Research Update: Nickel filling in nanofeatures using supercritical fluid and its application to fabricating a novel catalyst structure for continuous growth of nanocarbon fibers Open Access

Continuous growth of high-quality nanocarbon fibers, such as carbon nanotubes (CNTs) and carbon nanofilaments (CNFs), is becoming one of the major concerns in terms of their electrical wire application. Several methods, such as laser vaporization,^1,2 arc discharge,^3,4 and catalytic chemical vapor deposition (CCVD),^5,6 have been reported for carbon growth. It is, however, still difficult to fabricate long and high-quality nanocarbon fibers using these methods because carbon source gas depleted and/or catalysts are coated with excess carbon during the growth of nanocarbon fibers. To perform both continuous supply of carbon source gas and the elimination of excess carbon at the same time, a part of authors (Hikata et al.) proposed a carbon transmission method (CTM).⁷ In the CTM, a reactor cell is separated into a carbon source gas environment and an inert gas environment by a membrane which catalyst fibers are inlaid in. This room separation allows the continuous supply of carbon source gas to one end of the fibers so that nanocarbon fibers grow at the other end that is exposed to the inert gas environment.

In the CTM, the diameter of the growing nanocarbon fibers is determined by the diameter of the catalyst fibers. The catalyst fibers were previously fabricated with a wire drawing technique, where the lower limit of the catalyst fiber diameter (approximately 1 μm)⁷ was too large to grow single nanocarbon fibers. We propose a novel catalyst structure, where a self-supporting anodized alumina is used as a membrane and alumina nano-through-holes are filled with a catalyst metal. Due to the large thickness of a self-supporting alumina membrane, the nanoholes have an extremely high aspect ratio (EHAR). Obviously, how to fill a catalyst metal into the nano-through-holes is a key in fabricating this catalyst structure.

In this study, supercritical fluid chemical deposition (SFCD) was used for filling the catalyst (Ni in our case). The SFCD is a technique to fill/deposit a metal through the reduction of a metallo-organic complex being dissolved in supercritical CO₂fluids. Supercritical fluids (the critical point of CO₂ is 7.38 MPa and 31.2 °C)⁸ have low viscosity and excellent diffusivity similar to a gas, high density and solvent capability similar to a liquid, and zero surface tension. These unique properties make supercritical fluids a promising medium for delivering reactant molecules to surfaces that have complex micro/nanosized patterns. Indeed SFCD is capable of depositing metal inside nanosized features.^9–13 In the present work, we chose Ni as a catalyst metal because good filling characteristics of SFCD Ni into nanofeatures have been demonstrated.^9,14 The aspect ratios reported in the past researches are not large enough (e.g., approximately 5)¹⁴, and therefore, Ni fill characteristics itself are of crucial research subject. From these reasons, in the present study, we aimed (1) to fill Ni in EHAR nano-through-holes of self-supporting alumina membrane and (2) to confirm the growth of nanocarbon fibers using the fabricated catalyst membrane.

In order to investigate filling characteristics, we first used Si nanotrenches. These nanotrenches were designed for studying metal filling performance in large-scale integration processing and eased our cross-sectional observations. For Ni SFCD, a batch reactor¹⁵ was used and nickel-dipivaloylmethanate [Ni(C₁₁H₁₉O₂)₂, Ni(dpm)₂] was used as a precursor. Before deposition, Ni(dpm)₂ was weighed and was placed together as a substrate in the reactor cell. The volumetric concentration of the Ni(dpm)₂ was fixed at 27 mg/ml CO₂. After filling the cell with gaseous hydrogen to 0.7 MPa, liquid CO₂ was admitted to 10 MPa. The cell was then heated to a target temperature and the temperature was held for a fixed deposition time of 15 min, followed by depressurization and cooling. The samples were characterized using a scanning electron microscope (SEM). The chemical composition was analyzed by using an energy dispersive X-ray spectrometer (EDX) equipped with a SEM. The samples for cross-sectional observation were prepared by using a mechanical cutting and/or a cross section polisher.

Figure 1 shows cross-sectional SEM images of Ni deposited in nanotrenches. Ni was clearly deposited and was filled in the nanotrenches at any temperatures employed in this study. At deposition temperatures of 180–240 °C [Figs. 1(a)–1(c)], longitudinal voids were observed at the trench center. These discontinuous voids form when the trench entrance is pinched off, which show that Ni was deposited in a nearly conformal manner as in usual vapor deposition. When the deposition temperature was 280 °C [Fig. 1(d)], excellent filling of Ni without voids or seams was achieved.

