8763
2008, 112, 8763–8766 Published on Web 05/22/2008
Controllable Preparation of Copper Tetracyanoquinodimethane Nanowire and the Field Emission Study Fei Tian,†,‡ Wei Liu,†,‡ and Chun-Ru Wang*,† Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, Beijing National Laboratory for Molecular Sciences, and Graduate School of Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: March 25, 2008; ReVised Manuscript ReceiVed: May 6, 2008
We report a facile method to synthesize single crystalline copper 7,7,8,8- tetracyanoquinodimethane (CuTCNQ) nanostructures using the chemical vapor deposition (CVD) method. It was revealed that, through simply adjusting the temperature-rising rate in CVD process, the CuTCNQ nanostructures were controllable in both the size and morphology. The single crystal of the CuTCNQ nanostructures was characterized by SEM, TEM, XRD, XPS, IR, etc. The field emission property of the CuTCNQ was studied and the result revealed that field emission property of CuTCNQ depends on its morphology. In the past decade, one-dimensional (1D) organic nanomaterials have become the focus of intensive research because of their unique chemical, physical, and mechanical properties.1,2 Compared with inorganic nanomaterials, 1D organic nanomaterials which have many unique properties such as flexibility, high photoconductivity, and nonlinear optical effects have attracted interest for potential applications in functional nanoscale electronic and optoelectronic devices. Many 1D organic nanomaterials, such as polypyrrole nanowires, polydiacetylene nanowires, polyacetylene nanofibers, etc., have been extensively studied in recent years.3–5 Among the organic one-dimensional nanomaterials, copper 7,7,8,8-tetracyanoquinodimethane (CuTCNQ) has attracted intense interest due to its intriguing structural, electronic, optical properties, and potential applications in electrical memory devices, sensors, magnetic devices, and field emission.6–12 CuTCNQ possesses not only a very stable molecular structure but also novel electronic properties as an organic charge-transfer complex. It has two polymorphs, defined as the high-conductivity phase I and the low-conductivity phase II, respectively. The high-conductivity phase I CuTCNQ has a high density of charge carriers that leads to potential applications in optoelectronics and electronics, so most recent studies of CuTCNQ have focused on this polymorph. In this paper, we also focus on the highconductivity phase I CuTCNQ and make a comparative study on the field emission performances of different sizes of CuTCNQ. Recently, various methods have been reported to synthesize the 1D micro/nanostructures of CuTCNQ, e.g., the vaporinduced reaction method, electrochemical method, vapor-solidphase method, and solution-solid-coalescing method.13–17 Here we report a simple vapor-induced reaction method to prepare the CuTCNQ nanostructure array, i.e., evaporating the TCNQ * To whom correspondence should be addressed. E-mail: crwang@ iccas.ac.cn. † Institute of Chemistry. ‡ Graduate School of Chinese Academy of Sciences.
10.1021/jp8025754 CCC: $40.75
materials in vacuum and depositing them on Cu-containing substrates. Through tuning the temperature-rising- rate, different CuTCNQ nanostructures can be prepared. For the synthesis of CuTCNQ, TCNQ powder (2 mg) was loaded in a quart crucible and placed in the center of a quartz tube, and several copper foils were selected as substrates to set in the quartz tube with a distance at ca. 1 cm far from the TCNQ sample. To avoid any possible contamination, the copper substrates have been cleaned successively in an ultrasonic bath of acetone for 20 min, 0.1 M HCl for 20 min, deionized water for 10 min, and ethanol for 15 min before using. The quartz tube together with the TCNQ sample and copper foils were connected to a vacuum system. The pressure was maintained at about 2 Pa for 20 min, and then a furnace was set outside of quartz tube. The relative position between the furnace and the quartz tube was elaborately adjusted to allow the TCNQ sample and the substrates locating in different temperature range, e.g., the temperature of copper substrates changes from room temperature to ca. 120 °C while the TCNQ sample being heated up to 250 °C. We made dozens of tests to calibrate the temperature difference between the TCNQ sample and Cu substrates, and inadvertently observed that the morphology of CuTCNQ nanostructures is strongly dependent on the temperature-rising rate, so we prepared the CuTCNQ samples at several different conditions and made a comparative study on their morphology and field emission performances. Figure 1 showed the SEM studies of three CuTCNQ samples prepared with the above-mentioned method at different temperature-rising rates. By adjusting the applied currents on the furnace, we increased the temperature on the TCNQ area from ca. 24- 250 °C in 6 min (sample I), 8 min (sample II), and 12 min (sample III) respectively, and in the meantime the temperature of the copper substrates was measured to increase to ca. 120 °C. In this process, it can be easily observed that the TCNQ sample was evaporated and deposited on Cu substrates, which altered the substrates’ color from light yellow to blue slowly. 2008 American Chemical Society
8764 J. Phys. Chem. C, Vol. 112, No. 24, 2008
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Figure 2. Spectroscopic characterizations of CuTCNQ nanostructures: (a) FTIR of CuTCNQ, (b) XPS spectrum showing the Cu 2p3/2 and 2p1/2 split of CuTCNQ nanorod, (c) TEM image of an individual CuTCNQ nanorod, and the inset shows the electron diffraction pattern, and (d) XRD profile of CuTCNQ nanostructures.
