials 15 (2003) 353. [2] G. Shen, D. Chen, Y. Bando, D. Golberg, Journal of Materials Science and Technology 24 (2008) 541. [3] A. Datta, S.K. Panda, S. Chaudhuri, Journal of Physical Chemistry C 111 (2007) 17260. [4] K. Das, S. De, Journal of Physical Chemistry C 113 (2009) 3494. [5] Z.G. Chen, J. Zou, G. Liu, X.D. Yao, F. Li, X.L. Yuan, T. Sekiguchi, G.Q. Lu, H.M. Cheng, Advanced Functional Materials 18 (2008) 3063. [6] Z.G. Chen, L.N. Cheng, H.Y. Xu, J.Z. Liu, J. Zou, T. Sekiguchi, G.Q. Lu, H.M. Cheng, Advanced Materials 22 (2010) 2376. [7] U.K. Gautam, X.S. Fang, Y. Bando, J.H. Zhan, D. Golberg, ACS Nano 2 (2008) 1015. [8] Y. Baek, Y. Song, K. Yong, Advanced Materials 18 (2006) 3105.
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Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Structure and field-emission properties of W/WO2.72 heterostructures fabricated by vapor deposition Xinli Liu, Min Song n, Shiliang Wang, Yuehui He State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China
H I G H L I G H T S
G R A P H I C A L
W/WO2.72 heterostructures were synthesized by the chemical vapor deposition process. The long and straight central axial W whiskers grow along [1 1 0] direction. WO2.72 grows on side surface of the W whiskers, with [0 1 0] as the growth direction. The heterostuctures have enhanced field emission property over the W whiskers.
A SEM image shows that a large number of the nanowires grow on the side surface of the W whisker stems.
art ic l e i nf o
a b s t r a c t
Article history: Received 28 March 2013 Received in revised form 15 May 2013 Accepted 21 May 2013 Available online 29 May 2013
One dimensional W/WO2.72 heterostructures were successfully synthesized using WO3 as the raw material by a simple two-step chemical vapor deposition process. The morphology and microstructure of the W/WO2.72 heterostuctures were characterized using scanning electron microscopy and transmission electron microscopy. The results indicate that the long and straight central axial W whiskers grow along [1 1 0] direction, while the branched WO2.72 nanowires grow on the side surface of the W whiskers along the radial direction, with [0 1 0] as the growth direction. The as-synthesized heterostuctures exhibit enhanced field emission property over the single W whiskers, and could be used as a candidate for fieldemission devices and ultrahigh sensitivity sensors due to their unique composition and structure. & 2013 Elsevier B.V. All rights reserved.
Keywords: Heterostructure Tungsten Tungsten oxide Field emission
A B S T R A C T
1. Introduction One-dimensional (1-D) nanostructures attract extensive attentions and show potential applications as functional materials due to their fascinating properties [1]. Among 1-D nanostructures, 1-D heterostructures with modulated compositions and interfaces show unique characteristics and potential applications as nanoscaled building blocks for future optoelectronic devices and systems [2].