Better filling characteristics at a higher temperature do not agree with the common sense in vapor deposition. During a SFCD process, different physicochemical processes compete, namely, dissolution of the precursor in CO₂, diffusion-transport of the precursor into nanostructures, and deposition reaction. From preliminary experiments, we found that Ni is deposited from Ni(dpm)₂ above 170 °C, and obviously, diffusivity becomes higher with increasing temperature. Therefore, dissolution behavior, especially its high temperature dependence, is of interest, whereas high temperature solubility of metal chelates is not well documented.¹⁶ We dissolved Ni(dpm)₂ in CO₂ and monitored the change of the CO₂ color through a window of a view cell using a digital camera. The pressure of the view cell was fixed at 10 MPa and temperature was increased gradually from room temperature to 200 °C with a sheath heater embedded in the cell wall. The intensities of red (R), green (G), and blue (B) signals of CO₂ were determined by using Adobe Photoshop. Figure 2 shows the intensities of R, G, and B signals of CO₂ at each temperature. Insets show macroscopic appearances of CO₂. When CO₂ was admitted to the cell at room temperature, CO₂ became purplish. Accordingly, the intensities of the G and B signals became less intensive than that of pure liquid CO₂. (Liquid and supercritical CO₂ is transparent and colorless.) This indicates that the Ni(dpm)₂ was dissolved in CO₂ at room temperature. No change of the RGB signals was observed up to 40 °C. When the temperature reached 50 °C, the G and B signals dramatically decreased and CO₂ became orange. This is known as critical opalescence and evidences that Ni(dpm)₂-dissolving CO₂ are in the supercritical state (not in the gas or liquid state). At a temperature range of 60–160 °C, colorless CO₂ was observed and the precursor reappeared in CO₂. The CO₂ became purplish again at 170 °C and the purple color became deeper with increasing temperature. These results show that the precursor solubility decreases at medium temperatures (60–160 °C) and increases again with increasing temperature. That is, the precursor precipitates or condenses during temperature rise.

Namely, the filling mechanism is discussed as follows. At room temperature, when CO₂ is filled in a cell, Ni(dpm)₂ dissolves well in CO₂ (Fig. 3(a)). Ni(dpm)₂ is then diffusion-transported to the nanostructures. When the temperature is increased, Ni(dpm)₂ condenses in the nanostructures due to the solubility decrease (Fig. 3(b)). Capillary condensation of the precursor would also take place.¹³ Finally, the Ni(dpm)₂ remaining in the nanostructures converts to metal, resulting in an excellent filling (Fig. 3(c)).

From these results, to fill a metal in nanostructures, it was found to be important to dissolve the precursor in CO₂ at a lower temperature and to condense the precursor in the nanostructures before reaching the decomposition temperature of the precursor. Moreover, a higher temperature also is needed to ensure the decomposition of the precursor. From the observation of precursor dissolution, Ni(dpm)₂ was found to be a suitable precursor for sound filling in nanostructures.

Next, we performed Ni fill into the alumina nano-through-holes at 280 °C. The thickness of an alumina membrane used in this study was 0.3 mm. The nanoholes formed in the alumina membrane have a diameter of 10–20 nm and pierce through the membrane. The conditions for Ni SFCD were the same as those used for the Si wafers. After filling Ni, the surface of the membrane was mechanically polished in order to remove the Ni film deposited at the surface. The fabricated catalyst membrane was used for nanocarbon fiber growth using the atmospheric pressure CCVD. (The CTM was not performed in this study.) Gaseous C₂H₂ was used as the source of carbon. Before supplying C₂H₂, the catalyst membrane was heated to 700 °C in Ar. The C₂H₂ gas diluted (5%) with Ar was then supplied at a flow rate of 50 sccm for 30 min.

Figure 4 shows a cross-sectional backscattered electron (BSE) image of Ni deposited in the alumina nano-through-holes, where extremely good filling characteristics are exhibited. Note that the deposited Ni appears discontinuous because the polished surface was not perfectly parallel to the longitudinal direction of the holes. The filling length was approximately 2 μm from the top of the hole, meaning that an EHAR of 200 for filling Ni was achieved by using SFCD. A cross-sectional SEM image of the fabricated catalyst membrane after the carbon growth is shown in Fig. 5(a). Filamentous carbon having a diameter of approximately 50 nm was formed on the membrane. Raman spectrum showed strong D band (1339 cm⁻¹) and G band (1580 cm⁻¹) peaks (Fig. 5(b)). Also, no multiple radial breathing mode peaks were observed (inset in Fig. 5(b)). This indicates that the grown CNFs include defects. SEM-EDX analyses of the cross-section revealed that elemental carbon existed in the Ni nanofibers and the CNFs grew from the top of the Ni nanofibers (Fig. 5(c)). The diameter of the CNFs formed at the surface of the membrane was larger (50 nm) than that of the holes (10 nm). This indicates that excess carbon thickened the original CNFs grown directly from the Ni catalysts.

We proposed a novel catalyst structure for CTM nanocarbon growth. A self-supporting anodized alumina membrane was used and catalyst Ni was filled into the alumina nano-through-holes by using SFCD. The conditions for SFCD metal fill were discussed in terms of the dissolution and condensation of the precursor. CNFs were grown using the catalyst structure, and the direct growth of CNFs from Ni in anodized alumina nano-through-holes was confirmed. This catalyst membrane is expected to be used for the continuous growth of high-quality nanocarbon fibers using CTM.