Tuning Morphologies and Field-Emission Properties of CuTCNQF4

Apr 1, 2009 - CAS Key Laboratory of Organic Solid, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy...
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J. Phys. Chem. C 2009, 113, 7044–7051

Tuning Morphologies and Field-Emission Properties of CuTCNQF4 and AgTCNQF4 Nanostructures Canbin Ouyang,†,‡ Yanbing Guo,†,‡ Huibiao Liu,*,† Yingjie Zhao,†,‡ Guoxing Li,†,‡ Yongjun Li,† Yinglin Song,§ and Yuliang Li*,† CAS Key Laboratory of Organic Solid, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China, Graduate UniVersity of Chinese Academy of Sciences, Beijing 100190, P. R. China, School of Physical Science and Technology, Suzhou UniVersity, Suzhou, Jiangsu ProVince, 215006, P.R. China ReceiVed: December 23, 2008; ReVised Manuscript ReceiVed: March 4, 2009

The organic charge-transfer (CT) complexes nanostructures of CuTCNQF4 and AgTCNQF4 (copper/silver tetrafluorotetracyanoquinodimethane) were successfully fabricated by organic solid-phase reaction. The morphologies of CuTCNQF4 and AgTCNQF4 nanostructures can be easily tuned by controlling the reaction temperature. The electron field-emission properties on these nanostructures were investigated. The current density and turn-on field are 4.25 mA/cm2 and 5.48 V/µm for the nanowires of CuTCNQF4, and 1.82 mA/ cm2 and 5.21 V/µm for AgTCNQF4, respectively. The results demonstrated that the nanostructures of complexes CuTCNQF4 and AgTCNQF4 were potential candidates for the field-emission cathode. Introduction Small-molecule organic conductors and semiconductors as bulk materials have attracted wide attention. These organic complexes have been identified to have potential in molecular electronic devices, optical switching, and nonlinear optical devices.1-5 Organic charge-transfer (CT) complexes have been intensely studied because of their quasi-one-dimensional and high electronic conductivity which can control many physical properties.6-8 In particular, CT complexes with well-defined architecture have shown prominent merits over their bulk counterparts for applications in electrical and optical memory devices, sensors, and magnetic devices.9-11 However, the studies on the fabrication of nanostructures and properties of CT complexes on nanoscale are still limiting. Tetracyanoquinodimethane (TCNQ)-based solid-state chemistry represents an area of considerable current interest in the field of materials science.12-14 Although successful attempts to impart TCNQbased CT complexes devices with high-quality performance, a few methods for development are the exploration of materials based on other acceptors,15-17 which may significantly enhanced the understanding of their intriguing physical and chemical properties, such as electronic, magnetic and optic etc. However, there are still challenges on how to develop new methods and use new molecules for their growth on nanoscale, meanwhile efficiently integrate them into a unique property on the surface of the complex. An active area of research has been synthesis and fabrication of low-dimensional organic semiconductors for applications in field-emission cold cathodes.18-23 Recently, we reported the field-emission properties of metal-organic CT complexes of CuTCNQ and AgTCNQ and CuTCNAQ nanostructures.21,23 The outstanding current density and the low turn-on field showed the great potential applications of organic CT complexes in fieldemission flat displays.24 These results encourage us to synthesize * Authors for correspondence. E-mail: [email protected]; [email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences. § Suzhou University.

