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2007, 111, 3544-3547 Published on Web 02/09/2007
Controlled Growth and Field-Emission Properties of the Organic Charge-Transfer Complex of K-(BEDT-TTF)2Cu(SCN)2 Nanorod Arrays Changshui Huang,†,‡ Yang Zhang,§ Huibiao Liu,† Shuang Cui,†,‡ Chunru Wang,† Lei Jiang,† Dapeng Yu,§ Yuliang Li,*,† and Daoben Zhu*,† Beijing National Laboratory for Molecular Science (BNLMS), CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, P. R. China, Graduate School of Chinese Academy of Sciences, Beijing 100080 , P. R. China, and Department of Physics, Peking UniVersity, Beijing, 100080, P. R. China ReceiVed: NoVember 10, 2006; In Final Form: January 24, 2007
We have demonstrated a simple and convenient method for fabricating the nanorod arrays of the organic charge-transfer complex of κ-(BEDT-TTF)2Cu(SCN)2 supported on Pt foil and ITO glass. The morphologies of the κ-(BEDT-TTF)2Cu(SCN)2 nanorod arrays can be controlled by adjusting the growth current density. The field-emission properties of the κ-(BEDT-TTF)2Cu(SCN)2 nanorod arrays are also measured for showing an excellent field-emission performance. The results suggest that the κ-(BEDT-TTF)2Cu(SCN)2 nanorod arrays could be potentially useful electron field emitters.
BEDT-TTF-based organic charge-transfer complexes have been studied intensely since the discovery, a few years after the initial synthesis of neutral BEDT-TTF molecules,1 of metallic behavior down to low temperature.2 Many interesting solid-state physical properties of these complexes such as superconductivity,3,4 competing antiferromagnetism,5 spinpeierls,6 metal-to-insulator transitions,7 and ferromagnetism8 have been reported. However, most of the physical properties of the organic charge-transfer complexes were investigated on macroscopic single-crystalline samples9 or thin films.10 To date, various techniques, such as thermal sublimation in a high vacuum,11 chemical vapor deposition,12 Langmuir-Blodgett techniques,13 halide evaporation on an organic donor-treated polymer film,14 adsorption in solution,15 and electrodeposition16 have been used to synthesize the organic charge-transfer complexes. Difficulties remained in creating these materials on nanoscale and aligned patterned shape directly, which would be required for any potential application. We report here the first controlled growth of the κ-(BEDT-TTF)2Cu(SCN)2 freestanding nanorod arrays on the surface of indium tin oxide (ITO) glass and platinum foils by an electrocrystallization approach using differences of constant current density, and the fieldemission properties of these samples are studied. (BEDT-TTF)2Cu(SCN)2 nanorod arrays with areas of 1 × 0.5 cm2 were prepared by an electrocrystallization approach, which can easily tune the size and shape on uniformity. A Pt foil (cathode) and an ITO glass (anode) were used as the working electrodes. A drawing of the typical electrocrystallization cell was described in the Supporting Information. In brief, 3.5 mg (0.09 mmol) of BEDT-TTF, 8 mg (0.07 mmol) of CuSCN, 18 mg (0.19 mmol) of KSCN, and 8 mg (0.03 mmol) * Corresponding author. E-mail:
[email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. § Peking University.
10.1021/jp067449k CCC: $37.00
Figure 1. (A) Lower magnification and (B) higher magnification SEM images of the (BEDT-TTF)2Cu(SCN)2 nanorods a. (C) Lower magnification and (D) higher magnification SEM images of the (BEDTTTF)2Cu(SCN)2 nanorods b.
of 18-Crown-6 were dissolved in 10 mL of 1,1,2-trichloroethane (TCE) and stirred vigorously for 3 h at 50 °C. The mixture was cooled down to 30 °C, and the remainder of undissolved CuSCN and KSCN was precipitated on the bottom of the electrocrystallization cell. (BEDT-TTF)2Cu(SCN)2 was completely grown on the surface of the anode ITO glass after reaction for 4 h at 30 °C under a constant current density of 100 µA/cm2 and 200 µA/cm2, respectively. The (BEDT-TTF)2Cu(SCN)2 nanorods can be also obtained on the surface of the anode while the anode ITO glass was replaced with Pt foil. The morphology of the (BEDT-TTF)2Cu(SCN)2 nanorods was observed by scan electron microscopy (SEM). Figure 1 showed us a typical morphology of vertically aligned (BEDT© 2007 American Chemical Society
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J. Phys. Chem. C, Vol. 111, No. 9, 2007 3545
Figure 2. (A) Lower magnification and (B) higher magnification SEM images of the (BEDT-TTF)2Cu(SCN)2 nanorods c. (C) Lower magnification and (D) higher magnification SEM images of the (BEDT-TTF)2Cu(SCN)2 nanorods d. (E) TEM image and (F) SAED pattern of a typical (BEDTTTF)2Cu(SCN)2 nanorod c. (G) TEM image and (H) SAED pattern of a typical (BEDT-TTF)2Cu(SCN)2 nanorod d.
