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Carbon coated Nb2O5 nanowires as enhanced field emitters
Chemical Physics Letters 501 (2011) 431–436
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Carbon coated Nb2O5 nanowires as enhanced field emitters Rajiv Ramanujam Prabhakar a, Binni Varghese b, Wang Yuzhan b, Gao Xingyu b, Chorng Haur Sow a,b,⇑ a b
National University of Singapore Nanoscience and Nanotechnology Initiative, 2 Science Drive 3, Singapore 117542, Singapore Department of Physics, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
a r t i c l e
i n f o
Article history: Received 20 August 2010 In final form 11 November 2010 Available online 4 December 2010
a b s t r a c t Nb2O5 nanowires with controllable carbon coating thickness were prepared by plasma enhanced chemical vapour deposition. These hybrid nanowires were found to be efficient field emitters where the turnon and threshold fields decreased with increasing carbon coating thickness. Notably, high current density was obtained from these hybrid nanowires (a maximum of 59 mA/cm2 at 11 V/lm) which was an order of magnitude higher than pristine Nb2O5 nanowires. The emission sites were distributed uniformly and current stability tests showed a stable emission current density with <10% current fluctuation for a period of 400 min. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Field emission properties of 1-D nanostructures have been explored extensively for the development of cold cathode materials for applications in vacuum microelectronics [1,2]. Among these, carbon nanotubes (CNT) were found to be efficient field emitters and hence extensive research efforts have been devoted to study the field emission properties of CNTs [3–5]. When compared to CNT field emitters, metal oxide field emitters have controllable electrical properties and greater stability in harsh environments [6]. Therefore the field emission properties of metal oxide nanowires such as ZnO [7], MoO2 [8], MoO3 [9], In2O3 [10], CuO [11], RuO2 [12], SnO2 [13], and Nb2O5 [14] have been explored and were found to be efficient field emitters. Among these metal oxide nanowires, Nb2O5 nanowires have shown potential as field emitters with a reasonably high and stable emission current density and low turn-on field [14]. Efforts to further improve the field emission properties of various nanostructures by surface modification have been attempted by various researchers [15–17]. For instance the field emission characteristic of Si tips was found to improve significantly after coating with diamond like carbon (DLC) [15]. Recently, it has also been demonstrated that the field emission properties of ZnO [16] and SiC [17] nanowires can be improved by coating the nanowires with carbon. These findings suggest that such a hybrid nanomaterial could be an attractive material for applications as a field emitter. Hence following this strategy, coating Nb2O5 nanowires with carbon may exhibit potential as efficient electron field emitters.
⇑ Corresponding author at: National University of Singapore Nanoscience and Nanotechnology Initiative, 2 Science Drive 3, Singapore 117542, Singapore. Fax: +65 6777 6126. E-mail address: [email protected] (C.H. Sow). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.11.040
In this work, we report the enhanced field emission properties of carbon coated Nb2O5 nanowires. Uniform and thickness controlled carbon coating on Nb2O5 nanowires was achieved by using plasma enhanced chemical vapour deposition (PECVD) technique. The as obtained hybrid nanowires comprised of a crystalline Nb2O5 core and carbon shell structure. These carbon coated Nb2O5 nanowire arrays exhibited a low field emission turn on (6.6–7.1 V/lm) and threshold fields (8.6–9.9 V/lm) and these fields were found to be dependent on the thickness of carbon coating. Moreover a remarkably high field emission current density of 59 mA/cm2 at an applied field of 11 V/lm was obtained from the carbon coated Nb2O5 nanowire arrays. This is an order of magnitude higher than the current density obtained from the pristine Nb2O5 nanowire arrays. In addition, carbon coated nanowires also exhibited a stable emission current over a period of 400 min at a moderate applied field.
2. Experimental section Nb2O5 nanowires were prepared directly on Nb foils by a technique detailed in our previous report [14]. The as-prepared Nb2O5 nanowire samples were placed in a heating stage inside a PECVD system. The PECVD chamber was then evacuated and after reaching a base pressure of 106 torr, the samples were heated to a temperature of 650 °C. After which hydrogen gas was flowed into the system at a constant flow rate of 60 sccm and the chamber pressure was controlled and maintained at 500 mtorr for 4 min. After this step, a RF power of 80 watts was applied and hydrogen plasma was generated and the system was maintained at this condition for 10 min. To initiate the growth of carbon onto the nanowires, acetylene gas was passed at a flow rate of 15 sccm and the pressure was maintained at 1200 mtorr. During this step plasma power was increased to 100 watts. The flow of acetylene gas was
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maintained for different time intervals (5, 30, 60 min) to achieve carbon coating with different thicknesses. After growth, the flows of acetylene and hydrogen gases were stopped and the chamber was allowed to cool down to 30 °C under vacuum. Morphology and microstructure of the carbon coated nanowires were inspected using a field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL, JEM-2010F, 200 kV). Micro-Raman spectroscopy analysis was performed using a Renishaw System2000 (excitation wavelength used was 514 nm and laser spot size was 2 lm). Crystal structure was studied using X-ray diffraction (XRD, X’PERT MPD, Cu Ka (1.542 Å) radiation). Field emission measurements of the carbon coated nanowires were performed in parallel plate geometry [11]. The vacuum chamber pressure was maintained at 106 torr during the measurements and all the measurements were carried out at room temperature. An indium tin oxide (ITO) coated glass slide was used as an anode. The sample to anode spacing used was 100 lm. The emission area was circular in shape with an area of 23 mm2. In order to assess the uniformity of the emission sites, a phosphor (Zn:ZnO) coated ITO glass was used as the anode and fluorescent field emission images were captured during emission. The applied voltage between the electrodes was increased in steps of 10 V up to a maximum of 1100 V. The corresponding emission currents were measured using a Keithley 237 high voltage source-measurement unit.
