BK 21 Physics Research Division, Institute of Basic Sciences, Department of Energy Science, Sungkyunkwan University, Suwon 440–746, Republic of KoreaDepartment of Nanosciences, ICB CNRS-UMR 5209, University de Bourgogne 9 Avenue Alain Savary, Dijon 21078, France
r t i c l e i n f o
rticle history:eceived 27 March 2011eceived in revised form 21 August 2011ccepted 27 August 2011
a b s t r a c t
High quality single crystalline CuV2O6 (CVO) nanowires were successfully grown on the silicon substratesby simple thermal annealing of spin coated film in air. The synthesized nanowires have an average diame-ter of 100 nm, and length ranges between 1 and 5 �m, with growth direction along <2 0 1>. We found that
growth temperature play a vital role in controlling the phase and the diameter of nanowires in this pro-cess. We propose a growth mechanism based on the experimental results. Field emission properties wererevealed to be strongly morphology dependent. The nanowires in small diameter and high aspect ratioexhibited the best FE performance showing excellent field emission current densities 1.2 mA cm−2 witha low turn-on field of ∼1.2 V �m−1. The experimental results show that CVO nanowires are promisingcandidate in realizing high performance field emission display.
Field emission is technologically important for vacuum micro-lectronic devices such as field emission displays, X-ray source,icrowave devices, etc. [1–5]. FE emitters based on 1D metal oxides
uch as ZnO, CuO, WO3 and MoO3 [6–9] have gained great attrac-ion owing to the feasibility in precise structural control duringhe growth process and hence, a more predictable current–voltageharacteristic. However, these metal oxides have a high turn-onnd threshold electric fields which limit their practical application.etal vanadates, such as copper vanadates, bismuth vanadates,
nd zinc vanadates, have recently attracted great scientific atten-ion because of their unique properties such as high mechanicaltrength, electronic properties, photonic efficiency, and chemicaltability [10–14]. Of these metal vanadates, 1D nanostructures ofopper vanadate with good conductance are considered to be idealeld emission electron sources that can emit electrons at low elec-ric field [15]. The preparation of various 1D CVO nanostructuresuch as wires, rods, and tubes have been intensively studied with a
ide range of synthetic methods [16]. However, all of the reportedVO nanostructures involve the usage of sacrificial templates oruiding catalysts, whose removal may complicate the application
∗ Corresponding author at: BK 21 Physics Research Division, Institute of Basic Sci-nces, Department of Energy Science, Sungkyunkwan University, Suwon 440–746,epublic of Korea. Tel.: +82 31 290 5906; fax: +82 31 290 5947.
of the nanostructures. In this work we describe a facile syntheticroute to obtain high-quality single-crystalline CVO nanowires, byspin-coating CVO solution on the p-type Si wafers followed bythermal annealing, without using harmful reagents, hazardous sol-vents, or surfactants. Nanowires grown by this method have severaladvantages compared with other conventional techniques in termsof simplicity, low cost, and a high degree of compositional con-trol. The direct growth of nanowires on the p-type Si substrateoffers a much better electron transport capability by providing adirect conduction path, which is very helpful to lower the con-tact resistance between nanowires and substrates for enhancingFE of nanowires. We have studied the electrical and field-emissionproperties of nanowires. The high field emission current densityachieved makes the CVO nanowires a promising candidate for highthe current field emitter of flat panel displays and high brightnesselectron sources.
2. Experimental details
All chemicals, NH4VO3, Cu(NO3)2·6H2O, and HCl, were purchased from SigmaAldrich and used as received. For synthesis of the CVO precursor, 1 mmol of NH4VO3
was dissolved in HCl aqueous solution (4 mol L−1) and subjected to magnetic stirringfor 1 h, and 0.01 mmol Cu(NO3)2·6H2O was subsequently added. The synthesized
solution was spin coated at 2000 rpm for 30 s on the pre-cleaned Si substrates (thenative oxide layer was removed by etching the sample in 10% HF solution for 30 s)and was dried on a hot plate at 150 ◦C for 30 min. These samples were then sub-jected to annealing in an ambient atmosphere at 500, 550, 600, and, 650 ◦C for 2 h,respectively.
