Semiconductors are generally considered far superior to metals as thermoelectric materials because of their much larger Seebeck coefficients (S). However, a maximum value of S in a semiconductor is normally accompanied by a minuscule electrical conductivity (σ), and hence, the thermoelectric power factor (P = S 2σ) remains small. An attempt to increase σ by increasing the Fermi energy (E F), on the other hand, decreases S. This trade-off between S and σ is a well-known dilemma in developing high-performance thermoelectric devices based on semiconductors. Here, we show that the use of metallic carbon nanotubes (CNTs) with tunable E F solves this long-standing problem, demonstrating a higher thermoelectric performance than semiconducting CNTs. We studied the E F dependence of S, σ, and P in a series of CNT films with systematically varied metallic CNT contents. In purely metallic CNT films, both S and σ monotonically increased with E F, continuously boosting P while increasing E F. Particularly, in an aligned metallic CNT film, the maximum of P was ∼5 times larger than that in the highest-purity (>99%) single-chirality semiconducting CNT film. We attribute these superior thermoelectric properties of metallic CNTs to the simultaneously enhanced S and σ of one-dimensional conduction electrons near the first van Hove singularity.

First-principles total-energy calculations clarified the effect of charged Al nanoparticles on carrier accumulation in graphene by an external electric field. Carrier injection in graphene with Al nanoparticles is sensitive to the relative position of the Al nanoparticles to the gate electrode. The nanoparticles sandwiched between graphene and electrode prevent the carrier injection in graphene, while the nanoparticles adsorbed on the opposite side do not affect the Dirac point shift, resulting in the successive carrier injection in graphene.Because of the density of state difference, the capacitance of graphene with Al nanoparticle also depends on the electrode position.Graphene has been keeping a premier position not only in the low-dimensional sciences but also in the electronic device engineering due to its unique structural and electronic properties. [1][2][3][4][5][6][7][8] For the fundamental and applied sciences of graphene, it is mandatory to precisely control their electronic structure by external conditions, such as chemical modification, atom/molecule doping, and external electric field. In particular, the band gap engineering and the Fermi level tuning are highly important for fundamental study and application of graphene. In practical application of graphene, graphene inherently possesses hybrid structures with foreign materials in its device structures, such as insulating substrate, metal electrode, and atoms/molecules, which affect the electronic structure of graphene. [11][12][13][14][15][16][17][18] It has been demonstrated that graphene physically adsorbed on insulating substrates does not possesses the Dirac cone but quadratic dispersion band with a finite energy gap which depends on the surface morphology and atom species. [12][13][14][15] Atoms and molecules adsorbed on the graphene also modulate the Dirac cone of graphene by covalent and van der Waals interaction between adsorbates and graphene. [16][17][18] Although the carrier accumulation in pristine graphene and graphite under an external electric field has been steadily elucidated, 19,20

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Using the density functional theory combined with an effective screening medium method, we studied the electronic structure of N-doped graphene under an external electric field. The electronic states near the Fermi level depend on the carrier concentration reflecting their wave function distribution. The electronic states associated with the dangling bond shift upward with increasing electron concentration, following the upward shift of the Fermi level. The electronic states associated with nonbonding π states almost retain their energy upon hole/electron doping by the external electric field.

Using the density functional theory combined with the effective screening medium method, we studied geometric structures of Al nanoparticles adsorbed on graphene surfaces under electron/hole injection by a counter electrode. The equilibrium spacing between graphene and an Al nanoparticle is sensitive to their mutual arrangement with respect to the electrode, carrier concentration, and carrier species. For most cases, owing to the Coulomb attractive interaction between accumulated carriers in graphene or an Al nanoparticle and the counter electrode, the Al nanoparticle is desorbed from the graphene surfaces at the carrier concentration of approximately 0.5 e or 0.5 h per unit cell. *