Figure 1. SEM images of CuTCNQ nanostructure prepared under the different temperature-rising rates. The TCNQ sample was heated to 250 °C in time of (a and b) 6 min, (c and d) 8 min, and (e and f) 12 min.
The sample shown in Figure 1a was CuTCNQ nanostructures produced with a very high temperature-rising rate, which showed rod-like morphology with a diameter ranging between 200 and 1000 nm, and their cross-sections were either square or octagon shapes.13 When the temperature-rising rate decreased, the CuTCNQ nanostructures became thinner nanorods in Figure 1c and needle-like nanowires in Figure 1e gradually. To compare the morphology of the three samples prepared at different temperature-rising rates, it was observed that not only the size of CuTCNQ nanostructures changed from short-flat nanorods (sample I) to slim nanorods (sample II), and then needlelike nanowires (sample III) but also the nanostructure’s ends became sharper and sharper. For example, as shown in Figure 1b, the CuTCNQ nanostructure’s ends had a thin protrudent edge originally, but they became some irregular tips in Figure 1d and finally became multispines in Figure 1f. The composition, morphology, and crystalline structures of the as-prepared CuTCNQ nanostructures were characterized by various spectroscopic techniques. Figure 2a is a typical FTIR spectrum of the CuTCNQ nanorods (sample I), in which the band at 2199 cm-1 was assigned as a C-N stretching mode, and the weak absorptions at 1353, 1577, and 1506 cm-1 were from C-C ring-stretching.18 XPS spectrometry was adopted to characterize the valence status of Cu in CuTCNQ nanostructures. As shown in Figure 2b, the Cu 2p3/2 and Cu 2p1/2 peaks in XPS spectrum indicated a Cu+ valence in the CuTCNQ molecules, and no any shoulders or satellites appeared in the spectrum due to Cu or Cu2+, which suggested the high purity of CuTCNQ products. Figure 2c shows a typical TEM image of an individual CuTCNQ nanorod collected from the sample II. The uniform and sharp edges suggested a single crystalline structure of the nanorods, which was confirmed by the selected area electronic diffraction study as shown in the insert of Figure 2c. To further characterize the crystalline structure of the as-prepared CuTCNQ, XRD spectrometry was performed and shown in Figure
2d. Based on the analyses of low-index diffraction peaks in the spectrum, the CuTCNQ phase I structure with a tetragonal unit cell of a ) 3.89, b ) 11.26, and c ) 11.27 Å was suggested.19 Briefly, through the combined SEM, TEM, IR, XPS, and XRD spectroscopic characterizations, the as-prepared sample by this method was assigned as single crystalline CuTCNQ of phase I. The morphology changing of CuTCNQ nanostructures reflects the formation mechanism of CuTCNQ nanostructures in the current conditions. As stated above, we should heat the quartz tube continuously for preparing the CuTCNQ nanostructures. In this process, the solid TCNQ sample began to vaporize above a certain temperature point. Initially they may not react with the copper substrates due to the low energy, but along with the temperature increase, some TCNQ molecules, which catch enough energy, gradually reacted with the heated copper substrates and formed CuTCNQ at some points. Starting here we may discuss how the temperature-rising rate affects the morphology of CuTCNQ nanostructures. In the case of a high temperature-rising rate, the high energetic TCNQ molecules increased quickly, and they bombarded the nearly naked copper foils and reacted to form CuTCNQ simultaneously on many points of the copper substrate. Thus along with the resulting CuTCNQ molecules covering the copper substrates, many adjacent reacting points fused and grew into large-diameter CuTCNQ nanorods. On the contrary, in the low temperaturerising rate case, only little TCNQ molecules in vapor had enough reacting energies to form CuTCNQ, so the chemical reaction occurred only in some special active points of the copper foils and grew to thinner CuTCNQ rods. After some time, although the rising temperature may provide more high energetic TCNQ molecules in the vapor, the already formed CuTCNQ nanowires by that time prohibited them from approaching the copper substrates, so no thick nanorods formed in this case. Regarding the ends of CuTCNQ rods, the different tip morphology can be explained by above bottom growth mechanism of the CuTCNQ nanostructures. For the case of thick nanorods as shown in Figure 1b, TCNQ molecules had more chance to react with the copper beneath the edge sites of CuTCNQ nanorods, so a thin wall was formed on the up ends of CuTCNQ nanorods. As for the case of thin nanorods shown in Figure 1f, the small area beneath CuTCNQ nanorods made
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J. Phys. Chem. C, Vol. 112, No. 24, 2008 8765 CuTCNQ nanorods became coarse spines, and the turn-on electric field was reduced to 17.3 V µm-1. As for sample III, the protrudent tips of the CuTCNQ nanostructures became multiple long and sharp needles, so that its turn-on electric field was further reduced to 7.78 V µm-1, which is nearly the same as the modified nanodiamond cone arrays23 but is lower than many other inorganic emitters such as carbon nitride. It is obvious that the field emission property of CuTCNQ nanostructures depends on the end morphology of the materials. The emission characteristics were analyzed using the Fowler-Nordheim model24 described as follows: J ) 1.54 × 10-6F2Ø-1 exp(-6.8 × 107Ø3/2ν(y0)/F), where J is the current density from the emitting tip, F is the local electric field, Ø is the work function of the sample, and ν(y0) is Nordherm function. For an isolated model: F ) βV/d, where V is the applied voltage, d is the distance between the anode and the cathode, and β is the F-N enhancement factor. Supposing the current density (J) of the sample is all the same, we get
ln(I/V2) ) 1/V(-6.8 × 107dØ3⁄2/β) + offset
(1)
ln(I/V2)
Figure 3. (a) Field emission J-E curve of sample I, (b) F-N plot of sample I, (c) field emission J-E curve of sample II, (d) F-N plot of sample II, (e) field emission J-E curve of sample III, and (f) F-N plot of the sample III.
