Controlled synthesis of three dimensionally oriented charge transfer salts nanowires arrays

Received on May 25, 2006; Supported by the National Natural Science Foundation of China  (20131040, 20418001, 20473102 and 50372070).

One-dimensional (1D) nanomaterials, such as nanowires, nanorods and nanotubes, have received considerable attention in the last decades for their novel physical and chemical properties, and potential application in many fields^[1]. Nanowires arrays are particularly attractive for high-density magnetic recording media^[2], sensors^[3], lightweight materials microwaveguides^[4], and optoelectronics devices^[5]. Dimensionality and size are two key factors that govern the properties of nanostructures^[6]. The requirement for dimensional control is especially true for one-dimensional (1D) structures, such as nanowires, nanotubes and nanorods, for which the control of nanowires (nanorods, nanotubes) multi-dimensional oriented arrays seems to be particularly difficult. The ability to order them into large multi-dimensional arrays onto various types of substrates represent essential tasks to fulfill in order to create a future generation of smart and functional materials and in the exploitation of the properties of nanomaterials. In order to fabricate 1D nanowires oriented arrays, templates^[7], patterning techniques^[8], epitaxial electrodeposition^[9], or catalysts^[10] are usually required to control the directional growth. Unfortunately, the introduction of templates and catalysts brings heterogeneous impurities and increases the difficulty for scale-up production. Some 3D nanostructural materials were synthesized, those works almost all are focused on the synthesis of inorganic and polymeric nanomaterials^[11]. However exact controllable for growing organic nanomaterials on multidimensional oriented arrays still remain blank. Therefore, the development of a mild, easily controlled, catalyst-free and template-free method to grow 3D nanowires arrays is of great significance.
    As a well-known metallic organic charge transfer (CT) salt, TCNQ (7,7,8,8-tetracyanoquinodimethane) salts based on copper (CuTCNQ), has been studied intensively^[12]. CuTCNQ have exhibited semiconducting properties as well as on/off switching characteristics and memory effects, which are applied for optical and electrical record media^[13]. We have previously explored a synthetic route to produce CuTCNQ nanowires on many substrates by organic vapor-solid reaction and the field emission properties of the nanowires show itself an excellent potential candidate for field emitters^[14]. The results encourage us to possibly organize the CuTCNQ nanowires into 3D nanowires arrays and grow large-scale multi-dimensional nanomaterials directly onto substrates from the molecular scale to the nanoscale by controlling the experimental conditions (such as the temperature of reaction and copper quantum et al.). Recently, we developed a simple method based on the organic vapor-solid reaction for producing new aggregate nanostructures of the complex of TCNQ molecule and copper. The method of organic vapor-solid reaction that we report here has potential as a generic means for fabricating 3D nanowires arrays for various materials, and may also offer an opportunty for further applications.
    We employ a copper layer with thickness of 60 nm deposited on the surface of Si (100) as copper source. The copper layers were prepared by using known methods^[15]. In a typical experiment, the furnace temperature was first increased to 85°C. Then the TCNQ powder is loaded into a ceramic boat and the Si (100) substrate with a copper layer is placed on the top of the ceramic boat, which is placed at the center of a quartz tube. After the quartz tube was inserted into a horizontal tube furnace, the furnace temperature is rapidly increased to 150°C and then kept at that temperature for 1 h. Finally, the temperature was allowed to descend to room temperature. In the process, the argon gas flowing rate was kept at 60 standard cubic centimeters per minute. During the evaporation, the TCNQ vapor reacts with copper of the surface substrate. After the reaction, the Si (100) substrate was coated with a layer blue-black film, that is, constructing of CuTCNQ nanowires. The full characterization data of the products indicate the typical characteristics of CuTCNQ (See supporting information).
    As expected, well-aligned CuTCNQ nanowires 3D arrays are grown on the surface Si(100) and arranged in very uniform arrays (Fig. 1). Fig. 1A shows a general view of the as-prepared CuTCNQ 3D nanowires arrays on the surface of Si(100). Fig. 1B shows the SEM images with higher magnifications. It is interested in that those nanowires formed pyramid-like bundles by the top of nanowires adhering to each other, and a new nanowire grew form the top of pyramid-like bundle, which seemed an antenna on the top of pyramid. Those bundles aligned to oriented 3D pyramid with antenna arrays on the surface of Si(100). From the typical area of CuTCNQ 3D nanowires arrays (Fig. 1C), those nanowires exhibited a uniform diameter of about 40 nm. The top of bundles consisted of a block of CuTCNQ, and the nanowires formed bundles by the block conglutinated with one another, also the antenna-like nanowires grew from the block with a uniform diameter and length of about 40 nm and 700nm, respectively (Fig. 1D). The results of energy-depressive spectrum (EDS) shows that the CuTCNQ nanowires are composed of copper, carbon and nitrogen elements (Fig. 2). The products were also characterized by FT-IR spectra, which provide evidence for bonding Cu and TCNQ. CuTCNQ nanowires exhibit a strong, broad n (C≡N) absorption band at 2202 cm^-1. Compared with the spectrum of pure TCNQ molecule, the spectrum of CuTCNQ complex shows an obvious change. The absorption at 825 cm^-1 is consistent with the presence of TCNQ^- and not TCNQ, TCNQ^2- or mixed-valence stacks of TCNQ^- and TCNQ, ^[14] that is very sensitive to changes in oxidation state. The absorption of CN group at 2202 cm^-1 indicates that the CuTCNQ nanowires is phase I of CuTCNQ.^[14]