M. Shahid et al. / Materials Chemistry and Physics 131 (2011) 184–189 185
F ◦ ) 550 ◦ ◦
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3
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ig. 1. FE-SEM images of CuV2O6 nanowires at different temperatures: (a) 500 C, (bf synthesis temperature after 2 h growth. Each error bar represents the diameter annd length.
. Results and discussion
Images from FE-SEM analysis (FE-SEM: JEOL JSM–7401F) of theVO nanowires are shown in Fig. 1(a–c). The FE-SEM analyseslearly indicate that the diameter and the length of the as-grownVO nanowires can be easily controlled by the growth tempera-ure. The effects of growth temperature on the average diameterf nanowire and length are shown in Fig. 1(d). As the tempera-ure increased from 500 ◦C to 600 ◦C, the diameter of nanowire alsoncreased. In contrast, the length of the nanowires was found toeak when grown at 550 ◦C and decreases when grown at 600 ◦C.n the other hand, when grown at 650 ◦C, the nanosheets were
ormed instead (see supporting information S2 (b)). These resultsuggest that the agglomeration of the activated nuclei may accountor the observed increase in the diameter of nanowires. Fig. 2 showsschematics for nanowires growth. We propose that four different
tages are involved during the growth of nanowires: In the firsttep a film of CVO solution is initially deposited on the substrate bypin coating, and subsequently dried on a hot plate. In the secondtep the decomposition of CVO solution in the presence of oxygent 150 ◦C allows nucleation of the film. In the third step, the filmreaks into nuclei and forms the basis for nanowire growth. In thenal step, when the temperature reaches 500 ◦C, nanowire growthommences from the nuclei as shown in Fig. 1(a).
The crystallinity and phase of as-grown nanowires were inves-igated by the XRD (D8 Focus 2200V, Bruker AXS, using Cu K�adiation (� = 1.5418 A) as shown in Fig. 3(a). The XRD patternslearly indicate the formation of triclinic crystal structures of CVOanowires, which is consistent with the standard JCPDS card (CVO
CPDS 74-2117 and 29-0588). It is also noted that further increase inhe growth temperature up to 650 ◦C (see supporting information2(a)), the crystal phase was transformed into Cu2V2O7 (JCPDS cardo. 73-1032). The chemical composition and oxidation state of thelements for the grown nanowires were confirmed by XPS. ThePS (Perkin–Elmer PHI 660) spectra of the CVO nanowires withxygen 1s, Vanadium 2p, and Copper 2p regions are shown in
ig. 3(b and c). The main oxygen peak in Fig. 3(b) at 530.2 eV wass expected for a vanadium–oxygen (V–O) bond. The shoulder atigher energy is due to oxygen bond to the surface OH group [17]. Inig. 3(b), the two peaks of vanadium correspond to a doublet V2p3/2
C, (c) 600 C. (d) Average diameter and length of the nanowires plotted as a functiongth distribution of 100 nanowires that were used to calculate the average diameter
(∼517.66 eV) and V2p1/2 (∼525.13 eV) spin–orbit split components[18,19]. The appearance of a weak band at 521.1 eV is attributed tothe X-ray satellite of the O (1s) core level. Comparison of the datawith that previously reported [20] indicates that the nanowiresgrown under these conditions are composed of vanadium in the+5 oxidation state. The Cu2p3/2 and Cu2p1/2 regions in Fig. 3(c)show distinctive signals at 934.45 and 954.46 eV. A strong satel-lite peak that appeared approximately at 942 eV can be assignedto Cu2+ [21]. The crystal structure of CVO nanowires was exam-ined by the TEM (FE-TEM JEM2100F) as shown in Fig. 4(a–c) withselected area diffraction (SAED) pattern. Lattice fringes are clearlyvisible in the high resolution TEM image of nanowires in Fig. 4(c);thus the single crystalline nature of the nanowires is confirmed.We found that the distance between the neighboring fringes is