3. Results and discussion For convenient discussion, Nb2O5 nanowires sample without carbon coating, samples with average carbon coating thickness of 2, 4 and 9 nm will be referred to as sample 1, sample 2, sample 3, and sample 4 respectively. The pristine Nb2O5 nanowires were grown uniformly across the Nb substrate and the length of the nanowires spanned from 5 to 25 lm (Supporting information S1). The SEM image of sample 3 is shown in Figure 1 (a). It is evident from the SEM images (Figure 1 (a) and Supporting information S1) that the 1D geometrical shape and density of the nanowires were unaffected by the carbon coating process for all carbon coated nanowires. It was also observed that the nanowires tend to bend near the tip after the carbon coating process. Figure 1 (b) shows the Raman spectrum for as-grown Nb2O5 nanowires and the carbon coated Nb2O5 nanowires. Previous Raman studies have been carried out on Nb2O5 and the major Raman bands have been assigned [14,18]. Using this information the various phonon modes of pristine Nb2O5 nanowires have been identified and indicated in the spectrum. The bands observed in 700–1000 cm1correspond to the longitudinal optical modes of the Nb–O stretching (m1) and the
corresponding transverse modes m2 (Eg) are observed in the range of 600–700 cm1. The bands observed in 200–300 cm1 are designated as the m6(T2u) mode. From the Raman spectrum of samples 2–4 it is observed that the intensity of Raman peaks corresponding to Nb2O5 decreased with increasing coated carbon thickness. The Raman spectra of samples 2–4 comprised of two new and prominent peaks centered at around 1350 and 1600 cm1 in addition to the Raman peaks of Nb2O5. These two peaks can be ascribed to the D mode and G mode of graphitic compound and hence providing evidence of carbon coating on the nanowires. The G mode is a bond stretching vibration of a pair of sp2 sites and the D mode is an A1g breathing vibration of a 6-fold aromatic ring which is activated by disorder [19,20]. The intensity of D peak ID and the intensity of the G peak IG were obtained from the Raman spectrum of sample 2, sample 3, sample 4 and the ratio ID/IG was calculated to be 0.79, 0.95, 1.10 respectively. These ratios reflect an increase in the intensity of the D peak relative to the intensity of the G peak with increasing carbon coating thickness suggesting an increase in the density of structural defects. In order to the prepare the sample for TEM inspection, the carbon coated nanowires were transferred to a TEM grid by first dipping the grid in ethanol and then gently rubbing on the surface of Nb foil on which the nanowires were grown. The HRTEM image of sample 4 was shown in Figure 2a and b and HRTEM image of sample 3 was shown in Figure 2c and d. TEM images clearly revealed a Nb2O5 nanowire core with carbon coating around it. It is also evident that the Nb2O5 nanowire core retained its crystalline structure and appeared to be unaffected due to carbon coating process. The thickness of carbon coating in sample 4 along the sidewalls was 9 nm. It is also evident from figures 2a and b that the apex of the nanowires was also coated with carbon and the thickness of carbon coating was roughly the same as the thickness of carbon coating along the sidewalls. The HRTEM image of sample 3 revealed that the thickness of the carbon coating was 4 nm along the sidewalls and the carbon coating on the apex of the nanowire was not uniform in comparison to sample 4. It is evident from these images that as the time of C2H2 exposure increased, the thickness of the carbon coating also increased. Therefore it is possible to control the carbon coating thickness by varying the time of C2H2 flow. The crystal structure of the as grown and the carbon coated nanowires were identified using X-ray diffraction (Supporting information S2). The peaks in the XRD pattern of the as-grown nanowires were indexed with the tetragonal phase of Nb2O5 (JCPDS:72-1484). An intense peak at 48.2° was observed and it can be attributed to the niobium substrate on which the nanowires were grown. All the different carbon coated nanowires were indexed with the tetragonal Nb2O5 phase, thus suggesting the absence of phase change of the nanowires after the carbon coating process. Phase change of the nanowires was suspected due to the
Figure 1. SEM image of (a) Sample 3 viewed at 45° from the substrate normal (b) shows the Raman spectrum of sample 1–4.
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Figure 2. (a) and (b) HRTEM image of sample 4 (c) and (d) HRTEM image of sample 3.
Figure 3. (a) Measured emission current density versus electric field (J vs E) for different carbon coated Nb2O5 nanowires and pristine Nb2O5 nanowires and the corresponding F–N plots are shown in the inset. (b) UPS spectra of pristine Nb2O5 nanowires and carbon coated Nb2O5 nanowires.
presence of H2 in the PECVD chamber. However the XRD data clearly showed the absence of any phase change of the nanowires. Figure 3a shows the emission current density (J) versus applied electric field (E) plot of pristine Nb2O5 nanowires and the different carbon coated nanowires. While obtaining the J–E curves, the voltage was repeatedly ramped up from 0 to 1100 V until reproducible J–E curves were obtained. For each carbon coated and pristine Nb2O5 nanowire emitters, 4 samples were tested in order to check the reproducibility of the J–E curves. The turn-on field (defined as the field required to produce an emission current density of 10 lA/ cm2) and the threshold field (defined as the field required to produce an emission current density of 5 mA/cm2) of sample 1, sample 2, sample 3 and sample 4 are listed in Table 1. It is evident from Table 1 that the turn-on field and the threshold field decreases with increasing carbon coating thickness. The maximum current density is taken as the current density obtained at an applied field of 11 V/lm (This is the maximum field which can be applied with our field emission setup). The maxium current density for samples 1–4 were 0.9, 11.8, 19.0, 59 mA/cm2 respectively. Notably, all 3 carbon coated nanowire samples provides a remarkably high emis-
Table 1 Field emission characteristics of pristine Nb2O5 and carbon coated Nb2O5 nanowires. Sample
Turn-on Field V/lm
Threshold Field V/lm
b
1 2 3 4
7.4 7.1 6.7 6.6
– 9.9 8.8 8.6
358 382 375 373
Workfunction eV
Maximum current density mA/cm2
3.90 4.40 4.25 4.15
0.9 11.8 19.0 58.9
sion current density (an order of magnitude higher) when compared to pristine Nb2O5 nanowires. Sample 4 provides the highest emission current density (59 mA/cm2 at 11 V/lm) and also exhibits the lowest turn-on (6.6 V/lm) and threshold field (8.6 V/lm) among the different samples investigated in this work. The maximum current density obtained from sample 1 was nearly 60 times lower than sample 4 which provided the highest current density among the carbon coated nanowires.
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The Fowler-Nordheim theory provides a quantitative description for the emission current density (J) produced by an applied electric field (E) [21]. The equation is given by
J¼
! ! 3 Ab2 E2 B/2 exp / bE
ð1Þ
Where / is the work function of the emitter and b is the field enhancement factor. A and B are universal constants whose values are 1.54 106 A eV V2 and 6.83 103 eV3/2 Vlm1 respectively. Eq. (1) can be rewritten in the form
ln
J E2
¼ ln
Ab2 /
!
