Synthesis of IrO 2 nanocrystals on carbon nanotube bundle arrays and their field emission characteristics
3. Results and discussion 1. FESEM images
Fig. 1(a) and (b) depicts the FESEM images of patterned CNT bundle arrays grown on a Si substrate. As shown inFig. 1(a) the diameter and the height of each bundle are, respectively, 25 and 10m with an inter-bundle distance of 10 m. The bundles are vertically aligned to the substrate surface with a hexagonal
arrange-Fig. 1. The FESEM images of (a) the patterned CNT bundle arrays and (b) vertically aligned CNT bundles. The inset shows the surface of the pristine CNTs. (c) The FESEM of IrO2NCs deposited on top of a CNT bundle. The inset shows that the walls of CNTs are covered by a large number of IrO2NCs.
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ment. The inset image ofFig. 1(b) is magnified from the surface of the CNTs before IrO2 deposition.Fig. 1(c) shows an image of an IrO2-coated CNT bundle. The morphology of CNTs retains the bundle-like structure after depositing IrO2NCs onto the surface.
The inset image inFig. 1(c) shows that the walls of the CNTs are densely covered with a large number of IrO2NCs.
3.2. Raman scattering analysis
Fig. 2(a)–(c) shows the Raman spectra of the CNTs, IrO2, and IrO2/CNTs composite, respectively. Raman signals from the Si substrate are marked by an asterisk. The spectrum of the CNTs (Fig. 2(a)) shows a D-band at ∼1355 cm−1 originating from the disordered carbon and a G-band at ∼1587 cm−1 corresponding to sp2-hybridized carbon[18]. The Raman spectrum of IrO2 (see Fig. 2(b)), obtained from IrO2nanocrystals on a Si substrate, shows three Raman modes: Egat 560 cm−1, B2g at 729 cm−1 and A1gat 750 cm−1and. The observation of these three Raman-active modes is well correlated with that of single crystals[19].Fig. 2(c) is the Raman spectrum of the IrO2/CNTs composite, which reveals three IrO2features (Egat 549 cm−1, B2gat 716 cm−1and A1gat 730 cm−1) and two CNTs signatures (D-band at∼1349 cm−1, and G-band at
∼1579 cm−1). The red-shifts in the peak positions and linewidth broadening for the IrO2features can be attributed to both the size and residual stress effects[20,21].
Fig. 2. Raman spectra of the (a) CNTs on Si substrate, (b) IrO2on Si substrate, and (c) IrO2/CNTs composite on Si substrate. Raman signals from the Si substrate are marked by an asterisk.
Fig. 3. (a) The low magnification TEM image and (b) the HAADF image of an IrO2-coated CNT. (c) The HRTEM image for a multi-walled CNT coated with IrO2. (d) The FFT image of (c).
662 Y.M. Chen et al. / Journal of Alloys and Compounds 487 (2009) 659–664
3.3. TEM analysis
The low-magnification TEM image of an IrO2-coated CNT (Fig. 3(a)) shows uniform distribution and random direction of IrO2 NCs depositing on the surface of the CNT. The sizes of IrO2 NCs are about 10–50 nm.Fig. 3(b) is the high angle annular dark field (HAADF) image of IrO2NCs on a CNT. The HAADF image indi-cates that the center portion still retains a tubular structure after depositing IrO2NCs. Energy-dispersive X-ray spectroscopy (EDX) measurements reveal that the NCs have an average atomic ratio of Ir to O of 1:2.Fig. 3(c) depicts the high resolution TEM (HRTEM) image for a CNT coated with IrO2NCs. The HRTEM image shows that the obtained CNT has a multi-walled structure. The lattice spacing between adjacent walls of the CNT has been determined to be 0.34 nm, which is in good agreement with the layer spacing of graphite (0 0 2) plane. The value of the (1 1 0) plane spacing of IrO2is determined to be 0.32 nm. The image also reveals a defect appearance due to lattice mismatch. A fast Fourier transform (FFT) image fromFig. 3(c), depicted inFig. 3(d), indicates that the IrO2
(1 1 0) layers are deposited onto the CNT (0 0 1) plane.
3.4. Field emission characteristics
Fig. 4displays the current density J as a function of electric field E for the pristine CNTs and the IrO2/CNTs composite. For the IrO2/CNTs composite, the value of the turn-on field, defined as the electric field required for driving a current density of 0.1A/cm2, is determined to be 0.9 V/m, which is much lower than that for pris-tine CNTs (2.5 V/m). In addition, a lower threshold field, defined as the magnitude of E as J approaches 1 mA/cm2, of 2.7 V/m is also achieved for the IrO2/CNTs composite comparing to the value of 3.6 V/m for pristine CNTs. The Fowler–Nordheim (F–N) plots for pristine CNTs and IrO2/CNTs composite, as shown in Fig. 5, are used to determine the threshold field at which emission begins. According to the F–N plots, after passing a turning point at 1.8 V/m for CNTs (0.8 V/m for IrO2/CNTs), a linear relation between ln(J/E2) and 1/E is observed, indicating that the FE is intrin-sically driven by the electric field. The F–N plot is fitted by the FN equation expressed as[22,23],J = A
ˇ2E2 and B are constants relating to the emitter parameters. We assume that A = 1.56× 10−6eV A V−2, B = 6.83× 103 (eV)−3/2Vm−1, and the factor describes the geometrical efficiency of FE [23,24].