n
Corresponding author. Tel.: +86 73188877677; fax: +86 73188710855. E-mail addresses: [email protected], [email protected] (M. Song). 1386-9477/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2013.05.017
The well-designed and controlled heterostructures usually have special properties because the cores and branches are composed of various materials with different characteristics. For example, core/ shell structured CdS/ZnS heterojunctions have enhanced photoluminescence efficiencies and electrical response, compared to those of the single CdS nanorods [3]. Core/shell structured TiO2/CdS nanorods show potential applications for the reversible conversion between solar energy and electrical energy, due to their long-lived charge-separated state and large photo current density [4]. As one of the best electrode materials for high-performance super capacitors, core/shell structured Co3O4/NiO nanowire arrays exhibit excellent super capacitor performance with high specific capacitance and good cycling stability. The dual phase ZnS with tetrapod tree-like
X. Liu et al. / Physica E 53 (2013) 260–265
heterostructures and branched architectures have excellent fieldemission properties due to their specific crystallographic feature and cone shape patterned branch with nanometer-sized tips [5,6]. The hierarchical core-shell structures consisting of primary ZnS nanotubes, indium (In) core nanowires, and ZnS nanowire secondary branches present multifold enhanced field-emission property [7]. Till now, many methods have been developed to fabricate 1-D nanostructures, such as vapor phase methods, solution phase methods, lithography processes, electro spinning and template methods [2]. Various types of the 1-D heterostructures have been reported, including core/shell and core/multi-shell nanocable structures, coaxial nanowire heterojunctions, epitaxial nanorod heterostructures, and hierarchical heterostructures [8]. For example, Shen et al. successfully synthesized co-axial and C-coated ZnS core/shell nanocables by thermal evaporation of a mixture of ZnS and SnS powders in a graphite crucible [9]. Ren et al. reported the fabrication of a hierarchical ZnO/In2O3 nanostructure using a simple evaporation method [10]. It should be noted that most of the 1-D heterostructures are the combination of various semiconductors, and the reports on metal/metal oxide 1-D heterostructures are very rare. Till now, most reported metal/metal oxide heterostuctures are limited to Zn/ZnO [10]. Tungsten (W) is a refractory metal with a series of outstanding physical and chemical properties, such as the highest melting point of about 3420 1C and the lowest vapor pressure among all metals, the decent strength and rigidity at room and elevated temperatures, the excellent corrosion resistance against metal and oxide vapors and a near-ideal midgap work function [8,11]. W can be used in metal gates in the applications of complementary metal oxide semiconductor (CMOS) technology in nanodevices smaller than 100 nm [8], and can also be used as field emission (FE) gun [11]. Previous studies have revealed that individual tungsten nanowires possess an ultra-high field enhancement factor and a high stability of the FE current density [11,12]. Tungsten oxides are particularly applicable in flat panel displays, photoelectrochromic smart windows, optical modulation devices, writing–reading–erasing optical devices, lithium-ion batteries, dye-sensitized solar cells, catalysts and gas sensors [13–19]. When the size decreases to the nano-scale, tungsten oxide nanostructures exhibit some fascinating properties and applications superior to their bulk counterparts, such as ultra sensitive and highly selective gas sensors [20], and excellent field-emission properties [21,22]. Although 1-D nanostructures of W and WOx have been studied extensively, with a variety of nanometer-scaled structures of W and WOx being synthesized and their structures and properties being also investigated thoroughly, few studies on W/WOx heterostructures have been reported. Baek et al. reported the fabrication of a novel W/WO3 hierarchical heterostructures, with single-crystalline W nanothorns grown on WO3 nanowhiskers using Ni as the catalyst by a two-step evaporation process [8]. However, the growth process and morphology cannot be controlled effectively [8]. Our previous work showed that W whiskers can grow on Si, SiO2 or Al2O3 substrate, and WO2.72 nanowires can grow on W substrate using the vapor deposition method [11,23]. It is therefore reasonable to propose that WO2.72 nanowires can grow on W whiskers. In this paper, a novel hierarchical heteronanostructure with single-crystalline WO2.72 nanothorns growing on W whiskers (referred to W/WO2.72) was fabricated successfully. The morphology, microstructure, growth mechanism and field emission property of the heteronanostructure were investigated.
2. Experimental The heterostructures were fabricated using chemical vapor deposition in a system similar to that reported previously [23].