new structures of metal-organic CT complexes for applications in further field emitters. Tetrafluorotetracyanoquinodimethane (TCNQF4) has a practicality structure with four fluorine atoms which has shown more efficient electron-acceptor properties than TCNQ and been extensively studied on preparation of charge transfer complexes with organic and metal electron donor and physical properties such as conductivity,25 semiconductor,26,27 magnetism28 and switches29 as bulk materials. However, studies on growth of nanostructures and their field emission properties based on complexes nanostructures arrays of tetrafluorotetracyanoquinodimethane (TCNQF4) have not been reported. Herein, we develop a facile way for fabrication of CuTCNQF4 and AgTCNQF4 complexes films on the nanometer scale by tuning the reaction temperature. Field-emission measurements show that the nanostructures films of CuTCNQF4 and AgTCNQF4 complexes are excellent potential candidates for field-emission cathodes. Experimental Section TCNQF4 was synthesized by previous synthetic routes30 and recrystallized from CH2Cl2. Copper and silver foils were used after being cleaned in an ultrasonic bath of acetone for 20 min, 0.1 M HCl for 15 min, deionized water for 10 min, and ethanol for 10 min. CuTCNQF4 and AgTCNQF4 nanostructures with large area of 1 × 2 cm2 were prepared by combination of controlling the time and temperature of organic solid-phase reaction by which the size and shape could be easily tuned for uniformity. In a typical procedure, 2.0 mg of TCNQF4 powder was loaded into a ceramic boat and then placed at the center of a quartz tube (800 mm in length and 35 mm in inner diameter) which was inserted into a horizontal tube furnace. The copper or silver foil was placed on the top of the ceramic boat. The powder of TCNQF4 was evaporated and then deposited onto the surface of copper or silver foil at appropriate temperature for 30 min. Finally, the reaction temperature was allowed to cool to room temperature. In the growth process of CuTCNQF4 or AgTCNQF4 complex nanostructures, the reaction temperature was kept at 170, 200, and 220 °C, respectively, with a increasing

10.1021/jp8113545 CCC: $40.75  2009 American Chemical Society Published on Web 04/01/2009

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Figure 1. (a) FT-IR spectra of TCNQF4 solid and films of CuTCNQF4 and AgTCNQF4 complexes prepared at 200 °C; (b) Raman spectra of TCNQF4 solid and films of CuTCNQF4 and AgTCNQF4 complexes prepared at 200 °C.

Figure 2. XPS spectra of (a) Cu 2p and (c) N 1s for CuTCNQF4 complex film prepared at 200 °C; XPS spectra of (b) Ag 3d and (d) N 1s for AgTCNQF4 complex film 200 °C.

rate of 6 °C/min, and the argon gas flowing rate was kept 40 standard cubic centimeters per minute (sccpm). The nanostructures of CuTCNQF4 and AgTCNQF4 were characterized by Fourier transform infrared (FT-IR) spectra, (KBr pellets on a Perkin-Elmer System 2000 spectrometer). FTIR spectrum of TCNQF4 was performed by using a pressed sheet of the mixture of TCNQF4 powder and KBr; FT-IR spectrum of CuTCNQF4 or AgTCNQF4 was performed by using films of CuTCNQF4 or AgTCNQF4 nanostructures grown on the surface of copper or silver substrate at 200 °C. Raman spectra were taken on a Renishaw-2000 Raman spectrometer at resolution of 2 cm-1 by using the 514.5 nm line of an argon ion laser as the excitation source. The chemical composition of CuTCNQF4 or AgTCNQF4 films grown on the surface of copper or silver substrate at 200 °C was confirmed by X-ray photoelectron spectroscopy (XPS) (ESCALab220I-XL). Scanning electron microscopy (SEM) images were obtained with a JEOL JSM 6700F field-emission scanning electron microscope. Transmission electron microscopy (TEM) images, the selected area

electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS) were taken with a JEOL 2010 microscope operating at 200 keV. The field-emission property of CuTCNQF4 or AgTCNQF4 nanostructures films was measured using a two-parallel-plate configuration in a homemade vacuum chamber at a base pressure of ∼1.0 × 10-6 Pa at room temperature. The sample was attached to one of two stainless-steel plates as a cathode with the other plate as the anode. The distance between the electrodes was 300 µm. A dc voltage sweeping from 0 to 5000 V was applied to the sample at a step of 50 V. The emission current was monitored using a Keithley 6485 picoammeter. Results and Discussion Figure 1a shows the FT-IR spectra of films of CuTCNQF4 and AgTCNQF4 complexes obtained at a reaction temperature of 200 °C, which provide evidence for bonding Cu/Ag and TCNQF4. Compared with the spectrum of pure TCNQF4