Fig. 1 SEM images of CuTCNQ 3D nanowires arrays as-grown on the surface of Si(100). (A) A general view of the sample. (B) A high magnification SEM image. (C) A typical area of CuTCNQ 3D nanowires arrays. (D) A single CuTCNQ nanowires on the top of bundle nanowires.

Fig. 2 Energy-depressive spectrum of CuTCNQ 3D nanowires arrays.

    The growth of CuTCNQ 3D nanowires arrays can be explained on the basis of initial nucleation and nuclei movement on the surface of Si (100). In the synthesis process, the TCNQ powders are rapidly heated to 85°C to vaporize and react with copper atoms on the Si(100) surface where organic vapor solid reaction occurred to form CuTCNQ complexes. The initial CuTCNQ aggregates nanocluster on the Si(100) surface, which is used as the nucleation centers determining the growth of CuTCNQ nanowires, and the CuTCNQ nanorods grow from the nucleation sites. The nanorods gradually grow for yielding nanowires with increasing reaction time. With the reaction temperature rapidly increasing, the quantum of TCNQ vapor increased, the rate of producing TCNQ vapor is higher than the rate of reacted TCNQ vapor with copper. These TCNQ vapors that did not participate in reaction with Cu agglomerated small liquid droplets on the top formed CuTCNQ nanowires arrays. These small TCNQ liquid droplets on the top are able to composite large droplets because of high density of CuTCNQ nanowires and the short distance among those nanowires, these droplets can further induce on the top to form bundles of CuTCNQ nanowires. Finally the large liquid droplets were acted as nucleation center to grow new nanowires arrays until the copper on the surface of Si(100)were consumed completely and the CuTCNQ 3D nanowires arryas were successfully fabricated. The dimensional arrays of the TCNQ nanowires are also found to strongly depend on reaction temperature. Under the same condition, the furnace temperature increases to 150°C from room temperature, the CuTCNQ 1D nanowires arrays were formed on the surface of Si(100) (Figure 3). The SEM images show that the diameters of uniform nanowires are about 55 nm (Figure 3B). The results show that the different dimensional arrays formed may be due to the rate of TCNQ vapor yielded from starting temperature of 25°C is slower than that of TCNQ vapor yielded at a furnace temperature from 85°C to 150°C.

Fig. 3 (A) Low magnification SEM image of CuTCNQ 1D nanowires arrays. (B) High magnification SEM image of CuTCNQ nanowires.

    In summary, oriented charge transfer salt CuTCNQ 3D nanowires arrays were easily controlled to synthesize by organic vapor-solid phase reaction without template and catalysts. This approach would be led to the development of a novel general concept-purpose-built materials, which is dedicated to design of functional materials with the appropriate controlling orientation and dimension in order to probe, tune, and optimize their physical properties. Considering the unique properties of CuTCNQ complex, its 3D nanowires arrays should be of importance for both theoretical investigations and practical applications.

Acknowledgments   The authors would like to thank Prof. Lifeng Chi and Dr. Xiaochun Wu for offering the substrates with copper layer.

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