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The synthesis process took place in a conventional tube furnace without vacuum pump. Tungsten trioxide powders (99.9% in purity) were used as the raw material and Si wafers (6 mm 4 mm 1 mm) were used as the substrate. WO3 powder and Si substrates were placed on both sides of the small exit (3 mm 3 mm) of a quartz tube (20 mm in diameter), and the quartz tube was placed in the horizontal tube furnace (60 mm in diameter and 600 mm in length). High-purity Ar with a constant flow of 200 standard cubic centimeters per minute (sccm) was introduced into the quartz tube and H2 with a constant flow of 100 sccm was introduced into the furnace tube. The pressure in the reactor was 1 bar during the whole reaction process. Then the temperature of the furnace was increased from room temperature to 950 1C with a rate of 10 1C/min. When the furnace was heated to 600 1C, Ar gas contained water vapor was introduced into the system (the content was controlled by water bath's temperature). After maintained at 950 1C for 4 h, the furnace was naturally cooled down to 700 1C and the H2 flow was shut off. Then the furnace was holding at 700 1C for 30 min, 2 h and 4 h. After the furnace was naturally cooled down to room temperature, the samples were taken out for characterization. The final products were investigated using X-ray diffraction (XRD, D/max 2550VB with Cu Kα-radiation), scanning electron microscopy (SEM, Nova Nano SEM 230) and transmission electron microscopy (TEM, JEOL-2100F) operating at 200 kV. Field-emission property of the W/WO2.72 heterostructures was tested at room temperature, using a two-parallel-plate configuration in a lab-built high vacuum system with a pressure of 10−7 Pa. The Si substrate was fixed on a Mo holder that acts as cathode. A rod-shaped copper probe with an equivalent cross-sectional area of 0.8 mm2 was used as anode. The distance between the anode and cathode was kept at 320 μm. A high voltage source measurement system was used to sweep the applied voltage from 0 to 4000 V with an increment of 100 V per step. At the same time, the emission current was measured.
3. Results and discussion 3.1. Morphology and structure characterizations Fig. 1a is a typical SEM image of the 1-D heterostructures holding at 700 1C for 4 h, showing that a large number of nanowires grow on the side surface of the W whisker stem. The whisker stem is of a diameter of 200–500 nm and a length of 5–15 mm. The branched nanowires are straight and smooth with a typical length of several hundred nanometers and a diameter ranging from 20 to 50 nm. Fig. 1b is the SEM image of the heterostructures holding at 700 1C for 2 h, clearly showing that much less nanowires grow on the W whisker stem, with the length and diameter being much smaller than those of the heterostructures synthesized for 4 h. Fig. 1c is the SEM image of the 1-D heterostructures holding at 700 1C for only 30 min, from which a small number of nanowires with quite short lengths were observed, indicating that the growth of the nanowires on W whiskers has just started. Further observation on the heterostructure in Fig. 1c indicates that the nucleation and growth of nanowires on W whiskers are not homogenous, leading to the inhomogeneous length and diameter of nanowires (Fig. 1a) during the following growing process. One should be noted that the diameter of the W stems in Fig. 1a differs from that in Figs. 1b and c. In general, the diameter of the W whiskers are controlled by growth temperature and the distance between Si substrate and the exit of the quartz tube substrate, as shown by Liu et al. and Wang et al. [11,24]. In this study, the difference in diameter of the W stems is due to the distance between the Si substrate and the exit of the quartz tube substrate, since the growth temperature
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Fig. 2. (a) XRD patterns of the synthesized heterostuctures and (b) enlarged view of the peaks between 201 and 301.
Fig. 1. SEM images of the synthesized heterostuctures holding at 700 1C for (a) 4 h, (b) 2 h and (c) 30 min.
remains the same for each case. The XRD patterns of the product synthesized for 4 h are shown in Fig. 2a. It can be seen that the dominant peaks can be indexed to the body-centered cubic (bcc) tungsten with the lattice parameter of a ¼0.316 nm (JCPDS: 040806). Apart from the main peaks from the W stem, the peaks between 201 and 301 (Fig. 2b) can be well indexed by the monoclinic WO2.72 structure with the lattice parameters of a¼ 1.828 nm, b¼ 0.3775 nm, c ¼1.398 nm and β¼115.21 (JCPDS 5-392). It can thus be concluded that the heterostructures are composed of W and WO2.72. Detailed morphology and crystal structure characterizations were carried out using TEM. Fig. 3a is a bright field TEM image of the W/WO2.72 heterostuctures, showing that some nanowires grow on the whisker stem. High-resolution TEM (HRTEM) image (Fig. 3b) shows that a thin amorphous film exists on the W whisker stem and also on the branch surface. This amorphous coating is the result of the superfluous tungsten oxide provided during the growth process. Fig. 3c presents an HRTEM image of the W whisker stem. Two sets of the orthogonal parallel lattice fringes with the spacing of 0.22 and 0.22 nm were marked by white lines in the image, corresponding to the (1 1 0) and (1 1 0) atomic planes of bcc W, respectively. The W whisker grows along [1 1 0] direction, as indicated by the white arrow. The corresponding selected-area electron diffraction (SAED) (Fig. 3d) further proves
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Fig. 3. (a) A typical TEM image and (b) a HRTEM of the heterostuctures, (c) a HRTEM image and (d) the corresponding SAED pattern of the W whisker stem, (e) a HRTEM image and (f) the corresponding SAED pattern of the W/WO2.72 branch.