3
B/2 bE
ð2Þ
The Fowler–Nordheim(F–N) plot (inset of Figure 3a)shows a linear variation of ln(J/E2) versus 1/E for all samples implying that the electron emission follows the Fowler–Nordheim theory. From the slope of the linear region in the F–N plot, the field enhancement factor can be calculated by
3 B /2 b¼
Slope
ð3Þ
In order to calculate the field enhancement factor, the work function of the nanowires must be determined. In addition, the work function would also be able to provide a better insight into the efficient field emission properties of carbon coated nanowires. Therefore ultraviolet photoelectron (UPS) spectroscopy study was carried out in order to estimate the work functions of the pristine Nb2O5 nanowires and nanowires coated with different amount of carbon. From the low kinetic energy cutoff of the UPS spectra in Figure 3b, the work functions of sample 1–4 were obtained and tabulated in Table 1. In Figure 3b, the kinetic energy scale was corrected for an applied bias voltage of –5 V during the measurements. The work function of the carbon coated nanowires was found to decrease with increasing carbon coating thickness. The work function reduction of carbon coated nanowires can be attributed to the presence of clusters of sp3 like defects in a sp2 graphite network and these clusters decrease the potential barrier required for electrons to escape to vacuum level [22–24]. In order to obtain increasing carbon coating thickness, the nanowires were subjected to longer time of C2H2 exposure in the presence of H2 in the PECVD chamber. Increasing H2 exposure could increase the sp3 content [25] and hence increasing the sp3 like defects which reduced the work function of carbon coated nanowires with increasing carbon coating thickness. The work function of samples 2–4 were comparable to the work function of multi walled carbon nanotubes (4.30 eV) which have been demonstrated to be very effective field emitters and suitable for practical applications [26,27]. The F–N plots for all samples showed two distinct slopes, one in low-field regime and the other in high-field regime. At low fields, the field emission mechanism followed the F–N equation and at high fields it deviated from the F–N theory but still followed a linear variation on the F–N coordinate [28,29]. This behavior in the high-field regime can be attributed to mechanisms such as space charge effect and emitter current saturation which could contribute to space charge effect [30–32]. The value of the slope in the low-field regime of the F–N plot and also the respective work function values of samples 1–4 were used to estimate the field enhancement factors using Eq. (3). The field enhancement factors have been listed in Table 1 and found to be similar for all four samples. The field enhancement factor b depends on the geometry of the emitters and since the geometrical shape of the emitters is the same for both pristine and carbon coated Nb2O5 nanowire samples, the b values are similar. The threshold fields of carbon coated nanowires were found to decrease with increasing carbon coating
thickness. Similar field emission characteristic has been previously reported for amorphous carbon, where the threshold field was found to decrease with increasing carbon film thickness (for carbon films with less than 50 nm thickness) [33] and the emission mechanism can be explained based on the space-charge induced band bending interlayer model [34,35]. According to this model, the heterojunction and band bending within film are the key factors. Band bending occurs across the thickness of the carbon film and this results in the internal field being maximum near the heterojunction [35]. This internal electric field is much greater than the applied field and hence enabling the electrons to tunnel through the barrier [34]. . Figure 4a shows the fluorescent field emission image of sample 3 and Figure 4b shows the fluorescent field emission image for sample 1 both at the same applied electric field (10 V/lm). It is evident from these images that the emission sites in sample 3 are more uniformly distributed than sample 1. Emission current stability is a crucial factor for field emitters when they are considered for applications in microwave amplifiers, field emission display and field emission gun [36]. Hence current stability analysis is important for the characterization of field emitters. For the emission current stability test, a fixed voltage was applied between the cathode and anode, and the corresponding current was recorded as function of time. For all carbon coated nanowires samples, the current stability test was run for a period of 400 min. Figure 4c shows the current density versus time relation at an applied field of 7.2 V/lm for sample 3. The initial current density was at 0.13 mA/cm2 and after 20 min, the current density stabilizes at 0.10 mA/cm2. The current density was found to be stable with less than 10% current fluctuation until the end of the testing period. Figure 4(d) shows the current density versus time relation for sample 3 at a higher applied field of 8 V/lm. The initial current density was 1.47 mA/ cm2 and it slowly decreased to 0.51 mA/cm2 after a period of 4 h and stabilized at this value with less than 10% current fluctuation until the end of the testing period. The emission current stability test for sample 2 was shown in Supporting information S3(A). The initial current density was 0.15 mA/cm2 and there was a steady degradation of emission current during the testing period of 400 min. At the end of testing period the emission current density was found to be 0.05 mA/cm2. The emission current stability test for sample 4 was shown in Supporting information S3(B). The initial current density was 0.8 mA/cm2 and after 3 h it reduced to 0.35 mA/cm2 and stabilized at this value until the end of the stability testing period with minimal current fluctuation (<10%). It is evident from the emission current density vs time plots of the carbon coated nanowires (Figure 4 (d) and S3) that there is a steady degradation of emission current density initially. Similar emission current degradation has been observed for CNTs and such degradation can be most likely due to joule heating at high emission current densities and also ion bombardment from background gas in the vacuum chamber [37,38]. From the current stability tests for the different carbon coated nanowire samples, it was found that sample 3 and sample 4 were stable field emitters in comparison to sample 2. Since increasing H2 exposure in PECVD for carbon coating could increase the sp3 content of carbon, samples 3 and 4 are likely to have larger sp3 content than sample 2 [25]. It has also been experimentally verified that field emission current degradation of DLC coated Mo was due to decrease in the proportion of sp3 bonds [39]. This suggests that the discrepancy of electron emission stability of sample 2 in comparison to samples 3 and 4 can be attributed to its lower sp3 content. In order to estimate how the field emission characteristics of carbon coated Nb2O5 nanowires compare to other 1-D nanomaterials, a brief summary of its F–E characteristics are provided in Table 2. Although the field emission performance of the carbon coated Nb2O5 nanowires may not compete with CNT based field
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Figure 4. (a) Fluorescent field emission images of sample 3 at an applied field of 10 V/lm. (b) Fluorescent field emission images of sample 1 at an applied field of 10 V/lm. The FE current density as a function of time for sample 3 at an applied field of (c) 7.2 V/lm and (d) 8 V/lm.
