Tuning CuTCNQ Nanostructures on Patterned Copper Films - The

In this contribution, we have demonstrated the ability to tune the morphologies of organic charge transfer complex (CuTCNQ) nanomaterials by controlli...
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J. Phys. Chem. C 2008, 112, 17625–17630

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Tuning CuTCNQ Nanostructures on Patterned Copper Films Huibiao Liu,†,* Xiaochun Wu,‡ Lifeng Chi,‡,* Dingyong Zhong,‡ Qing Zhao,§ Yuliang Li,*,† Dapeng Yu,§ H. Fuchs,‡ and Daoben Zhu† CAS Key Laboratory of Organic Solid, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China, Physikalisches Institut, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster, and Center for Nanotechnology (CenTech), 48149, Germany, Department of Physics, Peking UniVersity, Beijing, 100871, P.R. China ReceiVed: June 10, 2008; ReVised Manuscript ReceiVed: September 10, 2008

In this contribution, we have demonstrated the ability to tune the morphologies of organic charge transfer complex (CuTCNQ) nanomaterials by controlling the shape and thickness of copper patterns on silicon (100) at mild experimental conditions. The results showed that the CuTCNQ nanorods grew on the copper patterns (65 and 70 nm) and the CuTCNQ nanoparticles generated on the thin copper patterns (26 and 37 nm). Excellent field emission properties were observed in these nanostructures of different morphologies. Importantly, the field emission current density of those nanomaterials is higher than that of organic semiconductor nanomaterials and many inorganic semiconductor nanomaterials. Introduction 1D nanomaterials have attracted great interests of researchers in different fields due to their special structural properties, their importance in basic scientific research, and their potential technological applications.1 1D nanomaterials are ideal systems for investigating the dependence of electrical transport, optical, and mechanical properties on size and dimensionality.2 The control of the shape and the orientation of 1D nanomaterials represent essential tasks to generate nanodevices. Various methods have been exploited to control the morphologies and structures of inorganic, polymeric, and organic nanomaterials.3-8 Efforts have been directed toward developing new processing techniques for controlling structures in nanoscale. Among the organic materials, the organic charge transfer salt CuTCNQ (TCNQ ) 7,7,8,8-tetracyanoquinodimethane) has showed unique electrical properties and applied for optical and electrical recording media.9 They show high densities of charge carriers and lead to high conductivity and optical field-induced phase transitions. Directed self-assembly CuTCNQ material can provide controlled fabrication of a nanometer-sized building block in view of achieving improved thermal and chemical stability and enhancing its performance. The resistivity of CuTCNQ salt switches from a high impedance state to a low impedance state within a few nanoseconds by applying an electric field. Therefore, CuTCNQ salts are able to be a novel class of excellent field emission materials. In this article, we extend our previous work10 and described a simple method for controlled growth of organic charge transfer salt CuTCNQ nanorods on patterned copper layers on silicon (100). In general, it is not possible to obtain well-defined CuTCNQ rodlike structure unless CuTCNQ can be grown by a solid reaction technique. The shape and size of CuTCNQ nanorods are controlled by the morphologies and thickness of copper pattern on silicon (100). The CuTCNQ nanorods grew on thick copper * To whom correspondence should be addressed. E-mail: [email protected] (Y.L.), [email protected] (L.C.), [email protected] (H.L.). † Chinese Academy of Sciences. ‡ Wilhelms-Universita ¨ t Mu¨nster. § Peking University.

