ABSTRACT
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.
ABSTRACT
Group IV materials are promising candidates for highly reliable and human-friendly thin-film thermoelectric generators, used for micro-energy harvesting. In this study, we investigated the synthesis and thermoelectric applications of a Ge-based ternary alloy thin film, Ge1-x-ySixSny. The solid-phase crystallization of the highly densified amorphous precursors allowed the formation of high-quality polycrystalline Ge1-x-ySixSny layers on an insulating substrate. The small compositions of Si and Sn in Ge1-x-ySixSny (x < 0.15 and y < 0.05) lowered the thermal conductivity (3.1 W m-1 K-1) owing to the alloy scattering of phonons, while maintaining a high carrier mobility (approximately 200 cm2 V-1 s-1). The solid-phase diffusion of Ga and P allowed us to control the carrier concentration to the order of 1019 cm-3 for holes and 1018 cm-3 for electrons. For both p- and n-type Ge1-x-ySixSny, the power factor peaked at x = 0.06 and y = 0.02, reaching 1160 µW m-1 K-2 for p-type and 2040 µW m-1 K-2 for n-type. The resulting dimensionless figure of merits (0.12 for p-type and 0.20 for n-type) are higher than those of most environmentally friendly thermoelectric thin films. These results indicate that group IV alloys are promising candidates for high-performance, reliable thin-film thermoelectric generators.
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.
ABSTRACT
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.
ABSTRACT
The layer exchange technique enables high-quality multilayer graphene (MLG) on arbitrary substrates, which is a key to combining advanced electronic devices with carbon materials. We synthesize uniform MLG layers of various thicknesses, t, ranging from 5 nm to 200 nm using Ni-induced layer exchange at 800 °C. Raman and transmission electron microscopy studies show the crystal quality of MLG is relatively low for t ≤ 20 nm and dramatically improves for t ≥ 50 nm when we prepare a diffusion controlling Al2O3 interlayer between the C and Ni layers. Hall effect measurements reveal the carrier mobility for t = 50 nm is 550 cm2/Vs, which is the highest Hall mobility in MLG directly formed on an insulator. The electrical conductivity (2700 S/cm) also exceeds a highly oriented pyrolytic graphite synthesized at 3000 °C or higher. Synthesis technology of MLG with a wide range of thicknesses will enable exploration of extensive device applications of carbon materials.
ABSTRACT
Metal-induced layer-exchange growth of amorphous carbon (a-C) is a unique technique for fabricating high-quality, uniform multilayer graphene (MLG) directly on an insulating material. Here, we investigated the effect of transition-metal species on the interaction between metals and a-C in the temperature range of 600-1000 °C. As a result, metals were classified into four groups: (1) layer exchange (Co, Ni, Cr, Mn, Fe, Ru, Ir, and Pt), (2) carbonization (Ti, Mo, and W), (3) local MLG formation (Pd), and (4) no graphitization (Cu, Ag, and Au). Some layer-exchange metals allowed for low-temperature MLG synthesis at 600 °C, whereas others allowed for high-quality MLG with a Raman G/D peak ratio of up to 8.3. Based on the periodic table, we constructed metal selection guidelines for growing MLG on an insulator, opening the door for applications that combine advanced electronic devices with carbon materials.
ABSTRACT
Infection Control Committee (ICC) of Akita University Hospital was established in 1980's, when methicillin-resistant Staphylococcus aureus (MRSA) spread in teaching hospitals in Japan. After 20 years from the establishment of ICC, we needed to shake up some of the outdated infection control systems. Infection Control Team (ICT) was established in 2002 to reinforce ICC. ICT was consisted of five infection control doctors (ICD), two infection control nurses (ICN), two medical technicians in bacteriological division of clinical laboratory, one pharmacist and three administrative officials in the hospital. Monitoring of multiple drug-resistant pathogens, antibiotic use, surgical site infections and bloodstream infections are mandatory. Personnel training and educational activity are also required. After the establishment of ICT, inappropriate use of antibiotics and prevalence of MRSA were dramatically decreased. The hospital saved more than 30,000,000 yen annually. However, with a great regret, we experienced an outbreak of MRSA in a department of our hospital in 2003. MRSA infection was judged as a cause of death in three patients in the outbreak. MRSA was thought to transmit via medical personnel since pulsed-field gel electrophoresis revealed common genotype in 11 out of 15 patients studied. Prevention of healthcare-associated infections is a crucial in the management of hospitals. In this paper, we verified the efforts to control the outbreak and analyzed factors interfering infection control activities. A crucial role of a clinical laboratory in controlling healthcare associated infections was also discussed.