TTF)2Cu(SCN)2 nanorod arrays. As shown in Figure 1A and B, the diameter of the (BEDT-TTF)2Cu(SCN)2 nanorods a grown on the ITO glass under the constant current density of about 100 µA/cm2 was in the range of 100-200 nm. The diameter at the top of the (BEDT-TTF)2Cu(SCN)2 nanorods a was bigger than that in the middle of the (BEDT-TTF)2Cu(SCN)2 nanorods a. The shape of the (BEDT-TTF)2Cu(SCN)2 nanorods was changed by adjusting the current density. Figure 1C and D displayed the (BEDT-TTF)2Cu(SCN)2 nanorods b grown on the ITO glass under the constant current density of about 200 µA/cm2. The diameter of (BEDT-TTF)2Cu(SCN)2 nanorods b was in the range of 90-150 nm. However, the diameter at the top of the (BEDT-TTF)2Cu(SCN)2 nanorods b was smaller than that in the middle of the (BEDT-TTF)2Cu(SCN)2 nanorods b, which was different from (BEDT-TTF)2Cu(SCN)2 nanorods a. Novel shapes of the (BEDT-TTF)2Cu(SCN)2 nanorods were obtained while the anode ITO glass was replaced with Pt foils. As shown in Figure 2A and B, the (BEDT-TTF)2Cu(SCN)2 nanorods c grown on Pt foil through the electrocrystallization technique under a constant current density of about 100 µA/ cm2 looked like a seaflower. The diameter of the (BEDTTTF)2Cu(SCN)2 nanorods c was in the range of 500-1000 nm. The top of the (BEDT-TTF)2Cu(SCN)2 nanorods c was concave with the width of the edge in the range of 100-200 nm. The images of (BEDT-TTF)2Cu(SCN)2 nanorods d grown on Pt foil through the electrocrystallization technique under a constant current density of about 200 µA/cm2 were displayed in Figure 2C and D. The diameter of the (BEDT-TTF)2Cu(SCN)2 nanorods d was in the range of 90-150 nm. Although the arrangement of the (BEDT-TTF)2Cu(SCN)2 nanorods d was rather compact, the diameter at the top of the (BEDT-TTF)2Cu(SCN)2 nanorods d was smaller than that in the middle of the (BEDT-TTF)2Cu(SCN)2 nanorods d. Further characterization of the nanorods grown on Pt foils was performed by transmission electron microscope (TEM). Figure 2E showed a typical (BEDT-TTF)2Cu(SCN)2 nanorod c with a width of 500-1000 nm. Figure 2G showed a typical (BEDT-TTF)2Cu(SCN)2 nanorod d with a width of 90-130 nm. The selected area electron diffraction (SAED) pattern was carried out (shown in Figure 2F and H), demonstrating that the (BEDT-TTF)2Cu(SCN)2 nanorods c and d were crystal. The SAED pattern also revealed that the (BEDT-TTF)2Cu(SCN)2 nanorod grows along [001].
Figure 3. (A) FT-IR spectrum and (B) powder X-ray diffractogram of the (BEDT-TTF)2Cu(SCN)2 nanorods a, b, c, and d.
