Impact of Amorphous-C/Ni Multilayers on Ni-Induced Layer Exchange for Multilayer Graphene on Insulators

Abstract

Layer exchange growth of amorphous carbon (a-C) is a unique technique for fabricating high-quality multilayer graphene (MLG) on insulators at low temperatures. We investigated the effects of the a-C/Ni multilayer structure on the quality of MLG formed by Ni-induced layer exchange. The crystal quality and electrical conductivity of MLG improved dramatically as the number of a-C/Ni multilayers increased. A 600 °C-annealed sample in which 15 layers of 4-nm-thick a-C and 0.5-nm-thick Ni were laminated recorded an electrical conductivity of 1430 S/cm. This value is close to that of highly oriented pyrolytic graphite synthesized at approximately 3000 °C. This improvement is likely related to the bond weakening in a-C due to the screening effect of Ni. We expect that these results will contribute to low-temperature synthesis of MLG using a solid-phase reaction with metals.

Figure 1

(a) Schematic of the sample preparation procedure. (b) Scanning electron microscopy (SEM) image and (c) energy dispersive X-ray (EDX) spectrum of the sample for n = 15 nm and t = 4 annealed at 600 °C after Ni removal. The EDX spectrum was obtained in a field of view of 1 mm square.

Figure 2

Characterization of the cross-section of the sample for n = 15 nm and t = 4 annealed at 600 °C before Ni removal. (a) Bright-field TEM image. (b) EDX elemental map. (c) SAED pattern taken from the region including the Ni and MLG layers with a selected area of 200 nm diameter. (d) Dark-field TEM image using the C{002} plane reflection. (e, f) High-resolution lattice images showing the (e) lower and (f) upper parts of the MLG layer, respectively.

Figure 3

Raman study and the electrical properties of MLG formed by layer exchange. (a) Raman spectra obtained from the back side of the samples prior to Ni removal. (b) I_G/I_D ratio of the samples determined by the Raman spectra shown in (a), and (c) electrical conductivity σ of MLG after Ni removal, as a function of n and t.