Metal Catalysts for Layer-Exchange Growth of Multilayer Graphene

Thickness Dependency of Battery Anode Properties in Multilayer Graphene

With the development of practical thin-film batteries, multilayer graphene (MLG) is being actively investigated as an anode material. Therefore, research on determining a technique to fabricate thick MLG on arbitrary substrates at low temperatures is essential. In this study, we formed an MLG with controlled thickness at low temperatures using a layer exchange (LE) technique and evaluated its anode properties. The LE technique enabled the formation of a uniform MLG with a wide range of thicknesses (25-500 nm) on Ta foil. The charge/discharge characterization using coin-type cells revealed that the total capacity, which corresponded to Li intercalation into the MLG interlayer, increased with increasing MLG thickness. In contrast, cross-sectional transmission electron microscopy showed a metal oxide formed at the MLG/Ta interface during annealing, which had small Li capacity. MLG with sufficient thickness (500 nm) exhibited an excellent Coulombic efficiency and capacity retention compared to bulk graphite formed at high temperatures. These results have led to the development of inexpensive and reliable rechargeable thin-film batteries.

Si1-xGex anode synthesis on plastic films for flexible rechargeable batteries

SiGe is a promising anode material for replacing graphite in next generation thin-film batteries owing to its high theoretical charge/discharge capacity. Metal-induced layer exchange (LE) is a unique technique used for the low-temperature synthesis of SiGe layers on arbitrary substrates. Here, we demonstrate the synthesis of Si_1-xGe_x (x = 0-1) layers on plastic films using Al-induced LE. The resulting SiGe layers exhibited high electrical conductivity (up to 1200 S cm^-1), reflecting the self-organized doping effect of LE. Moreover, the Si_1-xGe_x layer synthesized by the same process was adopted as the anode for the lithium-ion battery. All Si_1-xGe_x anodes showed clear charge/discharge operation and high coulombic efficiency (≥ 97%) after 100 cycles. While the discharge capacities almost reflected the theoretical values at each x at 0.1 C, the capacity degradation with increasing current rate strongly depended on x. Si-rich samples exhibited high initial capacity and low capacity retention, while Ge-rich samples showed contrasting characteristics. In particular, the Si_1-xGe_x layers with x ≥ 0.8 showed excellent current rate performance owing to their high electrical conductivity and low volume expansion, maintaining a high capacity (> 500 mAh g^-1) even at a high current rate (10 C). Thus, we revealed the relationship between SiGe composition and anode characteristics for the SiGe layers formed by LE at low temperatures. These results will pave the way for the next generation of flexible batteries based on SiGe anodes.

Layer exchange synthesis of multilayer graphene

Low-temperature synthesis of multilayer graphene (MLG) on arbitrary substrates is the key to incorporating MLG-based functional thin films, including transparent electrodes, low-resistance wiring, heat spreaders, and battery anodes in advanced electronic devices. This paper reviews the synthesis of MLG via the layer exchange (LE) phenomenon between carbon and metal from its mechanism to the possibility of device applications. The mechanism of LE is completely different from that of conventional MLG precipitation methods using metals, and the resulting MLG exhibits unique features. Modulation of metal species and growth conditions enables synthesis of high-quality MLG over a wide range of growth temperatures (350 °C-1000 °C) and MLG thicknesses (5-500 nm). Device applications are discussed based on the high electrical conductivity (2700 S cm^-1) of MLG and anode operation in Li-ion batteries. Finally, we discuss the future challenges of LE for MLG and its application to flexible devices.

Multilayer graphene in situ formed in carbonized waste paper with the synergism of nickel and sodium

MLG is in situ formed with the synergism of nickel and sodium. The porous structure of amorphous carbon is effectively optimized with the formation of MLG and its capacitive performance is clearly improved.

Impact of the carbon membrane inserted below Ni in the layer exchange of multilayer graphene

High-quality multilayer graphene on glass is achieved at a low temperature (400 °C).

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

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.

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.