Table 2 Field emission properties of important metal oxides, carbon coated nanowires, DLC coated Si and carbon nanotubesa. Nanowires
Maximum current density mA/cm2
Threshold Fieldb
Sample 4 (Current work) C-coated ZnO [16] C-coated SiC [17] CNT [40] ZnO [41] SnO2 [13] MoO2 [9] In2O3 [10] CuO [11] DLC coated Si [15]
59 (11 V/lm) 1.2 (3.40 V/lm) 0.11 (5.25 V/lm) 100–1000 (10–15 V/lm) 11 (9 V/lm) 3.3 (5 V/lm) 16 (6 V/lm) 2 (12.30 V/lm) 0.45 (7 V/lm) Emission current = 4.5lA (40 V/lm)
8.6 (5 mA/cm2) Not defined Not defined 6.5 (10 mA/cm2) 5.3 (1 mA/cm2) Not defined 5.6 (10 mA/cm2) 11.3 (1 mA/cm2) Not defined Not defined
V/lm
with controllable thickness of carbon coating can be synthesized by simply varying time of exposure of C2H2. Field emission characterization revealed that carbon coated Nb2O5 nanowires have a low turn on and threshold fields and the capability to deliver high current densities (>50 mA/cm2). These nanowires were able to provide a stable emission current density at moderate applied fields and also a uniform distribution of emission sites across the emission area. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2010.11.040. References
a
The field at which maximum current density obtained is indicated in brackets. b The current density at which the threshold field is defined is indicated in brackets.
emitters, it is superior to other metal oxides and also carbon coated ZnO and SiC nanowires in terms of the maximum current density obtained as seen from table 2. However, metal oxide emitters present the advantages of controllable electrical properties and greater stability in harsh environments when compared to CNT based field emitters [6]. Sample 4 having the lowest turn-on and threshold fields among all the samples investigated in this study, would be able to provide an emission current density of 50 mA/cm2 at 55 V, when the cathode–anode separation distance is 5 lm. This cathode–anode separation distance can be achieved with present fabrication technologies [42]. This suggests that the carbon coated Nb2O5 nanowires could be a viable candidate for applications as a cathode in vacuum microelectronic devices. 4. Conclusion We have successfully coated Nb2O5 nanowires with carbon using plasma enhanced chemical vapour deposition. Nanowires
[1] X.S. Fang, Y. Bando, U.K. Gautam, C. Ye, D.J. Goldberg, J. Mater. Chem. 18 (2008) 509. [2] N.S. Xu, S.E. Huq, Mater. Sci. Eng., R 48 (2005) 47. [3] W.I. Milne et al., J. Mater. Chem. 14 (2004) 933. [4] X. Bai et al., J. Vac. Sci. Technol. B 25 (2007) 561. [5] Y.H. Yang, C.Y. Wang, U.S. Chen, W.J. Hsieh, Y.S. Chang, H.C. Shih, J. Phys. Chem. C 111 (2007) 1601. [6] G. Shen, P.C. Chen, K. Ryu, C.J. Zhou, J. Mater. Chem. 19 (2009) 828. [7] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, Appl. Phys. Lett. 81 (2002) 3648. [8] F. Wang, B.Q. Lu, Physica B 404 (2009) 1901. [9] J. Zhou, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Z.L. Wang, Adv. Mater. 15 (2003) 1835. [10] S.Q. Li, X.Y. Liang, T.H. Wang, Appl. Phys. Lett. 87 (2005) 143104. [11] Y.W. Zhu et al., Nanotechnology 16 (2005) 88. [12] C.L. Cheng, Y.F. Chen, R.S. Chen, Y.S. Huang, Appl. Phys. Lett. 86 (2005) 103104. [13] L.J. Li et al., Mater. Lett. 61 (2007) 4152. [14] B. Varghese, C.H. Sow, C.T. Lim, J. Phys. Chem. C 112 (2008) 10008. [15] S.H. Ahn, K.R. Lee, K.Y. Eun, D. Jeon, Surf. Coat. Technol. 120 (1999) 734. [16] L. Liao, J.C. Li, D.F. Wang, C. Liu, C.S. Liu, Q. Fu, L.C. Fan, Nanotechnology 16 (2005) 985. [17] Y. Ryu, B. Park, Y. Song, K.J. Yong, J. Cryst. Growth 271 (2004) 99. [18] A.A. McConnell, J.S. Anderson, C.N.R. Rao, Spectrochim. Acta 32A (1976) 1067. [19] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [20] F. Tunistra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [21] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. London, A 119 (1928) 173. [22] A.N. Obraztsov, A.P. Volkov, A.I. Boronin, S.V. Kosheev, Diamond Relat. Mater. 11 (2002) 813. [23] A.N. Obraztsov, A.P. Volkov, I.Yu. Pavlovsky, Diamond Relat. Mater. 9 (2000) 1190.
436
R.R. Prabhakar et al. / Chemical Physics Letters 501 (2011) 431–436
[24] A.N. Obraztsov, I. Pavlovsky, A.P. Volkov, E.D. Obraztsova, A.L. Chuvilin, V.L. Kuznetsov, J. Vac. Sci. Technol. B 18 (2000) 1059. [25] J. Robertson, Mater. Sci. Eng., R 37 (2002) 129. [26] H. Ago, T. Kugler, F. Cacialli, W.R. Salaneck, M.S.P. Shaffer, A.H. Windle, R.H. Friend, J. Phys. Chem. B 103 (1999) 8116. [27] A.V. Eletski, Phys.Usp. 45 (2002) 369. [28] J.-W. Wu, H.C. Shih, W-T. Wu, Chem. Phys. Lett. 413 (2005) 490. [29] L.H. Chen et al., Appl. Phys. Lett. 82 (2003) 4334. [30] S. Dmitrijevic, J.C. Withers, V.P. Manmmana, O.R. Monteiro, J.W. Ager, I.G. Brown, Appl. Phys. Lett. 75 (1999) 2680. [31] P.G. Collins, A. Zettl, Phys. Rev. B 55 (1997) 9391. [32] J.M. Bonard, J.P. Salvetat, T. Stockli, W.A. De Heer, L. Forro, A. Chatelain, Appl. Phys. Lett. 73 (1998) 918. [33] R.D. Forrest, A.P. Burden, S.R.P. Silva, L.K. Cheah, X. Shi, Appl. Phys. Lett. 73 (1998) 3784.
[34] G.A.J. Amaratunga, S.R.P. Silva, Appl. Phys. Lett. 68 (1996) 2529. [35] S.R.P. Silva, R.D. Forrest, D.A. Munindradasa, G.A.J. Amaratunga, Diamond Relat. Mater. 7 (1998) 645. [36] W.I. Milene, A. Illie, J.B. Cui, A. Ferrari, J. Robertson, Diamond Relat. Mater. 10 (2001) 260. [37] V.K. Kayastha, B. Ulmen, Y.K. Yap, Nanotechnology 18 (2007) 035206. [38] W. Zhu, C. Bower, O. Zhou, G. Kochanski, S. Jin, Appl. Phys. Lett. 75 (1999) 873. [39] F.Y. Chuang, C.Y. Sun, H.F. Cheng, C.M. Huang, I.N. Lin, Appl. Phys. Lett. 68 (1996) 1666. [40] X. Xu, G.R. Brandes, Appl. Phys. Lett. 74 (1999) 2549. [41] J. Xiao, G.M. Zhang, X. Bai, Y. Wu, W. Zhang, X.Y. Zhao, D.Z. Guo, Physica E 41 (2008) 309. [42] Z. Pan et al., Adv. Mater. 12 (2000) 1186.