approximately 4.29 ´A (Fig. 4(c)), which is consistent with that ofthe (2 0 0) plane of CVO nanowires. The TEM analysis indicates thatthe growth direction of CVO nanowires is perpendicular to the(2 0 0) plane. TEM-EDX (Oxford Instruments Inca-FET-3) elemen-tal mapping was also carried out to investigate the distribution ofthe elements in the nanowires. The elemental mapping of V, Cu,and O from the nanowire is shown in Fig. 4(d), and the EDX of thecorresponding nanowire is shown in Fig. 4(e). The electrical trans-port property of the nanowires was studied on individual nanowire.The nanowires were first removed from the growth substrate bysonication in isopropyl alcohol and deposited onto a silicon sub-strate capped with a 200 nm silicon dioxide layer. Electron-beamlithography was used to define 4 terminal metal electrode win-dows, followed by the metal deposition of Ti/Au (5 nm/150 nm) byelectron beam evaporation (Fig. 5, inset). Electrical properties (I–V)were measured using four probe station which has Keithley 2612Source Meter. The typical current–voltage (I–V) behavior of indi-vidual nanowire is shown in Fig. 5. The resistivity (�) of nanowirewas calculated using the relation � = R × A l−1 where R is the resis-tance, A is an area of nanowire (width and height) and l is the lengthof the nanowire between the electrodes. The resistivity of single ananowire was found to be 200 � cm which is comparable to the
bulk resistivity of CVO (102–103 � cm) [22].
The electron field emission characteristics of CVO nanowiresgrown at different temperature were investigated in a high vac-uum chamber with a parallel diode-type configuration at a base
186 M. Shahid et al. / Materials Chemistry and Physics 131 (2011) 184–189
Fig. 2. The schematic for nanowire growth.
Fig. 3. XRD patterns of the CuV2O6 nanowires at different temperatures: (I) 500 ◦C, (II) 550 ◦C, (III) 600 ◦C, (() Cu2V2O7 and (*) Si substrate (a). (b) XPS spectrum of CuV2O6
nanowires grown at 550 ◦C, vanadium V 2p with oxygen O 1s, and (c) XPS spectrum of copper Cu 2p.
Table 1A comparison of turn on field strength, method of preparation, and stability of different field emitters.
Field emitters Method of growth Turn on field [v �m−1] Fluctuation (�J J−1av)
Tungsten-whisker [23] CVD 4.0 3.6%ZnS branched architectures [24] CVD 2.39 1.1%Boron nanowires [25] VLS 5.1 22%Cu nanowires [26] CVD 4.6 No dataCdS nanobelts [27] VLS 3.7 5%ZnO nanowires [28] Electrochemical deposition 1.8 No dataMoO2 nanowires [29] CVD 2.24 No dataMoO3 nanowires [30] CVD 3.5 No dataV2O5 nanowires [31] Hydrotherma1 2.28 Stable (no current fluctuation)CuV2O6 nanowires (this work) Chemical solution deposition 1.2 5%
M. Shahid et al. / Materials Chemistry and Physics 131 (2011) 184–189 187
Fig. 4. (a) Low magnification TEM image of CuV2O6 nanowire. (b) Corresponding SAED pattern of individual nanowire recorded along [0 1 0] zone axis. (c) HR-TEM of nanowires entratfi circlen
pa2a
Ff
howing high crystallinity. (d) TEM Element mapping showing the respective concgure legend, the reader is referred to the web version of the article.) Copper (whiteanowires are composed of V, Cu and O. The Ni peaks originate from the TEM grid.
