Low-Temperature (400 °C) Synthesis of Multilayer Graphene by Metal-Assisted Sputtering Deposition.

Low-temperature synthesis of multilayer graphene (MLG) is essential for combining advanced electronic devices with carbon materials. We investigated the vapor-phase synthesis of MLG by sputtering deposition of C atoms on metal-coated insulators. Ni, Co, and Fe catalysts, which have high C solid solubility, enabled us to form MLG at 400 °C. The domain size and surface coverage of MLG were determined by the supplied amount of C atoms and the thickness of the metal layer associated with the solid solution amount of C. An average domain size of 2.5 μm and surface coverage of approximately 50% were obtained for a 1 μm thick Ni layer. Transmission electron microscopy demonstrated the high crystalline quality of the MLG layer despite the low processing temperature. Therefore, this simple sputtering technique has great potential for integrating graphene-based devices on various platforms.

Introduction

Multilayer graphene (MLG) is expected to be applied to low-resistance wiring, heat spreaders, and anodes for lithium-ion batteries because of its high electrical/thermal conductivities, current-carrying capacity, and specific capacity.[1−4] To make use of these features, it is essential to incorporate MLG into existing devices such as large-scale integrated circuits (LSIs) or flat-panel displays.[5,6] To achieve this, MLG needs to be synthesized at low temperatures to prevent damaging the underlying devices (e.g., approximately 500 °C for LSIs and a glass substrate). Low-temperature synthesis of MLG on arbitrary substrates has been achieved through vapor-phase[7−14] and solid-phase crystallization[15,16] processes using metal catalysts. These techniques utilize a phenomenon in which carbon atoms dissolved in the metal are precipitated as graphene.[17] Chemical vapor deposition (CVD) was the most common method for MLG synthesis.[7−12] However, because CVD requires high temperature for gas decomposition, devices such as plasma aiding are necessary to avoid raising the temperature of the sample.

Sputtering is a very simple method and is commercially superior. Metal-induced solid-phase crystallization of sputtered amorphous carbon (a-C) films,[18,19] particularly via layer exchange,[20−22] is useful for fabricating thick MLG films on arbitrary substrates. Conversely, there are a few reports on the vapor-phase crystallization of MLG using the sputtering method, whereas the vapor-phase crystallization is generally more advantageous for lowering the synthesis temperature than solid-phase crystallization.[23] In this study, we explored the possibility of low-temperature vapor-phase synthesis of MLG by sputtering with metal catalysis. The metals with high carbon solid solubility allowed MLG synthesis at 400 °C.

Experimental Section

Sample Preparation

The sample preparation procedure is shown in Figure . (1) Fe, Co, Ni, and Cu (thickness: 50–1000 nm) were deposited on a SiO2 glass substrate at room temperature (RT). (2) The sample temperature was raised to 400 °C in 20 min and kept at 400 °C for 40 min to heat the sample uniformly. (3) C was sputtered on the metals for 1.4–27.3 min with a deposition rate of 2.2 nm/min. (4) The sample was naturally cooled to RT (for 3.5 h). All depositions were performed using radiofrequency (RF) magnetron sputtering (base pressure: 3.0 × 10–4 Pa) with Ar plasma. The RF power was set to 50 W for the metals and 100 W for C.

Figure 1

(a) Schematic of the sample preparation. (b) Profile of sample temperature: (1) Ni deposition at RT, (2) heating and keeping at 400 °C, (3) C deposition, and (4) cooling naturally to RT.

Material Characterization

The Raman spectroscopy was performed using a JASCO NRS-5100, whose laser wavelength was 532 nm and spot size was 1 μm. The Nomarski optical micrographs of the sample surface were obtained at a magnification of 1000×. Here, the MLG coverage was calculated using binarization processing on these micrographs. Scanning electron microscopy (SEM) analyses were performed using a JEOL JSM-7001F with an energy-dispersive X-ray (EDX) spectrometer (JEOL JEO-2300). Transmission electron microscopy (TEM) analyses were performed using an analytical TEM, FEI Tecnai Osiris operating at 200 kV, equipped with an EDX spectrometer (FEI Super-X system) and a high-angle annular dark-field scanning TEM system with a probe diameter of less than 1 nm.

Results and Discussion

We examined Fe, Co, Ni, and Cu as metal catalyst layers for sputtering synthesis of MLG because these metals are typically used for graphene synthesis.[19,22]Figure a–d shows that all samples have black spots on the surfaces after sputtering. Figure e shows that the black spots for the Fe, Co, and Ni samples exhibit peaks at approximately 1350, 1580, and 2700 cm–1, which correspond to the D (disordered mode), G (graphitic mode), and two-dimensional (D mode overtone) peaks in the graphitic structure.[24] Conversely, no Raman peaks were obtained from the bright area in these samples. Therefore, the black spots in the Fe, Co, and Ni samples were determined to be MLG. In contrast, for the Cu sample, a broad peak corresponding to a-C was obtained on the entire surface. From the observation using transmitted light, the black spots in the Cu sample were determined to be holes due to the agglomeration of the Cu layer. Therefore, the MLG was formed on the Fe, Co, and Ni layers but not on the Cu layer. This is because the solid solubility of C in Cu is much lower than that in the other metals.[25] Generally, the D band intensity of MLG increases as the synthesis temperature decreases.[12,22] Because the synthesis temperature of the current MLG is as low as 400 °C, disorders in MLG would be responsible for the large D band intensity (Figure e). The MLG domains are the largest for the Ni sample. Figure f,g shows that the domain size of the Ni sample is approximately 1 μm. Thus, MLG was synthesized at a low temperature of 400 °C by sputtering using metal catalysts with high solid solubility of C.