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Carbon coated Nb2O5 nanowires as enhanced field emitters Rajiv Ramanujam Prabhakar a, Binni Varghese b, Wang Yuzhan b, Gao Xingyu b, Chorng Haur Sow a,b,⇑ a b
National University of Singapore Nanoscience and Nanotechnology Initiative, 2 Science Drive 3, Singapore 117542, Singapore Department of Physics, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
a r t i c l e
i n f o
Article history: Received 20 August 2010 In final form 11 November 2010 Available online 4 December 2010
a b s t r a c t Nb2O5 nanowires with controllable carbon coating thickness were prepared by plasma enhanced chemical vapour deposition. These hybrid nanowires were found to be efficient field emitters where the turnon and threshold fields decreased with increasing carbon coating thickness. Notably, high current density was obtained from these hybrid nanowires (a maximum of 59 mA/cm2 at 11 V/lm) which was an order of magnitude higher than pristine Nb2O5 nanowires. The emission sites were distributed uniformly and current stability tests showed a stable emission current density with <10% current fluctuation for a period of 400 min. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Field emission properties of 1-D nanostructures have been explored extensively for the development of cold cathode materials for applications in vacuum microelectronics [1,2]. Among these, carbon nanotubes (CNT) were found to be efficient field emitters and hence extensive research efforts have been devoted to study the field emission properties of CNTs [3–5]. When compared to CNT field emitters, metal oxide field emitters have controllable electrical properties and greater stability in harsh environments [6]. Therefore the field emission properties of metal oxide nanowires such as ZnO [7], MoO2 [8], MoO3 [9], In2O3 [10], CuO [11], RuO2 [12], SnO2 [13], and Nb2O5 [14] have been explored and were found to be efficient field emitters. Among these metal oxide nanowires, Nb2O5 nanowires have shown potential as field emitters with a reasonably high and stable emission current density and low turn-on field [14]. Efforts to further improve the field emission properties of various nanostructures by surface modification have been attempted by various researchers [15–17]. For instance the field emission characteristic of Si tips was found to improve significantly after coating with diamond like carbon (DLC) [15]. Recently, it has also been demonstrated that the field emission properties of ZnO [16] and SiC [17] nanowires can be improved by coating the nanowires with carbon. These findings suggest that such a hybrid nanomaterial could be an attractive material for applications as a field emitter. Hence following this strategy, coating Nb2O5 nanowires with carbon may exhibit potential as efficient electron field emitters.
⇑ Corresponding author at: National University of Singapore Nanoscience and Nanotechnology Initiative, 2 Science Drive 3, Singapore 117542, Singapore. Fax: +65 6777 6126. E-mail address: [email protected] (C.H. Sow). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.11.040
In this work, we report the enhanced field emission properties of carbon coated Nb2O5 nanowires. Uniform and thickness controlled carbon coating on Nb2O5 nanowires was achieved by using plasma enhanced chemical vapour deposition (PECVD) technique. The as obtained hybrid nanowires comprised of a crystalline Nb2O5 core and carbon shell structure. These carbon coated Nb2O5 nanowire arrays exhibited a low field emission turn on (6.6–7.1 V/lm) and threshold fields (8.6–9.9 V/lm) and these fields were found to be dependent on the thickness of carbon coating. Moreover a remarkably high field emission current density of 59 mA/cm2 at an applied field of 11 V/lm was obtained from the carbon coated Nb2O5 nanowire arrays. This is an order of magnitude higher than the current density obtained from the pristine Nb2O5 nanowire arrays. In addition, carbon coated nanowires also exhibited a stable emission current over a period of 400 min at a moderate applied field.
2. Experimental section Nb2O5 nanowires were prepared directly on Nb foils by a technique detailed in our previous report [14]. The as-prepared Nb2O5 nanowire samples were placed in a heating stage inside a PECVD system. The PECVD chamber was then evacuated and after reaching a base pressure of 106 torr, the samples were heated to a temperature of 650 °C. After which hydrogen gas was flowed into the system at a constant flow rate of 60 sccm and the chamber pressure was controlled and maintained at 500 mtorr for 4 min. After this step, a RF power of 80 watts was applied and hydrogen plasma was generated and the system was maintained at this condition for 10 min. To initiate the growth of carbon onto the nanowires, acetylene gas was passed at a flow rate of 15 sccm and the pressure was maintained at 1200 mtorr. During this step plasma power was increased to 100 watts. The flow of acetylene gas was
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maintained for different time intervals (5, 30, 60 min) to achieve carbon coating with different thicknesses. After growth, the flows of acetylene and hydrogen gases were stopped and the chamber was allowed to cool down to 30 °C under vacuum. Morphology and microstructure of the carbon coated nanowires were inspected using a field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL, JEM-2010F, 200 kV). Micro-Raman spectroscopy analysis was performed using a Renishaw System2000 (excitation wavelength used was 514 nm and laser spot size was 2 lm). Crystal structure was studied using X-ray diffraction (XRD, X’PERT MPD, Cu Ka (1.542 Å) radiation). Field emission measurements of the carbon coated nanowires were performed in parallel plate geometry [11]. The vacuum chamber pressure was maintained at 106 torr during the measurements and all the measurements were carried out at room temperature. An indium tin oxide (ITO) coated glass slide was used as an anode. The sample to anode spacing used was 100 lm. The emission area was circular in shape with an area of 23 mm2. In order to assess the uniformity of the emission sites, a phosphor (Zn:ZnO) coated ITO glass was used as the anode and fluorescent field emission images were captured during emission. The applied voltage between the electrodes was increased in steps of 10 V up to a maximum of 1100 V. The corresponding emission currents were measured using a Keithley 237 high voltage source-measurement unit.