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Figure 3. SEM images of CuTCNQF4 (a) NCs, (b) higher magnification of NCs, (c) NRs, (d) higher magnification of typical nanorod, (e) NWs, (f) higher magnification of NWs.

molecule, the spectra of AgTCNQF4 and CuTCNQF4 complexes show obvious changes. CuTCNQF4 and AgTCNQF4 show strong stretches at 2215 cm-1 and 2213 cm-1, respectively, whereas the nitrile group stretch of pure TCNQF4 molecule is 2225 cm-1. The bands located at 1213 cm-1 for CuTCNQF4 and 1206 cm-1 for AgTCNQF4 can be assigned to C-F bending out of plane, which are shifted to higher wavenumbers by about 21 cm-1 and 14 cm-1, respectively, compared to that of TCNQF4, which suggests formation of a different oxidation state.31 Raman shift generally is used to estimate the degree of charge transfer between the donor (metal) and acceptor (TCNQ/ TCNQF4).31 Figure 1b provides a comparison of the Raman spectra of TCNQF4 powder and films of CuTCNQF4 and AgTCNQF4 complexes prepared at 200 °C. The three typical Raman vibrations (2227, 1664, and 1461 cm-1) present in the TCNQF4 spectrum include one at 1461 cm-1, which corresponds to the C)C ring-stretching mode of TCNQF4. This band is shifted by 10 cm-1 (CuTCNQF4) and 14 cm-1 (AgTCNQF4) as a result of reduction of TCNQF4 into [TCNQF4]- and formation of Cu/AgTCNQF4 complexes.32 Furthermore, there was a shift of 7 cm-1 at the 2220 cm-1 band (C≡N stretching), with a reducing intensity,32 also provides a spectroscopic fingerprint for the formation of Cu/AgTCNQF4 complexes.

The success of the preparing of CuTCNQF4 and AgTCNQF4 complexes are further confirmed by the XPS spectra (Figure 2). Panels a and b of Figure 2 present the spectra of Cu 2p and Ag 3d, respectively. The signals of 932.2 and 952.2 eV exhibit essentially identical binding energies for the Cu 2p1/2 and 2p3/2 orbits, which are consistent with the presence of Cu+. The 3d3/2 and 3d5/2 signals at 367.7 and 373.7 eV show essentially identical binding energies for the Ag 3d orbit, which are consistent with the presence of Ag+. In panels c and d of Figure 2, the peaks of N 1s at 398.6 eV for CuTCNQF4 and 398.5 eV for AgTCNQF4 are different from pure TCNQF4 of 399.8 eV. The result indicates having only one type of valence state of TCNQF4 in CuTCNQF4 and AgTCNQF4. The quantitative analysis based on the areas of the peaks give the atomic ratio of F:N:Cu/Ag as 4:4:1 in complexes of CuTCNQF4 and AgTCNQF4. These XPS spectra clearly demonstrate the successful preparation of complexes of CuTCNQF4 and AgTCNQF4 nanostructures. The typical characteristics of the CuTCNQF4 and AgTCNQF4 nanostructures are demonstrated by SEM. The morphologies of CuTCNQF4 and AgTCNQF4 nanostructures are controlled by adjusting the reaction temperature at 170, 200, 220 °C, respectively, at a rising rate of 6.0 °C/min. As shown in Figure 3a, the CuTCNQF4 nanostructure produced at 170 °C shows column-like morphology. Figure 3b displays that the diameter

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Figure 4. TEM images of CuTCNQF4 (a) NRs. (b) High magnification of the single nanorod. (b: Inset) Corresponding SAED pattern. (c) NWs. (d) High magnification of the single nanowire. (d: Inset) Corresponding SAED pattern.

