Makoto Takamura

Hydrogen storage with titanium-functionalized graphene

We report on hydrogen adsorption and desorption on titanium-covered graphene in order to test theoretical proposals to use of graphene functionalized with metal atoms for hydrogen storage. At room temperature, titanium islands grow on graphene with an average diameter of about 10 nm. Samples were then loaded with hydrogen, and its desorption kinetics was studied by thermal desorption spectroscopy. We observe the desorption of hydrogen in the temperature range between 400 K and 700 K. Our results demonstrate the stability of hydrogen binding at room temperature and show that hydrogen desorbs at moderate temperatures in line with what is required for practical hydrogen-storage applications.

Quantum hall effect and carrier scattering in quasi-free-standing monolayer graphene

The quantum Hall effect has been observed in quasi-free-standing monolayer graphene on SiC for the first time. This was achieved by decreasing the carrier density while applying gate voltage in top-gated devices. The charge neutrality point was also clearly observed, which has not been reported in top-gated structures. The mobilities at constant carrier densities did not show apparent temperature dependence up to 300 K, and conductivity was linearly dependent on carrier density. These results indicate that Coulomb scattering induced by charged impurities limits the mobility of quasi-free-standing monolayer graphene up to 300 K.

Effects of hydrogen intercalation on transport properties of quasi-free-standing monolayer graphene

We report that mobility in quasi-free-standing monolayer graphene grown on SiC(0001), when compared at the same carrier density, depends on the annealing temperature used for hydrogen intercalation. This was verified by measuring mobility in top-gated devices using quasi-freestanding monolayer graphene obtained by annealing at different temperatures. The density of charged impurities varies with annealing temperature, and it influences transport properties. Our systematic investigation shows that annealing temperatures between 700 and 800 °C are optimum for obtaining high-mobility quasi-free-standing monolayer graphene with the lowest number of charged impurities.

Epitaxial Trilayer Graphene Mechanical Resonators Obtained by Electrochemical Etching Combined with Hydrogen Intercalation

We report on the mechanical resonance properties of trilayer graphene resonators created by controlling of the layer number. We epitaxially create bilayer graphene and an interfacial buffer layer on a SiC substrate. Using hydrogen intercalation combined with electrochemical etching, we break the Si–C bonds between the buffer layer and SiC substrate surface so that the bilayer graphene and buffer layer turn into three graphene layers. The successful creation of the trilayer graphene resonators is directly observed with a transmission electron microscope. By investigating the frequency shift induced by the laser irradiation, we estimate the thermal expansion coefficient. We find that a quality factor shows a typical temperature dependence of monolayer graphene and carbon-nanotube resonators with a doubly-clamped beam structure. This implies that there exists a general energy loss mechanism for both nanotubes and few-layer-graphene doubly clamped resonators.

Selective charge doping of chemical vapor deposition-grown graphene by interface modification

The doping and scattering effect of substrate on the electronic properties of chemical vapor deposition (CVD)-grown graphene are revealed. Wet etching the underlying SiO2 of graphene and depositing self-assembled monolayers (SAMs) of organosilane between graphene and SiO2 are used to modify various substrates for CVD graphene transistors. Comparing with the bare SiO2 substrate, the carrier mobility of CVD graphene on modified substrate is enhanced by almost 5-fold; consistently the residual carrier concentration is reduced down to 1011 cm−2. Moreover, scalable and reliable p- and n-type graphene and graphene p-n junction are achieved on various silane SAMs with different functional groups.

Correlation between morphology and transport properties of quasi-free-standing monolayer graphene

We investigate the morphology of quasi-free-standing monolayer graphene (QFMLG) formed at several temperatures by hydrogen intercalation and discuss its relationship with transport properties. Features corresponding to incomplete hydrogen intercalation at the graphene-substrate interface are observed by scanning tunneling microscopy on QFMLG formed at 600 and 800 °C. They contribute to carrier scattering as charged impurities. Voids in the SiC substrate and wrinkling of graphene appear at 1000 °C, and they decrease the carrier mobility significantly.

Energy dissipation in edged and edgeless graphene mechanical resonators

We examined the temperature (T) dependence of the inverse of quality factors (Q−1) of edged and edgeless graphene resonators to evaluate energy dissipation in these resonators. We found that Q−1 in an edgeless drumhead resonator shows a linear T dependence in a wide range of 20–300 K, while that in an edged doubly clamped resonator shows T2 and T0.3 dependence above and below ∼100 K, respectively. On the basis of these experimental results, and by comparing them with the previous experimental and numerical studies, we discuss the energy dissipation mechanisms in these resonators. The dissipation at free edges causes the T0.3 dependence in the lower temperature regime, and tensile strain due to the thermal contraction of the clamped-end metal will lead to the T2 behavior in the higher temperature regime. We demonstrate that elimination of these dissipation sources provides wide-ranging linear-T dependence of Q−1 in our drumhead resonators.

Energy Dissipation in Graphene Mechanical Resonators with and without Free Edges

Graphene-based nanoelectromechanical systems (NEMS) have high future potential to realize sensitive mass and force sensors owing to graphene’s low mass density and exceptional mechanical properties. One of the important remaining issues in this field is how to achieve mechanical resonators with a high quality factor (Q). Energy dissipation in resonators decreases Q, and suppressing it is the key to realizing sensitive sensors. In this article, we review our recent work on energy dissipation in doubly-clamped and circular drumhead graphene resonators. We examined the temperature (T) dependence of the inverse of a quality factor (Q-1) to reveal what the dominant dissipation mechanism is. Our doubly-clamped trilayer resonators show a characteristic Q-1-T curve similar to that observed in monolayer resonators: Q-1 ∝ T2 above ∼100 K and ∝ T0.3 below ∼100 K. By comparing our results with previous experimental and theoretical results, we determine that the T2 and T0.3 dependences can be attributed to tensile strain induced by clamping metals and vibrations at the free edges in doubly-clamped resonators, respectively. The Q-1-T curve in our circular drumhead resonators indicates that removing free edges and clamping metal suppresses energy dissipation in the resonators, resulting in a linear T dependence of Q-1 in a wide temperature range.

Bilayer-induced asymmetric quantum Hall effect in epitaxial graphene

The transport properties of epitaxial graphene on SiC(0001) at quantizing magnetic fields are investigated. Devices patterned perpendicularly to SiC terraces clearly exhibit bilayer inclusions distributed along the substrate step edges. We show that the transport properties in the quantum Hall regime are heavily affected by the presence of bilayer inclusions, and observe a significant departure from the conventional quantum Hall characteristics. A quantitative model involving enhanced inter-channel scattering mediated by the presence of bilayer inclusions is presented that successfully explains the observed symmetry properties.

Tuning of quantum interference in top-gated graphene on SiC

We report on quantum-interference measurements in top-gated Hall bars of monolayer graphene epitaxially grown on the Si face of SiC, in which the transition from negative to positive magnetoresistance was achieved varying temperature and charge density. We perform a systematic study of the quantum corrections to the magnetoresistance due to quantum interference of quasiparticles and electron-electron interaction. We analyze the contribution of the different scattering mechanisms affecting the magnetotransport in the