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Ultrathin Single-Crystalline Silver Nanowire Arrays Formed in an Ambient Solution Phase

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Science  12 Oct 2001:
Vol. 294, Issue 5541, pp. 348-351
DOI: 10.1126/science.1062126

Abstract

We report the synthesis of single-crystalline silver nanowires of atomic dimensions. The ultrathin silver wires with 0.4 nanometer width grow up to micrometer-scale length inside the pores of self-assembled calix[4]hydroquinone nanotubes by electro-/photochemical redox reaction in an ambient aqueous phase. The present subnanowires are very stable under ambient air and aqueous environments, unlike previously reported metal wires of ∼1 nanometer diameter, which existed only transiently in ultrahigh vacuum. The wires exist as coherently oriented three-dimensional arrays of ultrahigh density and thus could be used as model systems for investigating one-dimensional phenomena and as nanoconnectors for designing nanoelectronic devices.

Nanowires have attracted extensive interest in recent years because of their unusual quantum properties and potential use as nanoconnectors and nanoscale devices (1–3). To possess enhanced physical properties, the wires need to be of small diameter, high aspect ratio, and uniformly oriented (2). The stability of the nanowires is also a concern, because metal nanowires of ∼1 nm diameter are reported to have existed only transiently (<10 s in ultrahigh vacuum) (1, 3).

We have recently designed, synthesized, and characterized self-assembled organic nanotube arrays (Fig. 1A) formed by calix[4]hydroquinones (CHQs) (Fig. 1B) in the aqueous phase and found that they form chessboard-like arrays of rectangular pores (4, 5). The macrocyclic molecule CHQ is an electrochemically active calixarene derivative with four hydroquinone (HQ) subunits (6). HQs are widely used as reducing agents (7, 8); in particular, to reduce silver ions to form silver aggregates in acidic solution under photochemical conditions (7), because of the free energy gain through the reduction-oxidation process (9). The inner surface of the pores in the CHQ nanotubes is composed of aromatic π faces and OH groups. In the aqueous phase, partially mobile water molecules are present inside the pores because of the hydrophilic OH groups of CHQs and the water molecules bridging CHQ monomers, and thus help stabilize the silver ions in the pores. In particular, the rich π-electron density of HQ moieties in the CHQ nanotubes enables the CHQ nanotubes to capture metal ions in their pores with high affinity by cation-π interactions (10–12). Because the reduced neutral silver atoms do not exhibit strong interactions with the HQ moieties, they readily aggregate into nanowires (Fig. 1, C and D), as the size of the pores closely matches the primitive cell structure of the face-centered cubic (fcc) lattice of the bulk silver.

Figure 1

Organic nanotube templates and a silver nanowire inside the nanotube. (A) A schematic view of CHQ nanotube arrays based on the x-ray analysis (4) of the crystal (orthorhombic, space group Cmmm with lattice constants: a = 2.50 nm, b = 2.33 nm, c = 1.16 nm and α = β = γ = 90°) reveals that the pores of 0.6 × 0.6 nm2 (with van der Waals volume excluded) are separated by 1.7 nm from the neighboring ones. Water molecules are not shown for clarity. O atoms are shown in red. (B) CHQ monomer. (C and D) Top and side views, respectively, of a silver nanowire inside a CHQ nanotubetemplate. The solvent-accessible surface model in (C) and the stick model in (D) show schematic views of the CHQ nanotubes. The red color of the surface represents the negative electrostatic potential on oxygen atoms. The space-filled models (blue) represent the silver nanowires.

To prepare the nanowires, the CHQ nanotube powder was soaked in a 1 M AgNO3 aqueous solution for a few minutes, followed by ultraviolet irradiation for 30 s to accelerate the redox process. The resulting product was filtered and rinsed with distilled water. All the processes were carried out in the solution phase at standard temperature and pressure, whereas most of the previous methods involved extreme conditions such as high temperature (13), high pressure (2), or high vacuum (3).

