Fabrication of Epitaxial Interface between Transition Metal Cyanides

Nanoporous materials are currently attracting the interest of materials scientists, because nanopores can transfer and accommodate guest cations. We may realize a rapid cation transfer from one material to another through an epitaxial interface. In this study, we fabricated bilayer films of nanoporous transition metal cyanides by an electrochemical deposition technique. Scanning electron microscopy (SEM) images as well as X-ray diffraction patterns strongly suggest that the cyanide layers are epitaxially connected to each other. We compared the current–voltage (I–V) properties of the epitaxial bilayer films with those of the monolayer films.

A on each side. These compounds are currently attracting the interest of materials scientists, because they can be utilized as the positive electrode [3][4][5][6] of lithium ion secondary batteries and electronic paper. 7,8) Imanishi and coworkers 3,4) reported lithium intercalation into a host framework as well as chargedischarge behavior in a series of cyano-bridged transition metal compounds with M A ¼ V, Mn, Co, Ni, and Cu, and M B ¼ Fe. The charge capacity of the Cu compound reaches 140 mA hg À1 , which is comparable to that of the actually used material, LiCoO 2 (capacity is 140 mA hg À1 ). Recently, Moritomo and Shibata 9) and Shibata and Moritomo 10) have fabricated an all solid device using two cyanide films without an electrolyte solution, and realized a direct cation transfer from one film to another through a solid-solid interface. They demonstrated electromagnetism 9) as well as fast electrochromism, 10) although the interface was formed by the physical connection of the two cyanide films. Thus, the interface between the nanoporous materials is a key to controlling the cation transfer from one material to another.
In this paper, we report the fabrication and characterization of the bilayer films of nanoporous transition metal cyanides. In Fig. 1, we show the schematic structure of the epitaxial interface between the cyanides. On the basis of scanning electron microscopy (SEM) images as well as X-ray diffraction (XRD) patterns, we conclude that the surface layer is epitaxially grown on the buffer layer. We further investigated the electronic properties of the epitaxial bilayer films in the current-perpendicular-to-plane (CPP) configuration at 300 K and compared them with those of the monolayer films.
We further electrochemically synthesized monolayer FCr67, NMF83, and NCoF90 films on the ITO electrode in the respective aqueous solutions. The sawtoothed-type electrostatic potentials, between À0:8 and À0:2 V at 36 Hz for the FCr67 film and between À0:8 and À0:1 V at 36 Hz for the NMF83 and NCoF90 films, were applied. The h111i orientations of these films were worse than that of the NCF71 film (see Table I). The chemical compositions 13) of the monolayer films were determined by the inductively coupled plasma (ICP) method and the CNH organic elementary analyzer (Perkin-Elmer 2400 CHN elemental analyzer).
The upper panels of Fig. 2 show the cross-sectional SEM images of the (a) NCoF71/FCr67, (b) NCoF71/NMF83, and (c) NCoF71/NCoF90 bilayer films. The interface regions are  marked by dotted lines. The buffer NCoF71 layer consists of rodlike crystals, whose axial directions are perpendicular to the substrate. In all the bilayer films, the rodlike morphology is well transcribed to the surface layers. The lower panels of Fig. 2 show the surface SEM images of the bilayer films. The inset shows the surface of the buffer film, which is covered by single-crystalline trigonal pyramids. In all the bilayer films, the characteristic morphology seems to be transcribed to the surface layers, even though the size of the pyramids is slightly larger on the surface layers. These observations suggest that the surface layers epitaxially grow on the buffer layer. Now, let us investigate the structural properties of the films in more detail. The lower patterns of Fig. 3 show the XRD patterns of the monolayer films: (a) FCr67, (b) NMF83, and (c) NCoF90 films. The diffraction patterns of (a) FCr67 and (b) NMF83 revealed that the compounds are fcc wand have lattice constant (a) values of 10.59(2) and 10.53 (2) A, respectively. On the other hand, (c) NCoF90 is rhombohedral 14) and has the lattice constants a ¼ 10:37ð2Þ and ¼ 91:6ð2Þ . These lattice constants are slightly larger than that [a ¼ 10:25ð2Þ A] of the buffer NCoF71 film. The upper patterns of Fig. 3 show the XRD patterns of the bilayer films: (a) NCoF71/FCr67, (b) NCoF71/NMF83, and (c) NCoF71/ NCoF90 films. In the (a) NCoF71/FCr67 and (b) NCoF71/ NMF87 films, the (111) reflection is much stronger than that in the monolayer film. Similarly, both the ð1 11Þ and (111) reflections in the (c) NCoF71/NCoF90 film are much stronger than those iof the monolayer film. These observations clearly indicate that the h111i orientation is enhanced when the film is grown on the h111i-oriented buffer layer.