3. Results and discussion For convenient discussion, Nb2O5 nanowires sample without carbon coating, samples with average carbon coating thickness of 2, 4 and 9 nm will be referred to as sample 1, sample 2, sample 3, and sample 4 respectively. The pristine Nb2O5 nanowires were grown uniformly across the Nb substrate and the length of the nanowires spanned from 5 to 25 lm (Supporting information S1). The SEM image of sample 3 is shown in Figure 1 (a). It is evident from the SEM images (Figure 1 (a) and Supporting information S1) that the 1D geometrical shape and density of the nanowires were unaffected by the carbon coating process for all carbon coated nanowires. It was also observed that the nanowires tend to bend near the tip after the carbon coating process. Figure 1 (b) shows the Raman spectrum for as-grown Nb2O5 nanowires and the carbon coated Nb2O5 nanowires. Previous Raman studies have been carried out on Nb2O5 and the major Raman bands have been assigned [14,18]. Using this information the various phonon modes of pristine Nb2O5 nanowires have been identified and indicated in the spectrum. The bands observed in 700–1000 cm1correspond to the longitudinal optical modes of the Nb–O stretching (m1) and the
corresponding transverse modes m2 (Eg) are observed in the range of 600–700 cm1. The bands observed in 200–300 cm1 are designated as the m6(T2u) mode. From the Raman spectrum of samples 2–4 it is observed that the intensity of Raman peaks corresponding to Nb2O5 decreased with increasing coated carbon thickness. The Raman spectra of samples 2–4 comprised of two new and prominent peaks centered at around 1350 and 1600 cm1 in addition to the Raman peaks of Nb2O5. These two peaks can be ascribed to the D mode and G mode of graphitic compound and hence providing evidence of carbon coating on the nanowires. The G mode is a bond stretching vibration of a pair of sp2 sites and the D mode is an A1g breathing vibration of a 6-fold aromatic ring which is activated by disorder [19,20]. The intensity of D peak ID and the intensity of the G peak IG were obtained from the Raman spectrum of sample 2, sample 3, sample 4 and the ratio ID/IG was calculated to be 0.79, 0.95, 1.10 respectively. These ratios reflect an increase in the intensity of the D peak relative to the intensity of the G peak with increasing carbon coating thickness suggesting an increase in the density of structural defects. In order to the prepare the sample for TEM inspection, the carbon coated nanowires were transferred to a TEM grid by first dipping the grid in ethanol and then gently rubbing on the surface of Nb foil on which the nanowires were grown. The HRTEM image of sample 4 was shown in Figure 2a and b and HRTEM image of sample 3 was shown in Figure 2c and d. TEM images clearly revealed a Nb2O5 nanowire core with carbon coating around it. It is also evident that the Nb2O5 nanowire core retained its crystalline structure and appeared to be unaffected due to carbon coating process. The thickness of carbon coating in sample 4 along the sidewalls was 9 nm. It is also evident from figures 2a and b that the apex of the nanowires was also coated with carbon and the thickness of carbon coating was roughly the same as the thickness of carbon coating along the sidewalls. The HRTEM image of sample 3 revealed that the thickness of the carbon coating was 4 nm along the sidewalls and the carbon coating on the apex of the nanowire was not uniform in comparison to sample 4. It is evident from these images that as the time of C2H2 exposure increased, the thickness of the carbon coating also increased. Therefore it is possible to control the carbon coating thickness by varying the time of C2H2 flow. The crystal structure of the as grown and the carbon coated nanowires were identified using X-ray diffraction (Supporting information S2). The peaks in the XRD pattern of the as-grown nanowires were indexed with the tetragonal phase of Nb2O5 (JCPDS:72-1484). An intense peak at 48.2° was observed and it can be attributed to the niobium substrate on which the nanowires were grown. All the different carbon coated nanowires were indexed with the tetragonal Nb2O5 phase, thus suggesting the absence of phase change of the nanowires after the carbon coating process. Phase change of the nanowires was suspected due to the
Figure 1. SEM image of (a) Sample 3 viewed at 45° from the substrate normal (b) shows the Raman spectrum of sample 1–4.
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Figure 2. (a) and (b) HRTEM image of sample 4 (c) and (d) HRTEM image of sample 3.
Figure 3. (a) Measured emission current density versus electric field (J vs E) for different carbon coated Nb2O5 nanowires and pristine Nb2O5 nanowires and the corresponding F–N plots are shown in the inset. (b) UPS spectra of pristine Nb2O5 nanowires and carbon coated Nb2O5 nanowires.
presence of H2 in the PECVD chamber. However the XRD data clearly showed the absence of any phase change of the nanowires. Figure 3a shows the emission current density (J) versus applied electric field (E) plot of pristine Nb2O5 nanowires and the different carbon coated nanowires. While obtaining the J–E curves, the voltage was repeatedly ramped up from 0 to 1100 V until reproducible J–E curves were obtained. For each carbon coated and pristine Nb2O5 nanowire emitters, 4 samples were tested in order to check the reproducibility of the J–E curves. The turn-on field (defined as the field required to produce an emission current density of 10 lA/ cm2) and the threshold field (defined as the field required to produce an emission current density of 5 mA/cm2) of sample 1, sample 2, sample 3 and sample 4 are listed in Table 1. It is evident from Table 1 that the turn-on field and the threshold field decreases with increasing carbon coating thickness. The maximum current density is taken as the current density obtained at an applied field of 11 V/lm (This is the maximum field which can be applied with our field emission setup). The maxium current density for samples 1–4 were 0.9, 11.8, 19.0, 59 mA/cm2 respectively. Notably, all 3 carbon coated nanowire samples provides a remarkably high emis-
Table 1 Field emission characteristics of pristine Nb2O5 and carbon coated Nb2O5 nanowires. Sample
Turn-on Field V/lm
Threshold Field V/lm
b
1 2 3 4
7.4 7.1 6.7 6.6
– 9.9 8.8 8.6
358 382 375 373
Workfunction eV
Maximum current density mA/cm2
3.90 4.40 4.25 4.15
0.9 11.8 19.0 58.9
sion current density (an order of magnitude higher) when compared to pristine Nb2O5 nanowires. Sample 4 provides the highest emission current density (59 mA/cm2 at 11 V/lm) and also exhibits the lowest turn-on (6.6 V/lm) and threshold field (8.6 V/lm) among the different samples investigated in this work. The maximum current density obtained from sample 1 was nearly 60 times lower than sample 4 which provided the highest current density among the carbon coated nanowires.
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The Fowler-Nordheim theory provides a quantitative description for the emission current density (J) produced by an applied electric field (E) [21]. The equation is given by
J¼
! ! 3 Ab2 E2 B/2 exp / bE
ð1Þ
Where / is the work function of the emitter and b is the field enhancement factor. A and B are universal constants whose values are 1.54 106 A eV V2 and 6.83 103 eV3/2 Vlm1 respectively. Eq. (1) can be rewritten in the form
ln
J E2
¼ ln
Ab2 /
!