−7
ressure of 5 × 10 mbar. The field emission current was measuredt different voltages using an automatically controlled Keithley001 electrometer and DC power supply (HCN, 700–3500). Thes-grown nanowires were placed into the vacuum chamber. A
ig. 5. The typical characteristics of I–V curve of single nanowire. The bias variesrom −0.4 to 0.4 V. Right inset: Scanning electron microscope image of devices.
ions of Vanadium (red circles) (For interpretation of the references to color in thiss) and Oxygen (yellow) in the nanowire. (e) EDS spectrum indicates that individual
stainless-steel probe was used as an anode to induce electronsfrom the cathode nanowires. The distance between the cathodeand anode was 450 �m. A DC voltage sweeping from 40 to 3000 Vwas applied during the measurements. Fig. 6(a) shows a typicalplot of current density versus the electric field for CVO nanowiresgrown at different temperature. The turn-on field (defined at emis-sion current density of 10 �A cm−2) for samples grown at 500, 550,and 600 ◦C are 4.3, 1.2, and 2.1 V �m−1, respectively. The maxi-mum current density for a CVO sample grown at 550 ◦C was foundto be 1.2 mA cm−2
. We observed that CVO nanowires synthesizedby this simple spin coatable method show a much lower turn-on field compared to that of other metal oxides nanowires, assummarized in Table 1. This low turn-on field is mainly due tothe smaller diameters of the nanowires which is consistent withalready published works [32–34]. Furthermore, direct growth ofnanowires on p-type Si substrate facilitates fast transfer of elec-trons between the nanowires and substrate compared to nanowiresgrown by hydrothermal or solution methods. The measured currentwas observed to follow the Fower–Nordeim (F–N) law, where thecurrent density J is related to the applied electric field E as
J =(
AE2ˇ2
�
)exp
(−B�3/2
Eˇ
)
188 M. Shahid et al. / Materials Chemistry and Physics 131 (2011) 184–189
Fig. 6. (a) Field emission current density vs. electrical field from nanowire emitters grown at different temperatures, inset is an enlarged view of the low current emissionrange for the 500 ◦C sample. (b) Corresponding FN plots of ln (J/E2) vs. 1/E for nanowires. Excellent agreement with the F–N theory of field emission is observed. (c) Stabilityo ◦
wAfiaiigdcoti5ws
4
nddtoatoa
A
SiU
[[[[[[
[
[
[[[
[
[[
[
f the field-emission current from the nanowires grown at 550 C over time.
here J is the current density in amp cm−2,= 1.56 × 10−10 A V−2 eV, B = 6.83 × 109 V eV−3/2 V cm−1, ˇ is aeld enhancement factor, � is the work function (eV), and E is thepplied electric field. Fig. 6(b) shows a linear F–N behavior, indicat-ng a field emission signature from the CVO nanowire. The changesn the nanowire field emission characteristics induced by varyingrowth temperatures could ascribe the changing populationensity of the emitters to the diameters of the nanowires which isonsistent with our FE-SEM results (Fig. 1(a–c)). Emission stabilityf the CVO nanowires was examined by measuring emission overime. No obvious degradation of current density was observed dur-ng 1 h of continuous operation, with the current density at about0 �A cm−2, as shown in Fig. 6(c). The current fluctuation wasithin 5%, showing that CVO nanowires provide better emission
tability, which is important in practical applications.
. Conclusion
We have successfully grown high quality single crystalline CVOanowires by the thermal decomposition of spin coated film. Theiameter of nanowires increases with the growth temperatureue to the increased agglomeration of nuclei at higher tempera-ure. Field emission measurements reveal that the turn-on fieldf nanowire samples grown at 500, 550, and 600 ◦C are 4.3, 1.2,nd 2.1 V �m−1, respectively. The experimental results suggest thathis facile preparation method could be suitable for the growth ofther complex metal–oxide nanowires which can be used in manypplications.
cknowledgements
This work was supported by the Korean Ministry of Education,cience and Technology under grants NRF-2010-0029700 (Prior-ty Research Centers Program) and R31-2008-10029 (World Classniversity Program).
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.matchemphys.2011.08.055.