Figure 2

Characteristics of the samples after C deposition (1.4 min with 2.2 nm/min) for various metal catalysts (50 nm thickness). (a–d) Nomarski optical micrographs for (a) Fe, (b) Co, (c) Ni, and (d) Cu. (e) Raman spectra obtained at the black spots in (a–d). (f) Low- and (g) high-magnification SEM images of the Ni sample.

We investigated the effect of the thickness of C and Ni (tC and tNi) on MLG’s growth properties. We note that tC corresponds to the product of C deposition time and deposition rate (2.2 nm/min). Figure a shows that the surface morphology, i.e., the domain size and surface coverage of MLG, varies with both tC and tNi. For tC = 60 nm and tNi ≤ 200 nm, it was difficult to verify the MLG domain, which was likely because of thick a-C layers, as determined by the Raman study. The a-C layers were formed by the deposition of C atoms that cannot be dissolved in the thin Ni layers. We note that the D band intensity in the Raman spectra did not change with tC until the surface was covered by a-C. Figure b shows that the MLG domain size roughly increases with increasing tC except for tC = 3 nm. The sample with tC = 3 nm and tNi = 1000 nm exhibits the maximum domain size of 2.5 μm. This means that adding a small amount of C to a thick Ni layer is effective to obtain MLG with a large domain. Figure c shows that the MLG coverage increases with increasing tC. The coverage does not depend on tNi for tC ≤ 20 nm, whereas it increases with increasing tNi for tC ≥ 40 nm. The maximum coverage of approximately 50% is obtained for tC = 60 nm and tNi = 1000 nm. This is likely because a thicker Ni layer can dissolve more C atoms. In addition to increasing tC and tNi, optimizing the cooling rate will be also effective in improving the surface coverage.[18] Thus, the domain size and surface coverage of MLG were determined by the supplied amount of C atoms and the amount of C that Ni can dissolve. We tried to measure the MLG thickness using atomic force microscopy; however, it was difficult to identify MLG due to the Ni surface roughness. Considering that the amount of precipitated MLG depends on the solid solution amount of C in Ni, the MLG thickness will show a similar tendency to the surface coverage of MLG (Figure c). Generally, the surface of the MLG precipitated from metals has only C–C bonds and no functional group such as C–O bonds.[4] Therefore, the current MLG surface also seems to have no functional groups and consists of C–C bonds.

Figure 3

Effects of tC and tNi on the growth morphology of MLG. (a) Nomarski optical micrographs for the matrix composed of tC and tNi. (b) Domain size and (c) surface coverage of MLG as a function of tC, which were calculated by binarizing the optical micrographs.

We investigated the detailed cross-sectional structure of the sample for tC = 60 nm and tNi = 1000 nm, which has the maximum MLG coverage (Figure ). The bright-field TEM images and EDX mapping in Figure a–c show that the MLG domains with various sizes are locally formed on the Ni layer, which is consistent with the optical micrograph (Figure a). The Ni concentration in the MLG domain is below the detection limit of EDX as in the other precipitation methods. The entire sample surface is covered with sparse a-C, which could not dissolve in Ni. These TEM images also suggest that the MLG domains grew from the surface steps, i.e., grain boundaries in the Ni layer. This phenomenon is reasonable because the grain boundary diffusion is much faster than lattice diffusion and similar to other MLG precipitation methods.[15,16] The dark-field TEM image and the selected-area electron diffraction pattern in Figure d reveal that the MLG domain is a single crystalline layer and {002}-oriented parallel to the substrate. The high-resolution TEM image in Figure e clearly shows that the graphene layers are stacked and completely {002}-oriented. Although the presence of disorders is suggested from the Raman results (Figure e), the MLG domain contains no obvious defects, such as dislocations or stacking faults.

Figure 4

Characterization of the cross section of the sample for tC = 60 and tNi = 1000 nm. (a) Low- and (b) high-magnification bright-field TEM images. (c) EDX elemental mapping. (d) Dark-field TEM image using the C{002} plane reflection and selected-area electron diffraction pattern taken from the region including the Ni and MLG layers. (e) High-resolution lattice image of the MLG layer.

Conclusions

This sputtering method using metal catalysts enabled MLG to be formed at 400 °C. The domain size and surface coverage of MLG were determined by the supplied amount of C atoms and the Ni layer thickness associated with the solid solution amount of C. The maximum domain size and surface coverage of MLG were approximately 2.5 μm and 50%, respectively. Further investigations into the cooling rate, RF power, and Ni structure will allow us to improve the domain size and surface coverage or control the domain location. The findings in this study should encourage studies exploring the low-temperature synthesis of MLG using a simple sputtering method.