3
B/2 bE
ð2Þ
The Fowler–Nordheim(F–N) plot (inset of Figure 3a)shows a linear variation of ln(J/E2) versus 1/E for all samples implying that the electron emission follows the Fowler–Nordheim theory. From the slope of the linear region in the F–N plot, the field enhancement factor can be calculated by
3 B /2 b¼
Slope
ð3Þ
In order to calculate the field enhancement factor, the work function of the nanowires must be determined. In addition, the work function would also be able to provide a better insight into the efficient field emission properties of carbon coated nanowires. Therefore ultraviolet photoelectron (UPS) spectroscopy study was carried out in order to estimate the work functions of the pristine Nb2O5 nanowires and nanowires coated with different amount of carbon. From the low kinetic energy cutoff of the UPS spectra in Figure 3b, the work functions of sample 1–4 were obtained and tabulated in Table 1. In Figure 3b, the kinetic energy scale was corrected for an applied bias voltage of –5 V during the measurements. The work function of the carbon coated nanowires was found to decrease with increasing carbon coating thickness. The work function reduction of carbon coated nanowires can be attributed to the presence of clusters of sp3 like defects in a sp2 graphite network and these clusters decrease the potential barrier required for electrons to escape to vacuum level [22–24]. In order to obtain increasing carbon coating thickness, the nanowires were subjected to longer time of C2H2 exposure in the presence of H2 in the PECVD chamber. Increasing H2 exposure could increase the sp3 content [25] and hence increasing the sp3 like defects which reduced the work function of carbon coated nanowires with increasing carbon coating thickness. The work function of samples 2–4 were comparable to the work function of multi walled carbon nanotubes (4.30 eV) which have been demonstrated to be very effective field emitters and suitable for practical applications [26,27]. The F–N plots for all samples showed two distinct slopes, one in low-field regime and the other in high-field regime. At low fields, the field emission mechanism followed the F–N equation and at high fields it deviated from the F–N theory but still followed a linear variation on the F–N coordinate [28,29]. This behavior in the high-field regime can be attributed to mechanisms such as space charge effect and emitter current saturation which could contribute to space charge effect [30–32]. The value of the slope in the low-field regime of the F–N plot and also the respective work function values of samples 1–4 were used to estimate the field enhancement factors using Eq. (3). The field enhancement factors have been listed in Table 1 and found to be similar for all four samples. The field enhancement factor b depends on the geometry of the emitters and since the geometrical shape of the emitters is the same for both pristine and carbon coated Nb2O5 nanowire samples, the b values are similar. The threshold fields of carbon coated nanowires were found to decrease with increasing carbon coating
thickness. Similar field emission characteristic has been previously reported for amorphous carbon, where the threshold field was found to decrease with increasing carbon film thickness (for carbon films with less than 50 nm thickness) [33] and the emission mechanism can be explained based on the space-charge induced band bending interlayer model [34,35]. According to this model, the heterojunction and band bending within film are the key factors. Band bending occurs across the thickness of the carbon film and this results in the internal field being maximum near the heterojunction [35]. This internal electric field is much greater than the applied field and hence enabling the electrons to tunnel through the barrier [34]. . Figure 4a shows the fluorescent field emission image of sample 3 and Figure 4b shows the fluorescent field emission image for sample 1 both at the same applied electric field (10 V/lm). It is evident from these images that the emission sites in sample 3 are more uniformly distributed than sample 1. Emission current stability is a crucial factor for field emitters when they are considered for applications in microwave amplifiers, field emission display and field emission gun [36]. Hence current stability analysis is important for the characterization of field emitters. For the emission current stability test, a fixed voltage was applied between the cathode and anode, and the corresponding current was recorded as function of time. For all carbon coated nanowires samples, the current stability test was run for a period of 400 min. Figure 4c shows the current density versus time relation at an applied field of 7.2 V/lm for sample 3. The initial current density was at 0.13 mA/cm2 and after 20 min, the current density stabilizes at 0.10 mA/cm2. The current density was found to be stable with less than 10% current fluctuation until the end of the testing period. Figure 4(d) shows the current density versus time relation for sample 3 at a higher applied field of 8 V/lm. The initial current density was 1.47 mA/ cm2 and it slowly decreased to 0.51 mA/cm2 after a period of 4 h and stabilized at this value with less than 10% current fluctuation until the end of the testing period. The emission current stability test for sample 2 was shown in Supporting information S3(A). The initial current density was 0.15 mA/cm2 and there was a steady degradation of emission current during the testing period of 400 min. At the end of testing period the emission current density was found to be 0.05 mA/cm2. The emission current stability test for sample 4 was shown in Supporting information S3(B). The initial current density was 0.8 mA/cm2 and after 3 h it reduced to 0.35 mA/cm2 and stabilized at this value until the end of the stability testing period with minimal current fluctuation (<10%). It is evident from the emission current density vs time plots of the carbon coated nanowires (Figure 4 (d) and S3) that there is a steady degradation of emission current density initially. Similar emission current degradation has been observed for CNTs and such degradation can be most likely due to joule heating at high emission current densities and also ion bombardment from background gas in the vacuum chamber [37,38]. From the current stability tests for the different carbon coated nanowire samples, it was found that sample 3 and sample 4 were stable field emitters in comparison to sample 2. Since increasing H2 exposure in PECVD for carbon coating could increase the sp3 content of carbon, samples 3 and 4 are likely to have larger sp3 content than sample 2 [25]. It has also been experimentally verified that field emission current degradation of DLC coated Mo was due to decrease in the proportion of sp3 bonds [39]. This suggests that the discrepancy of electron emission stability of sample 2 in comparison to samples 3 and 4 can be attributed to its lower sp3 content. In order to estimate how the field emission characteristics of carbon coated Nb2O5 nanowires compare to other 1-D nanomaterials, a brief summary of its F–E characteristics are provided in Table 2. Although the field emission performance of the carbon coated Nb2O5 nanowires may not compete with CNT based field
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Figure 4. (a) Fluorescent field emission images of sample 3 at an applied field of 10 V/lm. (b) Fluorescent field emission images of sample 1 at an applied field of 10 V/lm. The FE current density as a function of time for sample 3 at an applied field of (c) 7.2 V/lm and (d) 8 V/lm.