The products were characterized by Fourier transform infrared (FT-IR) spectrum (Figure 3A). Two bands due to intramolecular vibrational transitions of BEDT-TTF are observed in the FTIR spectrum of (BEDT-TTF)2Cu(SCN)2.17 One is the split band in the region of 1174-1341 cm-1, and the other is the band of 883 cm-1. These bands around 1174-1341 cm-1 involve the hydrogen-related vibrational modes, such as C-C-H or H-C-C bending modes, in addition to the CdC stretching modes. The peak at 883 cm-1 can be assigned to the C-S stretching. Also, a strong ν (-NdCdS) band at 2080 cm-1 has been shown in Figure 3A. To further reveal the structure, powder X-ray diffraction (XRD) patterns were recorded on the (BEDT-TTF)2Cu(SCN)2 nanorods a, b, c, and d. It has previously been reported by Rovira etc.18 that the measured values of the interplanar spacing
3546 J. Phys. Chem. C, Vol. 111, No. 9, 2007
Figure 4. Possible formation process of the (BEDT-TTF)2Cu(SCN)2 nanorod arrays under different current densities.
d001 along with intensities of (00l) reflections allow the identification of the different phases of the BEDT-TTF-based charge-transfer salt. The corresponding patterns are shown in Figure 3B. It can be seen that, in Figure 3B, the intensive peaks at 2θ ) 7.10°, 14.32°, 21.56°, and 28.98° correspond to the (001), (002), (003), and (004) reflections of the κ-(BEDTTTF)2Cu(SCN)2 structure with parameters: a ) 16.25 Å, b ) 8.44 Å, c ) 13.12 Å; R ) γ ) 90°, and β ) 110.30°.19 Furthermore, the other peaks in Figure 3B were not related to the (00l) reflections of the (BEDT-TTF)2Cu(SCN)2 structure of any other phases.20 All of these results indicated that the charge-transfer complexes are found to electrocrystallize mainly κ-(BEDT-TTF)2Cu(SCN)2. In addition, the XRD patterns of the samples indicated the prensence of intensive (00l) reflections, which confirmed that the κ-(BEDT-TTF)2Cu(SCN)2 nanorods grow along the c axis.18 The size and shape of (BEDT-TTF)2Cu(SCN)2 were changed while the value of current density was adjusted. A possible formation process of the (BEDT-TTF)2Cu(SCN)2 nanorod arrays was illustrated schematically in Figure 4. The initial (BEDTTTF)2Cu(SCN)2 nanoclusters aggregate on the surface of the anode electrodes, which are used as the nucleation centers inducing the growth of (BEDT-TTF)2Cu(SCN)2. Compared with small current density, large current density results in many more nucleation centers of (BEDT-TTF)2Cu(SCN)2 and the (BEDTTTF)2Cu(SCN)2 are able to grow fast along the tip of the initial (BEDT-TTF)2Cu(SCN)2 where the charges are easy to aggregate. The aggregated charges provide the gravitation to induce the growth direction of the (BEDT-TTF)2Cu(SCN)2. The
Letters number of the nucleation centers determines the size of the (BEDT-TTF)2Cu(SCN)2. Therefore, the (BEDT-TTF)2Cu(SCN)2 will be formatted in the formation of nanorod arrays and the diameter of the (BEDT-TTF)2Cu(SCN)2 nanorods grown under large current density is smaller than that grown under small current density. However, when the current density is too small (see the Supporting Information), the number of the nucleation centers obtained is less and the (BEDT-TTF)2Cu(SCN)2 will grow slowly not only just along the tip but also toward the other direcrtion. So (BEDT-TTF)2Cu(SCN)2 of macroscopic size was obtained. When the current density is too large (see the Supporting Information), too many of the nucleation centers are obtained in the same regions so that the (BEDT-TTF)2Cu(SCN)2 are formatted in the formation of congested nanorods. In addition, the (BEDT-TTF)2Cu(SCN)2 nanorods can also be obtained on the surface of Pt foil. It will be interesting to explore the effect of the substrates, which results in nanorods of different morphologies being obtained on different substrates at the same conditions. Further investigation about the mechanism is underway. Because the charge-transfer salt (BEDT-TTF)2Cu(SCN)2 nanorods are vertically aligned, they are potential candidates for field-emission (FE) studies. In this work, the FE properties of the (BEDT-TTF)2Cu(SCN)2 nanorods were investigated for the first time. The field-emission characteristics were presented in Figure 5 by the curve of current density (J) versus applied field (E). The turn-on fields of the (BEDT-TTF)2Cu(SCN)2 nanorods c and d were about 11.57 v·µm-1 and 8.33 v·µm-1, respectively. Here, the turn-on field was defined as that at which a current density of 10 µA/cm2 was reached. This value was comparable to the values reported for some important materials such as the tungsten oxide nanowires net works,21 vertically aligned Co nanowires,22 RuO2 nanorods,23 IrO2 nanorods,24 and ZnO nanowires.25 The maximal current density of the (BEDTTTF)2Cu(SCN)2 nanorods c and d reached about 0.27 mA/cm2 at an applied field of 18.01 v·µm-1 and 1.79 mA/cm2 at an applied field of 14.67 v·µm-1, respectively. The inset in Figure 5 is the corresponding Fowler-Nordheim (FN) plot displayed with ln(I/V2) and 1/V, which shows a similarly linear relation, indicating that the field-emission process from the nanorod film is a quantum tunneling process. To compare with other materials such as graphite, the emission characteristics were analyzed using the FowlerNordheim model described as26
J ) E2loc exp[-6.8 × 107 φ3/2/Eloc] Here J is current density from the emitting tip, Eloc is the local electric field, and φ is work function of the sample. For an isolated hemisphere model
Figure 5. (A) Field emission J-E curve of the (BEDT-TTF)2Cu(SCN)2 nanorods c; the inset depicts the corresponding Fowler-Nordheim (FN) plots and the SEM image of the (BEDT-TTF)2Cu(SCN)2 nanorods c. (B) Field emission J-E curve of the (BEDT-TTF)2Cu(SCN)2 nanorods d; the inset depicts the corresponding Fowler-Nordheim (FN) plot and the SEM image of the (BEDT-TTF)2Cu(SCN)2 nanorods d.