patterns (65 and 70 nm) and CuTCNQ nanoparticles grew on thin copper pattern (26 and 37 nm). Excellent field emission properties are displayed by those nanorods. Experimental Section The products were characterized and analyzed by scanning electron microscopy (SEM), energy-depressive spectrum (EDS), Fourier transform infrared (FTIR) (PerkinElmer System 2000 spectrometer) spectroscopy, and X-ray photoelectron spectroscopy (XPS) (VG ESCALAB 250 imaging XPS spectrometer). The thicknesses of copper layer pattern were measured by AFM (Digital Instruments, Nanoscope IIIa, Dimension 3000, Santa Barbara, CA) operating in tapping mode, silicon cantilevers (Nanosensors) of spring constant 250-350 kHz were used. The field emission measurements were carried out in a vacuum chamber of 5 × 10-7 Pa at room temperature under a twoparallel-plate configuration. The silicon (100) slice with CuTCNQ nanorods was stuck onto a stainless-steel sample stage using conducting glue as the cathode. Another parallel stainless-steel plate served as the anode. The samples’ areas were about 0.03, 0.07, 0.0525, and 0.06 cm2 for sample A, B, C, and D, respectively. The distance between the anode and cathode is 300 µm for CuTCNQ. A voltage with a sweep step of 50 V was applied between the anode and cathode to supply an electric field. Microcontact printing of copper patterns: silicon (100) substrates were ultrasonicated successively in acetone (p.a.), chloroform (p.a.), isopropanol (p.a.), and water for 10 min. Then they were cleaned with a standard RCA procedure: 15 min immersion into a 70 °C hot 1:1:5 mixture of 25% NH4OH (Fluka, p.a.), 31% H2O2 (Fluka, p.a.) and water (Millipore, 18.2 MΩcm); 15 min immersion into a 70 °C hot 1:1:5 mixture of 37% HCl (Aldrich, ACS reagent), 31% H2O2 (Fluka, p.a) and water (Millipore, 18.2 MΩcm). They were finally rinsed with water, and then dried under a stream of nitrogen. MPTMS (3mercaptoproplytrimethoxysilan 95%, Aldrich) SAMs were formed on native SiO2 surfaces by vapor deposition in a vacuum chamber for 3 h at room temperature. PDMS stamps (sylgard 184, Dow Corning) were formed on a patterned silicon wafer

10.1021/jp805095w CCC: $40.75  2008 American Chemical Society Published on Web 10/17/2008

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SCHEME 1: Schematic Experimental Procedure for the Preparation of CuTCNQ Nanostructures

master (IMS Stuttgart, before first use master was rendered hydrophobic with fluoroalkyl-trichlorosilane vapor). Printing of copper patterns: copper (or chromium/copper) layers on hydrophobic stamps were deposited by thermal evaporation (0.1 nm/s). The thickness of metal was controlled by deposition time. A conformal contact between a copper-coated stamp and an MPTMS-modified silicon substrate was initiated and kept for 1 min, and then the stamp was peeled off from the substrate. Because of the stronger interaction between copper and the substrate, copper patterns were easily transferred to the substrate. Scheme 1 schematically describes the experimental procedure for the controlled synthesis of CuTCNQ nanostructures. The patterned copper layer with a thickness of 70 nm (sample A), 65 nm (sample B), 37 nm (sample C), and 26 nm (sample D) respectively on silicon (100) was fabricated using microcontact printing technique.11 In a typical experiment, the substrates were kept in a glass vessel. TCNQ powders were put into the bottom of the glass vessel and then rapidly heated to 150 °C under nitrogen gas flowing rate about 40 standard cubic centimeters per minute. Finally, the patterned copper layer was reacted with TCNQ and formed a layer of blue-black film of complex on the surface of silicon (100). The morphology of the film was investigated by scanning electron microscope (SEM), energydepressive spectrum (EDS), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) spectra. Results and Discussion Figures 1 and 2 show the SEM images of CuTCNQ complex in different shapes controlled by adjusting the thickness and morphologies of copper patterns. The CuTCNQ salt can form large area nanorods arrays by copper patterns on silicon (100) that have been confirmed by SEM measurements. The SEM images (parts A-F of Figure 1) show CuTCNQ nanorod arrays grown of a thickness of about 70 nm copper pattern on silicon (100) (sample A). The EDS results show that the CuTCNQ nanorods are composed of copper, carbon, and nitrogen elements (Figure S1 of the Supporting Information). The structure of the as-grown CuTCNQ nanorods on silicon (100) were determined by FTIR and XPS spectra. The FTIR spectrum of the CuTCNQ nanorods (part A of Figure 3) provides evidence for bonding between copper and TCNQ. The nanorods exhibit typical CuTCNQ absorption bands12 at 825, 1182, 1354, 1507, 1579,