little differences between coppers on edge sites and on center sites, so the spine-like ends reflect the multireacting points on copper substrates in the initial reacting stage. It was revealed that CuTCNQ nanostructures are excellent field emitters, since it is well known that the field emission performance of nanomaterials is closely related to their diameters, tip shapes, and morphology. From sample I to III, the CuTCNQ nanorods’ diameter became smaller, and the tips became sharper, we thus performed a comparative study for above-mentioned three CuTCNQ nanomaterials on their field emission properties. The field emission J-V curves were measured in homemade equipment. Briefly, the samples were set on sample holder at a high vacuum of about 1 × 10-6 Pa. The test anode was an aluminum stick with a diameter of 2 mm and the testing sample served as the cathode. The distance between the anode and the cathode was kept at 200 µm, and a voltage with a sweep step of 50 V was applied between the anode and cathode for supplying the required electric field. Figure 3 shows typical plots of the field emission (FE) current density versus the applied electric field for the three different CuTCNQ samples. An emission current density of 1 mA cm-2, which is defined as the minimum emission current required to produce a luminance of 300 cd m-2 from the VGAFED with a typical high-voltage phosphor screen efficacy of 9l mW-1, was achieved at 23.6 V µm-1 for sample I, 17.3 V µm-1 for sample II, and 7.78 V µm-1 for sample III. The turn-on electric field, or in other words, the field required to produce emission current density of 10 µA cm-2, was also varying a lot for the three samples. For sample I, the tip was almost flat with a short tubular-like protrusion, and the turn-on electric field was observed at 20.5 V µm-1 which is similar to the nanocrystalline diamond film with a flat surface.20–22 For sample II, the ends of
So the should have a linear relationship with (1/V),where I is the field emission current. As shown in Figure 3, the F-N plot for sample I was a perfect linear line, that for sample II was somewhat worse, and that for sample III showed a two-stage slope characteristic finally. This behavior was also observed previously in Liu’s report for their CuTCNQ nanowires,14 and is infact usually observed for other materials such as carbon nanotubes.25 However, as noted by Liu et al., although the F-N plot of sample III deviated from eq 1 in the low electric field, it still shows a linear relationship in the high electric field, so the field emission should be a quantum tunneling process. In eq 1, the -6.8 × 107 d Ø3/2/β can be estimated from the slope of the F-N plot of ln(I/V2) against (1/V). Since the work function Ø is constant for the same CuTCNQ materials, if we suppose that the β value of sample I is β1, then the β value for samples II and III are 1.24β1 and 3.92β1, respectively. This result strongly indicated that the field emission performance of CuTCNQ nanostructions depends on their morphology. In summary, CuTCNQ with different sizees and morphologies were controllably synthesized by a combined PVD and CVD method. The CuTCNQ materials were characterized to their own single crystalline structure. For CuTCNQ nanorods synthesized under different conditions, both their size and their end morphology are largely different. The field emission performance of these CuTCNQ materials was studied and revealed that the field emission property of the CuTCNQ nanostructures depends on their size and morphology strongly. Acknowledgment. C.R.W. thanks NSFC (Nos. 20573121 and 20121301) and the Major State Basic Research Program of China “Fundamental Investigation on Micro-Nano Sensors and Systems based on BNI Fusion” (Grant 2006CB300402) for financial support. References and Notes (1) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (3) Gan, H. Y.; Liu, H. B.; Li, Y. J.; Zhao, Q.; Li, Y. L.; Wang, S.; Jiu, T. G.; Wang, N.; He, X. R.; Yu, D. P.; Zhu, D. B. J. Am. Chem. Soc. 2005, 127, 12452. (4) Liu, W.; Cui, Z. M.; Liu, Q.; Yan, D. W.; Wu, J. Y.; Yan, H. J.; Guo, Y. L.; Wang, C. R.; Song, W. G.; Liu, Y. Q.; Wan, L. J. J. Am. Chem. Soc. 2007, 129, 12922.
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JP8025754