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Eloc )
V RRtip
Here V is the applied voltage, Rtip is the tip radius of curvature, and R is a modifying factor. From the above equations, we got
ln
()
I 1 ) (-6.8 × 107 RRtipφ3/2) + offset 2 V V
RRtipφ3/2 can be estimated from the slope of the F-N plot of ln(I/V2) against (1/V). In our case, taking R ) 10 (as used in another report27) and Rtip ) 70 nm, the evaluated work function of the (BEDT-TTF)2Cu(SCN)2 nanorods d is around 3.89 eV. This work function is smaller than that of graphite, which is typically around 4.34 eV,28 demonstrating that the (BEDTTTF)2Cu(SCN)2 nanorods have great potential as competitive candidates for field emitters. In summary, we have demonstrated a simple and convenient method of fabricating the nanorod arrays of a charge-transfer salt of (BEDT-TTF)2Cu(SCN)2 supported on Pt foil or ITO glass. Both TEM and diffraction investigations indicate the crystalline property of such nanorods. The morphologies of the (BEDT-TTF)2Cu(SCN)2 nanorod arrays can be controlled by adjusting the growth current density. The field-emission properties of (BEDT-TTF)2Cu(SCN)2 nanorod arrays are also measured for showing promising field-emission performance. The turn-on fields were comparable to those of many other important nanomaterials. The results suggest that the (BEDT-TTF)2Cu(SCN)2 nanorods could be potentially useful electron field emitters. Our findings also have important implications for the application of these materials based on BEDT-TTF for fieldemission devices. Acknowledgment. This work was supported by the National Natural Science Foundation of China (10474101, 20531060, 20473102, 20418001, 20421101, and 20571078) and the Major State Basic Research development Program (Grant Nos. 2006CB806201 and 2005CB623602). This project is partly supported by the National Center for Nanoscience and Technology, China. Supporting Information Available: Detailed experimental procedures for the sample preparation and characterization. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Masao, M.; Anthony, F. G.; Michael, P. C. J. Chem. Soc. Chem. Commun. 1978, 18. (2) (a) Kobayashi, H.; Kobayashi, A.; Saito, G.; Enoki, T.; Inokuchi, H. J. Am. Chem. Soc. 1983, 105, 297. (b) Laukhina, E.; Tkacheva, V.; Chekhlov, A.; Yagubskii, E.; Wojciechowski, R.; Ulanski, J.; VidalGancedo, J.; Veciana, J.; Laukhin, V.; Rovira, C. Chem. Mater. 2004, 16, 2471. (3) Kini, A. M.; Geiser, U.; Wang, H. H.; Carlson, K. D.; Williams, J. M.; Kwok, W. K.; Vandervoort, K. G.; Thompson, J. E.; Stupka, D. L.; Jung, D.; Whangbo, M.-H. Inorg. Chem. 1990, 29, 2555. (4) (a) Geiser, U.; Schlueter, J. A.; Wang, H. H.; Kini, A. M.; Williams, J. M.; Sche, P. P.; Zakowicz, H. I.; VanZile, M. L.; Dudek, J. D.; Nixon,
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