Here, we introduce the parameter S ( v  Table I. In the bilayer film, the magnitudes of S for the surface layers are comparable to that of the monolayer NCoF71 film. Thus, we concluded that the surface layers are epitaxially grown on the buffer NCoF71 film. We further investigated the electronic properties of the cyanide films in the CPP configuration. In our measurements, we slightly introduced the holes () into the transition metal sites by the oxidization of the film at 0.4 V versus a standard Ag/AgCl electrode in 0.5 mol NaNO 3 . The magnitude of was estimated from the total charge used in the oxidization process. The electronic contact on the surface side was formed by physical connection with the ITO electrode. 16) All the monolayer films show the ohmic behavior. To estimate the bulk resistivity of a monolayer film, we should subtract the contact resistance (R c ) between the cyanide film and the ITO substrate as well as the sheet resistance (R s ¼ 80 ) of the ITO electrode. To estimate R c , we investigated the film resistance against the film thickness. The magnitude of R c (¼ 5 cm 2 ) is one order smaller than that of the typical material resistance ($30-40 cm 2 for 1 m thickness; see Table II). We evaluated the resistivity   Table II. We consider that the electron hopping mechanism is responsible for the observed current-voltage (I-V ) behavior. 17) For example, in the NCoF71 film, the Fe site takes two valence states, i.e., Fe 2þ and Fe 3þ . 18) Thus, electron hopping (Fe 2þ {Fe 3þ ! Fe 3þ {Fe 2þ ) is possible via the thermal activation process. In Fig. 4, we show the I-V curves of the bilayer films: (a) NCoF71/FCr67, (b) NCoF71/NMF83, and (c) NCoF71/ NCoF90. All the bilayer films show the ohmic behavior. We decomposed the total resistance (R tot ) to the external component R ext , the material component [R mat 1 ðt 1 =SÞ þ 2 ðt 2 =SÞ; i (t i ) is the resistivity (thickness) of the i-th layer] and the interface component (R int ). We found that the resistance of the NCoF71/FCr67 film is dominated by R ext and R mat : R tot ¼ 1150 , R ext ¼ 760 , R mat ¼ 450 , and R int ¼ 40 . A similar trend is also observed for the NCoF71/ NMF83 (NCoF71/NCoF90) film; R tot ¼ 1510 ð1220Þ , R ext ¼ 720 ð680Þ , R mat ¼ 700 ð550Þ , and R ext ¼ 90 ðÀ10Þ .
Thus, the epitaxial interface shows a negligible resistance (R int $ 0 ) for the electron hopping process, reflecting the perfect cyano-bridged network. The negligible R int of the epitaxial interface makes a sharp contrast with that of the physically formed interface. 9,10) The resistance of the NCoF90/ NCoF90 bilayer film with the physically formed interface is dominated by R int ; R tot ¼ 12500 , R ext ¼ 1800 , R mat ¼ 1100 , and R int ¼ 9600 . 19) Such a large R int is ascribed to the disconnection of the cyano-bridged network.
In summary, we fabricated an epitaxial interface between nanoporous transition metal cyanides. The interface shows a negligible resistance (R int $ 0 ), reflecting a perfect cyanobridged network. Our technique is easy and low-cost, and is applicable to other network polymer materials.