that the tungsten whisker has a single crystalline bcc structure, and grows along the [1 1 0] direction. Fig. 3e presents the HRTEM image of the branched nanowire. The parallel lattice fringes with spacing of 0.38 nm correspond to the (0 1 0) atomic planes of monoclinic WO2.72, similar to our previous study [23], revealing that the branched WO2.72 nanowires grow along [0 1 0] direction. Fig. 3f is the corresponding SEAD pattern of the branched nanowire, further confirming that the WO2.72 branched nanowires are single crystalline and grow along the [0 1 0] direction. 3.2. Growth mechanism Fig. 4. Schematic of the growth mechanism of the W/WO2.72 heterostructures.
Based on the experimental results, the formation of W/WO2.72 heterostructures can be described by a two-step chemical vapor deposition process. Fig. 4 shows the schematic of the growth mechanism for W/WO2.72 heterostructures. (1) Step 1: the growth of W whiskers on Si substrate In this work, neither catalyst nor template was used, so the formation of tungsten whiskers is through direct vapor–solid (VS) mechanism. The detailed process can be described as follows. First, after introducing water vapor into the quartz tube by Ar carrier gas, the water vapor reacts with WO3 powders at 950 1C [25], leading to the generation of gaseous WO2(OH)2 inside the quartz tube through the following reaction: WO3 ðsÞ þ H2 OðgÞ↔WO2 ðOHÞ2 ðgÞ
ð1Þ
It should be noted that gaseous WO2(OH)2 is the most volatile compound formed in the system of W–O–H [25]. Second, the resultant WO2(OH)2 is transported to the substrate surfaces by the Ar carrier gas. Then, WO2(OH)2 reacts with H2 and generates W atoms via the following reaction : WO2 ðOHÞ2 ðgÞ þ 3H2 ðgÞ↔WðsÞ þ 4H2 OðgÞ
ð2Þ
Third, the generated tungsten atoms deposit on the Si substrate to form crystal nucleus, which gradually grow to form tungsten whiskers during the subsequent VS process. (2) Step 2: the growth of branched WO2.72 nanowire on the surface of W whisker stem
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The resultant gaseous WO2(OH)2, carried by Ar gas, flows out from the quartz tube, and cannot react with H2 once the H2 flow was shut off, resulting in the following chemical reactions [25]: 20WO2 ðOHÞ2 ðgÞ↔20WO2:9 ðsÞ þ O2 ðgÞ þ 20H2 O
ð3Þ
100WO2:9 ðsÞ↔100WO2:72 ðsÞ þ 9O2 ðgÞ
ð4Þ
Then the generated WO2.72 atoms deposit on the W whiskers, and gradually nucleate, absorb incoming WO2(OH)2 vapor and grow to form WO2.72 nanowires via the subsequent VS process. It should be noted that the W whisker might also provide W source for the nucleation of WO2.72, resulting from the oxidation of W whisker at 700 1C [26]. However, the W source from the oxidation of W whiskers should be negligible in this work since our parallel experiments showed that almost no WO2.72 naowires can be found on the side surface of the W whiskers if the WO3 are not supplied during the growth of WO2.72. It should be noted that the close-packed planes {0 1 0} of monoclinic WO2.72 results in the nanowires growing along the [0 1 0] direction to decrease the energy [27].