High-resolution electron microscope (HREM) images (14) and energy-dispersive x-ray spectroscopy (EDS) analysis of the silver nanowires in the CHQ nanotube bundles, together with the x-ray structure of the CHQ nanotube crystal, reveal that single-crystalline and monodispersed silver nanowires 0.4 nm in width are formed inside the pores of the CHQ nanotube arrays (Figs. 2 and 3). The EDS analysis shows that the nanowires are composed only of silver atoms (Fig. 2B). Because the organic elements of the CHQ nanotubes do not exhibit characteristic patterns in HREM imaging at 200 keV, the visible patterns (Fig. 2, C through E) emerge solely from the silver atoms. Figure 2C shows that the one-dimensional (1D) arrays are unidirectional and monodispersed. The uniformity of the HREM image of the nanowires was found up to a length of ∼750 nm, indicating the well-ordered 1D structure (15). The spots and lines in the HREM image (Fig. 2D) represent the positions of the silver atoms or lattice. The atomic spacing along the wire axis ([110] direction) is 0.275 nm (Fig. 2D), which is 5% shorter than the corresponding value of the fcc bulk along the [110] direction (0.289 nm). From the HREM image of the nanowire arrays piled over each other, the periodicity of 1.7 nm from axis to axis of neighboring wires is observed, as in Fig. 2E, where the distance between adjacent black stripes (1.2 nm) corresponds to the projected view of the pore-to-pore distance of CHQ nanotube arrays on the (110) plane in terms of the nanowire array reference basis (1.7 nm/Embedded Image = 1.2 nm) (16). This indicates the well-ordered 3D arrays of ultrahigh density. Two different types of indexing are used: the silver fcc lattice (shown in roman text) and the nanowire array reference basis (shown in italic text) (Fig. 3, A through C) (17).

Figure 2

HREM images and EDS elemental analysis of the nanowires. (A) Low-magnification image of the silver nanowires formed in CHQ nanotube templates (scale bar, 100 nm). (B) EDS elemental analysis indicating that the only metal present is silver. Lα1 and Lβ1 are the transitions responsible for the x-ray fluorescence lines of silver. (C) Magnified image of (A) showing that 1D structures are unidirectional and monodispersed (scale bar, 2 nm). (D) Magnified image of (C) at atomic resolution showing coherent orientations of silver atoms in different wires. The white arrow indicates the wire direction, [110] (scale bar, 1 nm). (E) Image of the nanowire arrays piled over each other (scale bar, 5 nm).

Figure 3

Structural analysis of the single-crystalline primitive silver nanowire arrays. (A) 3D model of the 4 × 4 silver nanowire arrays. The z axis corresponds to the [110] direction of the fcc lattice. (B) Zone axis notations (in italics) in terms of the nanowire array reference basis. The [100], [010], and [001] directions in this wire array reference basis correspond to [001], [1̄10], and [110] of the fcc lattice, respectively. (C) Top and side views of a primitive silver nanowire model. (D) Selected area diffraction pattern for zone [212] (scale bar, 2 nm−1) and simulated pattern (inset). (E) HREM image (scale bar, 1 nm) and simulated image (inset; sample thickness = 17 nm, defocus = 70 nm) corresponding to (D). (F) Nanowire array orientation corresponding to (E). (G) Nano–beam diffraction pattern for zone [453] (scale bar, 5 nm−1). (H) HREM image (scale bar, 1 nm) and simulated image (inset; sample thickness = 17 nm, defocus = 80 nm) corresponding to (G). (I) Nanowire array orientation corresponding to (H). (J) HREM image of a nanotube bundle with nanowires inside the pores (scale bar, 10 nm). Image (H) is the highly magnified HREM image of the dotted square shown in (J).