2202, and 2169 cm-1, respectively. The as-grown CuTCNQ nanorods show strong stretches at 2196 cm-1 (ν CtN). Compared with spectrum of pure TCNQ molecule, the spectrum of CuTCNQ nanorods shows obvious changes. The absorption at 825 cm-1 is consistent with the presence of TCNQ- and not TCNQ and TCNQ2- or mixed-valence stacks of TCNQ- and TCNQ,12 which is very sensitive to the changes in oxidation state. The absorption of CN group at 2202 cm-1 indicates that the CuTCNQ nanorods is a pure-phase I of CuTCNQ.13 The formation of CuTCNQ nanorods was further confirmed by the XPS spectra (parts B and C of Figure 3). In part B of Figure 3, the 2p3/2 and 2p1/2 signals (932.7 and 952.7 eV) exhibit essentially identical binding energies for copper 2p orbital in accord with copper(I).14 Likewise, the N1s orbital appearing as a single feature at 399.0 eV is indicative for only one type of TCNQ (part C of Figure 3). 15 The results indicated unambiguously that the nanorods were composed of copper and TCNQ. Most interestingly, different sizes of CuTCNQ nanorods were formed on the different shaped copper patterns even when the thickness of copper layer was kept constant. Part A of Figure 1 shows the morphologies of CuTCNQ complex on small squared patterns (φ: ca. 0.9 µm), and part B of Figure 1 is the corresponding high-resolution image. The oriented nanorods are fairly uniform in length but the diameter is smaller at the tip of nanorods than their bottom position. The CuTCNQ salt patterns have a flower-like structure, and the nanorods are petals of the flower, whereas the lengths of nanorods (ca. 500 nm) at the center are longer than that of nanorods (ca. 300 nm) at the brim. As shown in parts C and D of Figure 1, the nanoleaves were formed on the bigger square patterns (φ: ca. 5 µm), which are different from those CuTCNQ nanorods formed on the small square copper pattern and the linear copper pattern. The lengths of CuTCNQ nanorods (ca. 500 nm) grown on small square copper pattern are shorter than that of CuTCNQ nanorods (ca. 1 µm) grown on linear copper pattern. The CuTCNQ salt formed longer nanorods that rested against each other at the center of the linear pattern (parts E and F of Figure 1), whereas they grew paralleled to the brim. The lengths of those nanorods (ca. 1 µm) at the center are longer than those of at the brim (ca. 600 nm). In fact, the result of growth of nanorods on different patterns indicates that the size and shape of nanorods are able to adjust by the shape and thickness of the copper pattern.

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Figure 2. SEM images of CuTCNQ nanomaterials grown from the thickness of about 37 nm copper big squared pattern on silicon (100) (sample C): (A) small square copper pattern, (B) big square copper pattern, (C) linear copper pattern. The thickness of about 26 nm copper pattern on silicon (100) (sample D): (D) small square copper pattern, (E) linear copper pattern, (F) big square copper pattern.

Figure 1. SEM images of CuTCNQ nanorods grown from the thickness of about 70 nm copper pattern on silicon (100) (sample A): (A-B) small square copper pattern (φ: ca. 0.9 µm), (C-D) big square copper pattern (φ: ca. 5 µm), (E-F) linear copper pattern. The thickness of about 65 nm copper small squared and big squared pattern on Si(100) (sample B): (G-H) small square copper pattern, (I) big square copper pattern, (J) linear copper pattern.

Namely, the morphologies of CuTCNQ nanorods could be controlled by the modulation of copper patterns. The shapes of CuTCNQ salt present varied (parts G-J of Figure 1) with decreasing the thickness of copper pattern to 65 nm (sample B). Part G of Figure 1 shows a typical image of the nanorods that were formed on the small pattern with diameter and length of 50-120 and 400-800 nm respectively, and the low magnification SEM images are shown in part H of Figure 1. The CuTCNQ nanorods with diameter and length of 20-80 and 100-500 nm also were formed on the big square pattern (part I of Figure 1). However, to a linear pattern, only the CuTCNQ nanowires with diameter of about 50 nm were formed (part J of Figure 1). The observations show that it is very different on formation of size and shape of nanostructures on the square patterns and the linear pattern. When the layer thickness of copper pattern is decreased to about 37 nm (sample C), the morphologies of CuTCNQ salt varied with the shape of copper pattern (parts A-C of Figure 2). The nanoparticles with diameter of about 110 nm were