The work functions of IrO2and CNTs areϕIrO2 = 4.23 eV[17]and ϕCNT= 4.8 eV[25], respectively, andˇ is the field enhancement fac-tor. Theˇ and values can be extracted easily from a linear fit of the slope of the F–N plot of ln(J/E2) versus 1/E. The values ofˇ/ are determined to be 1.5× 103/1 and 7.4× 103/1× 10−4for the CNTs and the IrO2/CNTs composite, respectively.
One important issue in CNT-based emitters is the improvement of emission stability and lifetime.Fig. 6(a) and (b) shows the results of emission current density versus time for the pristine CNTs and the IrO2/CNTs composite, respectively. As shown inFig. 6(a), it clearly demonstrates that the emission current of the pristine CNTs is not stable under electric field of 3.6 V/m and decreases with the testing time. FESEM investigations (comparingFigs. 1(b) and 7(a)) indicate obvious changes have taken place during the stability mea-surement. After the emission stability testing process, microscale spot-like carbon particles have formed, which is attributed to the welding of neighboring CNTs onto the pristine CNTs. It is known that CNTs subjected to an appreciable electric field [26]have a tendency to be spot welded at the nanosized contact area due to local Joule heating. The high electric field applied to the dense CNTs emitter generated superheat at the sharp tips of the CNTs so that neighboring CNTs tend to fuse together forming larger spot-like carbon particles. It is expected that the superheat induced
weld-Fig. 4. Field emission current density J as a function electric field E for the pristine CNTs and the IrO2/CNTs composite.
Fig. 5. The F–N plots corresponding to the pristine CNTs and the IrO2/CNTs com-posite.
Fig. 6. Emission current stability of (a) the pristine CNTs and (b) the IrO2/CNTs composite under a continuously applied field at a pressure of 3× 10−7mbar.
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Fig. 7. The FESEM images of (a) the pristine CNTs and (b) the IrO2/CNTs composite after the emission stability testing processes.
ing of several neighboring CNTs in a certain region can happen in a rather short period of time, leading to a change in the surface morphology and a depreciation of the emission current. Therefore, it is clear that such a welding behavior can influence the emission current stability in a long term and reduce the lifetime of the CNTs emitter.
However, from Fig. 6(b), the IrO2/CNTs composite sample exhibits a stable long-term emission and the emission current fluctuation is within ±10% for duration of 8 h at a constant electric field of 2.7 V/m. No obvious degradation of current density is observed during the testing period, manifesting the durable and robust capability of the IrO2/CNTs composite. Fur-thermore, FESEM investigations (Figs. 1(c) and 7(b)) also reveal that there is no apparent morphological damage in the IrO2/CNTs composite after the emission stability testing process. Since the IrO2 possess a high conductivity and a low work function, it would be particularly suitable for operations under low driv-ing fields and hence low power consumption. Moreover, since the geometrical structure of the IrO2/CNTs composite provides a high field enhancement effect, this also shows the potential of the IrO2/CNTs composite for practical applications such as field-emission flat panel displays, X-ray sources and microwave devices.
4. Summary
In summary, IrO2 NCs are deposited on patterned vertically aligned CNT bundle arrays via MOCVD. The FESEM images reveal that the morphology of CNTs retains the bundle-like structure after depositing IrO2 NCs onto the surface and the walls of CNTs are densely covered with a large number of IrO2NCs. The Raman spec-trum shows a red-shift in peak position and linewidth broadening of IrO2 signatures with respect to that of the bulk counterpart, which can be attributed to the nanosized and residual stress effects for the as-deposited nanostructural IrO2. The HAADF image indi-cates that the center portion still retains a tubular structure after depositing IrO2NCs. EDX measurements reveal that the IrO2NCs have an average atomic ratio of Ir to O of 1:2. The J–E measure-ments yield a low turn-on field of 0.9 V/m at a current density of 0.1A/cm2, a low threshold field of 2.7 V/m at a current den-sity of 1 mA/cm2, and a high field enhancement factor of 7.4× 103 for the IrO2/CNTs composite. In addition, long-term stability has also been demonstrated. The enhancement of FE characteristics are attributed to the combined effects of the geometrical structure
of IrO2/CNTs composite, and the natural conducting and enhanced resistance to oxidation properties of IrO2. The results indicate that the IrO2/CNTs composite can be a potential candidate for field emis-sion devices.
Acknowledgments
This work is financially supported by the National Science Council of Taiwan under Contract Nos. NSC 97-2112-M-011-001, NSC-96-2221-E-011-078 and 97-2221-E-011-017.
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