The electron diffraction (ED) patterns of the HREM images, together with the image simulation, show that the silver nanowires are uniformly aligned within the pores of the CHQ nanotube bundles (Fig. 3, D through I). The analysis of the nanowire arrays along the [212] direction (Fig. 3, D through F) confirms the existence of single-crystalline silver nanowires. Although the 1D structures within carbon nanotubes have been characterized as either individual wires or as polydispersed and twisted bundles (18–20), the present silver nanowires samples are observed as coherently oriented arrays that show wire-wire periodicity as well as atomic periodicity. In Fig. 3D, the distance from the origin to the lattice point 3 4̄ 1̄ is 3.64 nm−1, corresponding to the reciprocal of the distance between two neighboring silver atoms, whereas the distance from the origin to the nearest lattice point 20 is 1.32 nm−1, corresponding to the reciprocal of the distance between two neighboring wires (0.76 nm) in Fig. 3E (21). Therefore, the figure exhibits a unique 3D wire-array ED pattern, showing lines of spots in the direction perpendicular to the wire axis (Fig. 3D). This pattern is somewhat similar to the case of MoS2 nanotube arrays (22). We have also observed other ED pattern images that demonstrate the existence of 3D wire arrays [see the supplementary material (23)]. The single-crystalline pattern of deformed hexagonal symmetry also matches the calculated pattern with the zone axis [453], with an error of only 2% (Fig. 3, G through I). The low-index orientation of the nanowire arrays exhibits pseudo bulk-like patterns (Fig. 3, G and H), which cannot be observed from any possible orientations of the bulk (fcc) silver crystal (21). Figure 3G, obtained from a small section of the clear 1D wire arrays shown at low magnification in Fig. 3J, indicates that the deformed hexagonal pattern originates from the array architectures. In addition, the HREM images do not show the surface reconstruction observed in gold nanobridges (24) and nanorods (25). Thus, all HREM images provide affirmative evidence that the silver structures exist as nanowires but not as bulk or electroplated surfaces. In particular, Figs. 2D, 3E, and 3H demonstrate the uniformly oriented 3D arrays with coherent orientations of silver atoms in neighboring wires (26).

The geometrical structures of the nanowires available from HREM images are compared with theoretical results, which we also used to investigate the electronic structure. We have carried out density-functional electronic structure calculations (27), using the ultrasoft pseudopotential plane wave method (28). A supercell was used, so that the neighboring nanowires were sufficiently separated. The calculated atomic layer spacing along the [110] wire axis (0.278 nm) was 4% smaller than the corresponding calculated bulk value (0.289 nm), which is in good agreement with the HREM result (0.275 nm). The interatomic spacing along the [001] direction was determined to be 0.395 nm, which is 3% smaller than the corresponding bulk value (0.409 nm). A slightly more substantial compression of the interatomic distances is observed across the cross section rather than along the wire axis. This is due to increased interactions between the inner and surface atoms caused by the high electron density near the surfaces in the case of metals, which has also been observed in silver nanoclusters (29). The calculated electronic structure shown in Fig. 4 suggests that the silver nanowire is metallic (30). The s bands cross the Fermi level at three points, suggesting the existence of three conducting channels for electronic transport as a quantum wire. On the other hand, the calculation predicts that the CHQ nanotube is an insulator (with a ∼3 eV band gap), which shields the conduction between nanowires.

Figure 4

Electronic band structure (A) and density of states (DOS) (B) of the single-crystalline primitive silver nanowire along the [110] direction.

Because the silver nanowires are confined and protected by the organic supporter, the shielding effect prevents them from oxidizing. In addition, the organic supporter provides long-term stability for the ultrathin linear nanowires, which would otherwise show only transient stability. In the samples stored under ambient air or aqueous environments for more than a month, no notable changes have been observed in the analysis of FTIR spectra and HREM images. Finally, considering the oxidation potential of the HQ moieties in the CHQ and the reduction potentials of metal ions, it should be possible to prepare nanowires in a similar way with other metallic elements, such as gold, palladium, platinum, and mercury.

  • * To whom correspondence should be addressed. E-mail: kim{at}postech.ac.kr

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