formed and aggregated on the small square pattern, in which CuTCNQ nanoparticles themselves were organized with a regular spacing resulting in a hierarchical self-assembly of nanoparticles on the pattern (part A of Figure 2). The nanotips with a diameter of about 30 nm were fabricated on the bigger square patterns as shown in part B of Figure 2. Some arrays of CuTCNQ nanotips were fabricated with the preservation of a short-range square order. In the case of linear pattern, some flakelike and rodlike shapes were present (part C of Figure 2). As shown in parts D-F of Figure 2, the CuTCNQ nanoparticles were formed on linear and square pattern using a thickness of about 26 nm copper patterns (sample D), whereas the diameters of those nanoparticles are different. The diameter of CuTCNQ nanoparticle grown on small square pattern is about 160 nm (part D of Figure 2), whereas the diameter of CuTCNQ nanoparticle grown on linear pattern and big square pattern is about 90 nm (part E of Figure 2) and 270 nm (part F of Figure 2), respectively. As shown above, CuTCNQ nanorods mainly show a vertical growth on the copper pattern surface. We are able to further control the growth orientation of CuTCNQ nanorods by depressing the vertical growing with a multilayer metal structure. The pattern design and lateral growing CuTCNQ nanowires is showed in part A of Figure 4. The lateral growth process is described as follows: First, a 30 nm chromium layer was deposited on a stamp; then an 80 nm copper layer was deposited on the top of chromium layer. After printing, the top of copper patterns was covered with the chromium layer. We hoped that, by covering a resistant layer on the top of the copper patterns, the vertical growth could be largely inhibited, whereas the lateral growth becomes dominate. In fact, the CuTCNQ nanowires can indeed grow on lateral range on surfaces of square and linrar patterns respectively as presented in parts B and C of Figure 4. Controlled growth of CuTCNQ nanorods can be explained as follows: TCNQ vapor generated through heating TCNQ

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Figure 3. (A) FTIR spectrum of CuTCNQ nanorods (sample A) (black line) and TCNQ powders (blue line). XPS data in the Cu 2p3/2 and 2p1/2 (B), and N1s (C) for CuTCNQ nanorods (sample A).

powders was transported in a flow reactor to a growth zone where the organic vapor solid reaction of TCNQ and copper occurred for forming CuTCNQ agglomerates to nanometer-scale nuclei on the surface of copper pattern. The initial CuTCNQ aggregates as the nucleation centers on the copper pattern surface determine the growth of CuTCNQ nanorods. The copper atoms on the surface of copper pattern diffuse and react with TCNQ molecules carried continuously to the surface of CuTCNQ nuclei by nitrogen gas flow. Then the CuTCNQ nanorods start to grow from the nucleation sites. The size of nanorods grown from these patterns varies with the thickness of copper patterns. In the case of the thin copper pattern, only the CuTCNQ nanoparticles were formed because the quantity of copper atoms on the surface of pattern is less than that of thick copper pattern. While increasing the thickness of copper pattern, the quantity of copper atoms increased, and the CuTCNQ nuclei gradually grow for formation more size nanorods. Because the CuTCNQ at the brim of pattern can grow along both the vertical and horizontal directions and those copper were easily used up, the length of nanorods grown