References
[1] W. Zhu, G.P. Kochanski, S. Jin, Science 282 (1998) 1471.[2] P. Zheng Wei, D. Zu Rong, W. Zhong Lin, Science 291 (2001) 1947.[3] S. Vandenbrouck, K. Madjour, D. Theron, Y. Dong, Y. Li, C.M. Lieber, C. Gaquiere,
IEEE Electron Device Lett. 30 (2009) 322.[4] D.J. Sirbuly, M. Law, P. Pauzauskie, H. Yan, A.V. Maslov, K. Knutsen, C.Z. Ning,
R.J. Saykally, P. Yang, PNAS 102 (2005) 7800.[5] Y. Zhu, Y. Bandog, L. Yin, D. Golberg, Nano Lett. 6 (2006) 2982.[6] A. Dev, S. Kar, S. Chakrabarti, S. Chaudhuri, Nanotechnology 17 (2006) 1533.[7] S. Jana, S. Das, N.S. Das, K.K. Chattopadhyay, Mater. Res. Bull. 45 (2010) 693.[8] D. Lu, A. Ogino, B. Liang, J. Liu, M. Nagatsu, Jpn. J. Appl. Phys. 48 (2009) 090206.[9] J. Yu, M. Shafiei, W. Wlodarski, Y.X. Li, K. Kalantar-Zadeh, J. Phys. D: Appl. Phys.
43 (2010) 025103.10] J. Yu, A. Kudo, Chem. Lett. 34 (2005) 850.11] H. Liu, D. Tang, Mater. Chem. Phys. 114 (2009) 656.12] C. Mao, X. Wang, X. Wu, J.J. Zhu, H.Y. Chen, Nanotechnology 19 (2008) 035607.13] F.A. Benko, F.P. Koffyberg, Can. J. Phys. 70 (1992) 99.14] N. Suresh Rao, O.G. Palanna, Bull. Mater. Sci. 18 (1995) 229.15] S.N. Cha, B.G. Song, J.E. Jang, J.E. Jung, I.T. Han, J.H. Ha, J.P. Hong, D.J. Kang, J.M.
Kim, Nanotechnology 19 (2008) 235601.16] R. Adelung, O. Cenkaktas, J. Franc, A. Biswas, R. Kunz, M. Elbahri, J. Kanzow, U.
Schürmann, F. Faupel, Nat. Mater. 3 (2004) 375.17] J. Mendialdua, R. Casanova, Y. Barbaux, J. Electron. Spectrosc. Relat. Phenom.
71 (1995) 249.18] Y. Chen, K. Xie, Z. Liu, Appl. Surf. Sci. 126 (1998) 347.19] H. Ma, S. Zhang, W. Ji, Z. Tao, J. Chen, J. Am. Chem. Soc. 130 (2008) 5361.20] C.J. Patridge, C. Jaye, H. Zhang, A.C. Marschilok, D.A. Fischer, E.S. Takeuchi, S.
Banerjee, Inorg. Chem. B. 48 (2009) 3145.21] E.A. Souza, R. Landers, M.H. Tabacniks, L.P. Cardoso, A. Gorenstein, Electrochim.
Acta 51 (2006) 5885.22] S. Gupta, Y.P. Yadava, R.A. Singh, Z. Phys. B: Condens. Matter. 67 (1987) 179.23] S. Wang, Y. He, X. Fang, J. Zou, Y. Wang, H. Huang, P.M.F.J. Costa, M. Song, B.
Huang, C.T. Liu, P.K. Liaw, Y. Bando, D. Colberg, Adv. Mater. 21 (2009) 2387.24] Z.G. Chen, L. Cheng, H.Y. Xu, J.Z. Liu, J. Zou, T. Sekiguchi, G.Q. Lu, H.M. Cheng,
Adv. Mater. 22 (2010) 2376.25] F. Liu, J. Tian, L. Bao, T. Yang, C. Shen, X. Lai, Z. Xiao, W. Xie, S. Deng, J. Chen, J.
She, N. Xu, H. Gao, Adv. Mater. 20 (2008) 2609.26] C. Kim, W. Gu, M. Briceno, I.M. Robertson, H. Choi, K. Kim, Adv. Mater. 20 (2008)