3.3. Field emission properties Field emission properties of the sample holding at 700 1C for 4 h were tested in a vacuum system and a plate configuration was used as the anode to absorb electrons. Fig. 5 presents the relationship between the emission current density and the electric field (J–E) of the heterostructures and the single W whisker array. Usually the turn-on field was defined as the electric field being required to produce an emission current density of 10 μA cm−2. It can be seen from Fig. 5a that the turn-on fields of the heterostructures and the single W whisker are 7.1 and 9.1 V/mm, respectively. The better field emission property of W/WO2.72 than that of single W whisker is suggested to be essentially due to the special branched architectures. The applied field in the first structure is enhanced by a W whisker stem that acts as a substrate for the secondary WO2.72 branches. A stronger field at the bottom of the branches is equivalent to an effectively higher applied bias, which then, in turn, is further enhanced by the 1D WO2.72 branches [7]. It is known that the field emission property depends significantly on the diameter and the density of the emitters [28]. In present work, the diameter of the W whisker is about several hundred nanometers, while the diameter of the WO2.72 is less than 50 nm. Therefore, the current density of the heterostructures is larger than that of the sample with single W whiskers and the current density will reach 10 μA cm−2 at a lower voltage, resulting in the turn on field being smaller for the W/WO2.72 heterostructures. Meanwhile, the WO2.72 nanowires grown on the W whisker can provide more emitters, which substantially increases the emission current, as shown in Fig. 5a. Moreover, previous studies also showed that the turn on field of tungsten oxide is smaller than that of the W nanowires [11,21,28]. This may be another reason for the difference in the turn on field of the single W whisker and W/WO2.72 heterostructures. The turn on field of the W/WO2.72 is closed to that reported for WO2.72 nanowire with diameter of 30 nm (6.2 V/μm) [29], but it is lower than that of W nanorods (8.0 V/μm) [28]. The field-emission behavior of the W/WO2.72 heterostructure was further analyzed by Fowler–Nordheim (FN) theory [30]. The corresponding FN plot (Fig. 5b) showed the relation between emitted current density J and the applied field E (which is defined as V/d, where d is the anode-sample distance) by following equation [29] 2 2
3=2
J ¼ ðAβ E =ϕÞexpð−Bϕ
=βEÞ
ð5Þ
Fig. 5. FE performance of the nanostructure arrays. (a) Current density versus electric field plot and (b) the corresponding FN plots of the nanostructures.
where ϕ is the work-function, β is the field enhancement factor, A and B are two constants (A¼1.54 10−6 A eV V−2 and B¼ 6.83 103 eV−3/2 V μm−1). By plotting ln(J/E2) as a function of 1/E, a linear relationship with the slope of −Bϕ3/2/β was obtained, as shown in Fig. 5b. The approximate linear relationship of the FN plots implies that the electron emissions of the heterostructures follow FN behavior. The calculated field enhancement factors of the heterostructures and W whisker are 684 and 457, respectively, using the work-function of bulk tungsten oxide (5.7 eV) [31] and bulk tungsten (4.5 eV) [32]. The field enhancement factor of W/WO2.72 heterostructures is larger than that of W whiskers, due to the smaller diameter of the WO2.72 nanowires compared to the W stems and the intrinsic property of the material. Based on those results, the as-synthesized W/WO2.72 heterostructures have the potential applications in field emission microelectronic devices, such as FE displays and microwave amplifiers. In addition, the growth process permits us to predetermine the field emission property, which is important to FE applications.
4. Conclusion In this paper, one dimensional W/WO2.72 hierarchical heterostuctures were successfully synthesized by a simple two-step vapor deposition process. The W whisker stem has a diameter of
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200–500 nm and a length of 5–15 mm, and grows along [1 1 0] direction. The branched WO2.72 nanowires grow on the side surface of the W whisker along the radial direction, with [0 1 0] as the growth direction. The WO2.72 nanowires have a length of several hundred nanometers and a diameter ranging from 20 to 50 nm. The W/WO2.72 hierarchical heterostuctures exhibit promising field-emission performance and could be used as a candidate for field-emission device and ultrahigh sensitivity sensors due to the unique composition and structure.
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