Liu et al. from the center of pattern is longer than those grown from the brim of the pattern. Our experiments show that the difference on the center and the brim is less with deceasing the diameter size of the same thickness of pattern. Therefore, the fairly uniform nanorods were easily formed from the small pattern. Namely, we are able to change the size and shape of CuTCNQ complex by controlling the thickness and size of copper pattern on the surface of silicon (100). As expected from the CuTCNQ nanorods, excellent field emission properties were observed for the CuTCNQ nanorods films. Part A of Figure 5 shows typical plots of the field emission current density versus the applied electric field of the CuTCNQ nanorods films obtained from different thickness copper pattern on the surface of silicon (100). Those CuTCNQ nanomaterials exhibit a turn-on field of 7.67 V µm-1 (for sample A), 7.1 V µm-1 (for sample B), 8.7 V µm-1 (for sample C), and 8.51 V µm-1 (for sample D), (part A of Figure 5), respectively, which are defined to be the macroscopic fields required to produce a current density of 10 µA cm-2. Although these values are higher than the best data from carbon nanotubes16 and SiC nanowires,17 they are lower than those of organic semiconductor nanowires18 and are similar to many other inorganic nanomaterials.19 The fact that the field emission of the sample B is better than that of other samples due to the difference shape and unit area density of CuTCNQ nanorods. From Figures 1 and 2, samples C and D consist of CuTCNQ nanoparticles, whereas the samples A and B are CuTCNQ nanorods. It is well-known that the field emission of nanorods is better than that of nanoparticles. Therefore, the CuTCNQ nanorods (samples A and B) exhibited a better property of field emission with a lower turn-on field, whereas samples C and D containing nanoparticles show a higher value of the turn-on field. The turn-on field of sample B is lower than that of sample A, which may originate from the difference of unit area density of uniform CuTCNQ nanorods, whereas samples C and D almost are the same value of turn-on field. Samples A, B, C, and D exhibit threshold fields of 14.5, 13.8, 17, and 16 V µm-1 respectively, which is defined to be the macroscopic fields requires to produce a current density of 10 mA cm-2. The maximum current density of sample A is 23 mA cm-2 at an applied field of 16 V µm-1, which is higher than that of organic semiconductor nanomaterials18 and many inorganic semiconductors19 and is almost same as that of amorphic diamond.20 As field emission materials, CuTCNQ nanomaterials have the advantage over other nanomaterials that they were prepared at low temperature, benchtop experimental conditions. The emission characteristics were analyzed using the Fowler-Nordheim model described as follows:21

J ) (Aβ2E2 ⁄ Φ) exp(-BΦ3⁄2βE) 10-6

V-2,

(1) eV-3/2

where A ) 1.54 × A eV B ) 6.83 × µm-1, Φ is the work function of the emitter, and β is the field enhancement factor. As shown in part B of Figure 5, for the four samples, the variation of ln(I/V2) with (1/V) is a rough straight line, indicating that field emission process from the CuTCNQ nanomaterials is a quantum tunneling process. Assuming Φ ) 2.77 eV for CuTCNQ,10 the field enhancement factors β are 346, 463, 269, and 318 for samples A, B, C, and D, respectively. The value of β relates to the structure, shape, size, alignment, crysallinity, aspect ratio, and so forth.19 The β values of samples A and B are higher those that of sample C and D. This mainly originates in the rodlike shape of samples A and B, which results in better field emission property. It is generally accepted that the intrinsic field enhancement of an 103

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Figure 4. (A) Schema for lateral growing CuTCNQ nanowires. (B-C) SEM images of CuTCNQ nanorods laterally grown on chromium (30nm)/ copper (80nm) bilayer patterns on silicon (100).

Conclusion In summary, we have demonstrated the ability to tune the morphologies of organic charge transfer complex (CuTCNQ) nanomaterials by controlling the shape and thickness of copper patterns on silicon (100). The results point to thick copper patterns (70 and 65 nm) generating CuTCNQ nanorods and thin patterns (37 and 26 nm) providing CuTCNQ nanoparticles. This method allows for the fabrication of other conducting nanorods. Excellent field emission properties were observed in these nanomaterials. The field emission current density of those nanomaterials is higher than that of organic semiconductor nanomaterials and many inorganic semiconductor nanomaterials. They should have great potential in vacuum device applications. Acknowledgment. The authors are most grateful to Professor Sishen Xie for his insight and encouragement. This work was supported by the National Natural Science Foundation of China (20531060, 20571078, and 20721061), the National Basic Research 973 Program of China (grants nos. 2006CB932100 and 2005CB623602), and by “Ministerium fu¨r Wissenschaft und forschung” of North Rhine-Westphalia (NRW), Germany. This project is partly supported by National Center for Nanoscience and Technology, China. Figure 5. (A) Field emission J-E curve of the CuTCNQ nanomaterials grown from different thickness copper pattern on silicon (100). (B) The corresponding FN plots.

individual nanorods is approximately proportional to the aspect ratio L/R, where L and R are the length and radius of the nanorods, respectively. In this work, the aspect ratio (L/R) of sample B is about 10, which is higher than that of sample A (ca. 8). Therefore, the β value of sample B is higher than that of sample A.

Supporting Information Available: EDS of sample A, AFM images of sample A, small square pattern and linear pattern, and A 30 nm chromium layer was deposited on a stamp, then an 80 nm copper layer was deposited on the top of the chromium layer. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) McEuen, P. L. Nature 1998, 393, 15. (b) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (c) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947.

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