Table 2 Field emission properties of important metal oxides, carbon coated nanowires, DLC coated Si and carbon nanotubesa. Nanowires
Maximum current density mA/cm2
Threshold Fieldb
Sample 4 (Current work) C-coated ZnO [16] C-coated SiC [17] CNT [40] ZnO [41] SnO2 [13] MoO2 [9] In2O3 [10] CuO [11] DLC coated Si [15]
59 (11 V/lm) 1.2 (3.40 V/lm) 0.11 (5.25 V/lm) 100–1000 (10–15 V/lm) 11 (9 V/lm) 3.3 (5 V/lm) 16 (6 V/lm) 2 (12.30 V/lm) 0.45 (7 V/lm) Emission current = 4.5lA (40 V/lm)
8.6 (5 mA/cm2) Not defined Not defined 6.5 (10 mA/cm2) 5.3 (1 mA/cm2) Not defined 5.6 (10 mA/cm2) 11.3 (1 mA/cm2) Not defined Not defined
V/lm
with controllable thickness of carbon coating can be synthesized by simply varying time of exposure of C2H2. Field emission characterization revealed that carbon coated Nb2O5 nanowires have a low turn on and threshold fields and the capability to deliver high current densities (>50 mA/cm2). These nanowires were able to provide a stable emission current density at moderate applied fields and also a uniform distribution of emission sites across the emission area. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2010.11.040. References
a
The field at which maximum current density obtained is indicated in brackets. b The current density at which the threshold field is defined is indicated in brackets.
emitters, it is superior to other metal oxides and also carbon coated ZnO and SiC nanowires in terms of the maximum current density obtained as seen from table 2. However, metal oxide emitters present the advantages of controllable electrical properties and greater stability in harsh environments when compared to CNT based field emitters [6]. Sample 4 having the lowest turn-on and threshold fields among all the samples investigated in this study, would be able to provide an emission current density of 50 mA/cm2 at 55 V, when the cathode–anode separation distance is 5 lm. This cathode–anode separation distance can be achieved with present fabrication technologies [42]. This suggests that the carbon coated Nb2O5 nanowires could be a viable candidate for applications as a cathode in vacuum microelectronic devices. 4. Conclusion We have successfully coated Nb2O5 nanowires with carbon using plasma enhanced chemical vapour deposition. Nanowires
[1] X.S. Fang, Y. Bando, U.K. Gautam, C. Ye, D.J. Goldberg, J. Mater. Chem. 18 (2008) 509. [2] N.S. Xu, S.E. Huq, Mater. Sci. Eng., R 48 (2005) 47. [3] W.I. Milne et al., J. Mater. Chem. 14 (2004) 933. [4] X. Bai et al., J. Vac. Sci. Technol. B 25 (2007) 561. [5] Y.H. Yang, C.Y. Wang, U.S. Chen, W.J. Hsieh, Y.S. Chang, H.C. Shih, J. Phys. Chem. C 111 (2007) 1601. [6] G. Shen, P.C. Chen, K. Ryu, C.J. Zhou, J. Mater. Chem. 19 (2009) 828. [7] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, Appl. Phys. Lett. 81 (2002) 3648. [8] F. Wang, B.Q. Lu, Physica B 404 (2009) 1901. [9] J. Zhou, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Z.L. Wang, Adv. Mater. 15 (2003) 1835. [10] S.Q. Li, X.Y. Liang, T.H. Wang, Appl. Phys. Lett. 87 (2005) 143104. [11] Y.W. Zhu et al., Nanotechnology 16 (2005) 88. [12] C.L. Cheng, Y.F. Chen, R.S. Chen, Y.S. Huang, Appl. Phys. Lett. 86 (2005) 103104. [13] L.J. Li et al., Mater. Lett. 61 (2007) 4152. [14] B. Varghese, C.H. Sow, C.T. Lim, J. Phys. Chem. C 112 (2008) 10008. [15] S.H. Ahn, K.R. Lee, K.Y. Eun, D. Jeon, Surf. Coat. Technol. 120 (1999) 734. [16] L. Liao, J.C. Li, D.F. Wang, C. Liu, C.S. Liu, Q. Fu, L.C. Fan, Nanotechnology 16 (2005) 985. [17] Y. Ryu, B. Park, Y. Song, K.J. Yong, J. Cryst. Growth 271 (2004) 99. [18] A.A. McConnell, J.S. Anderson, C.N.R. Rao, Spectrochim. Acta 32A (1976) 1067. [19] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [20] F. Tunistra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [21] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. London, A 119 (1928) 173. [22] A.N. Obraztsov, A.P. Volkov, A.I. Boronin, S.V. Kosheev, Diamond Relat. Mater. 11 (2002) 813. [23] A.N. Obraztsov, A.P. Volkov, I.Yu. Pavlovsky, Diamond Relat. Mater. 9 (2000) 1190.
436
R.R. Prabhakar et al. / Chemical Physics Letters 501 (2011) 431–436
[24] A.N. Obraztsov, I. Pavlovsky, A.P. Volkov, E.D. Obraztsova, A.L. Chuvilin, V.L. Kuznetsov, J. Vac. Sci. Technol. B 18 (2000) 1059. [25] J. Robertson, Mater. Sci. Eng., R 37 (2002) 129. [26] H. Ago, T. Kugler, F. Cacialli, W.R. Salaneck, M.S.P. Shaffer, A.H. Windle, R.H. Friend, J. Phys. Chem. B 103 (1999) 8116. [27] A.V. Eletski, Phys.Usp. 45 (2002) 369. [28] J.-W. Wu, H.C. Shih, W-T. Wu, Chem. Phys. Lett. 413 (2005) 490. [29] L.H. Chen et al., Appl. Phys. Lett. 82 (2003) 4334. [30] S. Dmitrijevic, J.C. Withers, V.P. Manmmana, O.R. Monteiro, J.W. Ager, I.G. Brown, Appl. Phys. Lett. 75 (1999) 2680. [31] P.G. Collins, A. Zettl, Phys. Rev. B 55 (1997) 9391. [32] J.M. Bonard, J.P. Salvetat, T. Stockli, W.A. De Heer, L. Forro, A. Chatelain, Appl. Phys. Lett. 73 (1998) 918. [33] R.D. Forrest, A.P. Burden, S.R.P. Silva, L.K. Cheah, X. Shi, Appl. Phys. Lett. 73 (1998) 3784.
[34] G.A.J. Amaratunga, S.R.P. Silva, Appl. Phys. Lett. 68 (1996) 2529. [35] S.R.P. Silva, R.D. Forrest, D.A. Munindradasa, G.A.J. Amaratunga, Diamond Relat. Mater. 7 (1998) 645. [36] W.I. Milene, A. Illie, J.B. Cui, A. Ferrari, J. Robertson, Diamond Relat. Mater. 10 (2001) 260. [37] V.K. Kayastha, B. Ulmen, Y.K. Yap, Nanotechnology 18 (2007) 035206. [38] W. Zhu, C. Bower, O. Zhou, G. Kochanski, S. Jin, Appl. Phys. Lett. 75 (1999) 873. [39] F.Y. Chuang, C.Y. Sun, H.F. Cheng, C.M. Huang, I.N. Lin, Appl. Phys. Lett. 68 (1996) 1666. [40] X. Xu, G.R. Brandes, Appl. Phys. Lett. 74 (1999) 2549. [41] J. Xiao, G.M. Zhang, X. Bai, Y. Wu, W. Zhang, X.Y. Zhao, D.Z. Guo, Physica E 41 (2008) 309. [42] Z. Pan et al., Adv. Mater. 12 (2000) 1186.