and length of CuTCNQF4 nanocolumns (NCs) are about 200 and 400 nm, respectively. Figure 3c shows that a large quantity of vertical nanorods (NRs) of CuTCNQF4 was fabricated at 200 °C. The uniform diameter and length of CuTCNQF4 NRs are about 120 nm and 1 µm, respectively. The results of EDS reveal that the CuTCNQF4 NRs are composed of copper, carbon, fluorine and nitrogen elements (see Supporting Information, Figure S1a). The quantitative analyses of EDS demonstrate that the atomic ratio of F:N:Cu is about 4:4:1. These EDS spectra clearly confirm the success of preparing CuTCNQF4 complexes. Figure 3d displays that the diameter of typical nanorods is 110 nm. Figure 3e shows that the lengths of CuTCNQF4 nanowires (NWs) prepared at 220 °C are in the range of 3-10 µm. Figure 3f indicates that the diameter of CuTCNQF4 NWs is about 80 nm. Further characterizations of the nanostructures are performed by TEM shown in Figure 4. Figure 4a indicates that the diameter and length of CuTCNQF4 NRs are about 110 nm and 1-2 µm, respectively. Figure 4b shows the TEM image of a typical single nanorod with diameter of 110 nm. The corresponding SAED pattern (the inset of Figure 4b) reveals that the CuTCNQF4 NRs are polycrystalline. Figure 4c shows that the diameter of CuTCNQF4 NWs is about 80 nm; the length is about several micrometers. Figure 4d displays that the diameter and length of a typical single nanowire are 80 nm and 5 µm, respectively. As shown in the inset of Figure 4b, the CuTCNQF4 NWs are also polycrystalline. Figure 5 shows the morphologies of AgTCNQF4 nanostructures prepared at different reaction temperatures. SEM images a and b of Figure 5 display the cake-like structures of AgTCNQF4 formed at 170 °C, and their widths are several micrometers. Increasing the reaction temperature to 200 °C, a large quantity of AgTCNQF4 NRs were fabricated (Figure 5c). The length of AgTCNQF4 NRs is up to 10 µm. Figure 5d shows that the diameter of AgTCNQF4 NRs is about 700 nm and the surface of AgTCNQF4 NRs is smooth. The results of EDS reveal that the AgTCNQF4 NRs are composed of silver, carbon,

fluorine, and nitrogen elements (see Supporting Information, Figure S1b). The quantitative analyses of EDS demonstrate that the atomic ratio of F:N:Ag is about 4:4:1. These EDS spectra clearly confirm the success of preparing AgTCNQF4 complexes. When the reaction temperature was increased to 220 °C, the AgTCNQF4 NWs were prepared (Figure 5e). The length of AgTCNQF4 NWs is several tens of micrometers, and the diameter is in the range of 200-300 nm (Figure 5f). Figure 6 shows TEM images of AgTCNQF4 nanostructures. As shown in Figure 6a, the length of AgTCNQF4 NRs is from 5 to 10 µm, and the average diameter is 700 ( 30 nm. Figure 6b reveals that the diameter of a typical, single AgTCNQF4 NR is 700 nm. Figure 6c shows that the diameters of AgTCNQF4 NWs are uniform. The diameter of a typical single AgTCNQF4 NW is 220 nm. The corresponding SAED patterns demonstrate that the AgTCNQF4 NRs (the inset of Figure 6b) and NWs (the inset of Figure 6d) are amorphous. On the basis of experimental results, we propose the growth processes of CuTCNQF4 and AgTCNQF4 nanostructures on the surface of copper/silver foil (Scheme 1). Upon heating, TCNQF4 vapor is generated and transported to the surface of copper/silver foil. TCNQF4 vapor impinges on the surface of copper/silver foil and the organic vapor-solid-phase reaction takes place, leading to the formation of Cu/AgTCNQF4 complexes, which lead to the nucleation of Cu/AgTCNQF4. Diffusion of TCNQF4 and copper/ silver atoms toward the nucleation sites triggers the kinetically controlled growth along one direction. When the temperature is lower, the reaction rate of Cu/Ag with TCNQF4 is slow. Therefore, the rate of crystal growth (Vcg) is almost equal to the rate of diffusion (Vd), which results in the formation of CuTCNQF4 NCs and AgTCNQF4 NKs. By increasing the reaction temperature, the reaction rate of Cu/Ag with TCNQF4 increases, and then the crystal growth is relatively fast with respect to the diffusion rate, which leads to the formation of CuTCNQF4 and AgTCNQF4 NRs. By continuing to increase the reaction temperature, the CuTCNQF4 and AgTCNQF4 NRs grow to NWs. As a result, the morphologies

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Figure 5. SEM images of AgTCNQF4 (a) nanocakes (NKs), (b) higher magnification of NKs, (c) NRs, (d) typical single nanorod, (e) NWs, (f) higher magnification of NWs.

of CuTCNQF4 and AgTCNQF4 nanostructures are able to be controlled by tuning the temperatures of the organic solid-phase reaction. The unique morphology and geometric shape of CuTCNQF4 and AgTCNQF4 make them promising for field-emission applications. In this work, the field emission properties of films of CuTCNQF4 and AgTCNQF4 nanostructures are demonstrated for the first time. Plots in Figure 7, a and c, show typical plots of the field-emission current density (J) versus the applied electric field (E) of the films of CuTCNQF4 nanostructure. The results of field-emission properties of CuTCNQF4 and AgTCNQF4 nanostructures are summarized in Table 1. Here, we define the turn-on field (Eto) and the threshold field (Ethr) as the electronic fields required to produce a current density of 10 µA/cm2 and 1 mA/cm2, respectively. For the CuTCNQF4 NCs, the Eto is 8.26 V/µm, and the Eto of CuTCNQF4 NRs and NWs decrease to 5.96 V/µm and 5.48 V/µm, respectively, which are lower than those of CuTCNQF4 NCs. The maximal current densities of CuTCNQF4 NCs, NRs, and NWs are 3.5, 3.5, 4.3 mA/cm2, respectively. For the CuTCNQF4 NCs, the Ethr is 12.6 V/µm, and the Ethr of CuTCNQF4 NRs is 11.2 V/µm. The Ethr of CuTCNQF4 NWs is 7.43 V/µm, which is lower than those of CuTCNQF4 NCs and NRs. The results indicate that the fieldemission properties of CuTCNQF4 NWs are much better than those of NRs and NCs, which is due to the diameter of NWs

being less than those of NRs and NCs (Figure 3). For the AgTCNQF4 NKs, the Eto is 12.1 V/µm, the maximal current density is only 255 µA/cm2. The Eto of AgTCNQF4 NRs and NWs decrease to 6.97 V/µm and 5.21 V/µm, respectively. The Ethr of AgTCNQF4 NRs and NWs are 10.3 V/µm and 13.4 V/µm, respectively. The results indicate that the field-emission properties of AgTCNQF4 NWs possess more efficiency than those of NRs and NCs, which is due to the diameter of AgTCNQF4 NWs being less than those of AgTCNQF4 NRs and NKs. As shown in Table 1, the results reveal that the fieldemission properties of CuTCNQF4 and AgTCNQF4 NWs are comparable to many organic nanomaterials.21-23,33 The field emission characteristics were further analyzed with the Fowler-Nordheim (F-N) theory34,35 described by

J ) E2loc exp(-6.8 × 107φ3/2 /Eloc)

(1)

Here, J is the current density from the emitting tip, Eloc is local electric field, and φ is work function of the sample. For an isolated hemisphere model:

Eloc ) V/RRtip

(2)

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Figure 6. TEM images of AgTCNQF4 (a) NRs. (b) High magnification of the single nanorod and the inset is the corresponding SAED pattern. (c) NWs. (d) High magnification of the single nanowire and the inset is the corresponding SAED pattern.

SCHEME 1: Schematic Proposed Growth Processes of CuTCNQF4 and AgTCNQF4 Nanostructures

Here, V is the applied voltage, Rtip is the tip radius of curvature, and R is a modifying factor. From the above equations, we get

ln(I/V2) ) 1/V(-6.8 × RRtipφ3/2) + offset

1/E (Figure 7b and d). The field-emission enhancement factor β is the inverse ratio of absolute value of the slope (S) of the F-N plot.34

(3)

RRtip can be estimated from the slope of the F-N plot (the insets of Figure 7a, c) of ln(I/V2) against (1/V). The plot of ln (I/V2) versus (1/V) yields a straight line, implying that a quantumtunneling process is responsible for them. Taking R ) 10 (as used in another report36), the Rtip of CuTCNQF4 NRs is 55 nm, and the evaluated work function is about 2.39 eV, whereas Rtip ) 350 nm for AgTCNQF4 NRs, the evaluated work function is around 1.07 eV, which is much lower than not only that of carbon nanostructures37 but also other important inorganic materials such as silicon nanostructures,38 ZnO,39 etc.40 For CuTCNQF4 NCs and NWs, taking φ ) 2.39 eV, we estimated the enhancement factor β by plotting ln(J/E2) versus

S ) -6.84 × 103φ3/2 /β

(4)

The CuTCNQF4 NWs has the lowest slope (S) value (35.6) compared to those of the NRs (46.0) and NCs (84.6), which indicates the β of NWs is higher than those of NRs and NCs. The AgTCNQF4 NWs have the lowest S (18.0) compared to those of the NRs (46.5) and NKs (103). The field-emission enhancement factor β is not only the inverse ratio of absolute value of the slope (S) of the F-N plot, but also the emitter radius (Rtip). As shown in Table 1, the S and Rtip of AgTCNQF4 NWs are less than those of NRs and NKs, which result in the excellent field-emission property of AgTCNQF4 NWs. For the CuTCNQF4 nanostructures, the Rtip of NWs is

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Figure 7. (a) Field emission J-E curve and (b) the corresponding F-N plot of CuTCNQF4 nanostructures; (c) Field emission J-E curve and (d) the corresponding F-N plot of AgTCNQF4 nanostructures.

TABLE 1: Results of Field Emission Properties of CuTCNQF4 and AgTCNQF4 Nanostructures

References and Notes

sample

Rtip (nm)

Eto (V/µm)

Ethr (V/µm)

S

CuTCNQF4 NCs CuTCNQF4 NRs CuTCNQF4 NWs AgTCNQF4 NKs AgTCNQF4 NRs AgTCNQF4 NWs

65 55 40 800 350 110

8.26 5.96 5.48 12.1 6.97 5.21

12.6 11.2 7.43 s 10.3 13.4

-84.6 -46.0 -35.6 -103 -56.5 -18.0

less than that of NRs, the aspect ratio of CuTCNQF4 NWs is larger than that of NRs and NCs, which leads to the fieldemission property of CuTCNQF4 NWs being better than those of NRs and NCs.41 Conclusion In summary, the metal-organic charge-transfer complexes of CuTCNQF4 and AgTCNQF4 have been successfully synthesized through organic solid-phase reaction. The morphologies of CuTCNQF4 and AgTCNQF4 nanostructures were able to be tuned facilely by adjusting the reaction temperature. The field emission properties of CuTCNQF4 and AgTCNQF4 nanostructures with various morphologies were investigated. The results revealed that the field-emission properties of CuTCNQF4 and AgTCNQF4 nanostructures depended on their morphologies and the NWs of CuTCNQF4 and AgTCNQF4 exhibited promising field-emission performance with high current densities and low turn-on fields compared to those of the other nanostructures. These nanostructures could find potential application as novel materials in cold-cathode-based electronics. Acknowledgment. This work was supported by the National Nature Science Foundation of China (20531060, 10874187, 20721061, and 20873155) and the National Basic Research 973 Program of China. Supporting Information Available: EDS of CuTCNQF4 NRs and AgTCNQF4 NRs. This material is available free of charge via the Internet at http://pubs.acs.org.

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