We present an infrared magneto-optical study of the highly thermoelectric narrow-gap semiconductor Bi2Se3. Far-infrared and mid-infrared (IR) reflectance and transmission measurements have been performed in magnetic fields oriented both parallel and perpendicular to the trigonal c axis of this layered material, and supplemented with UV-visible ellipsometry to obtain the optical conductivity σ1(ω). With lowering of temperature we observe narrowing of the Drude conductivity due to reduced quasiparticle scattering, as well as the increase in the absorption edge due to direct electronic transitions. Magnetic fields H c dramatically renormalize and asymmetrically broaden the strongest far-IR optical phonon, indicating interaction of the phonon with the continuum freecarrier spectrum and significant magnetoelectric coupling. For the perpendicular field orientation, electronic absorption is enhanced, and the plasma edge is slightly shifted to higher energies. In both cases the direct transition energy is softened in magnetic field. arXiv:0912.2769v2 [cond-mat.str-el]

The carrier type and density in Bi2Se3 single crystals are systematically tuned by introducing a calcium (Ca) dopant. A carrier density of ~1x1017 cm-3 which corresponds to ~25 meV in the Fermi energy is obtained in both n- and p-type materials. Electrical transport properties show that the insulating behavior is achieved in low carrier density crystals. In addition, both the band gap and reduced effective mass of carriers are determined.Comment: 11 page

The high carrier mobility of graphene makes it an attractive candidate for future electronic device applications.(1) In SiO2/Si-supported graphene devices, the mobility typically varies from 2000 to ∼2,0000 cm(2) V(-1) s(-1).(2) By removing SiO2,(3,4) much higher mobility (2 × 10(5) cm(2) V(-1) s(-1) in the latter) has been obtained, suggesting the importance of the Coulomb scattering in graphene transport. Although such elaborate device fabrication is clearly effective, the mobility of finished devices is fixed thereafter and can vary from device to device. In this work, we first demonstrate a significant enhancement in carrier mobility in SiO2-supported graphene decorated with a layer of ligand-bound nanoparticles (NPs) such as iron oxide, titanium dioxide, or cadmium selenide acting as a charge reservoir. By transferring charges between graphene and the NP reservoir through the molecules, we show a remarkable reversible tunability in mobility (4000-19000 cm(2) V(-1) s(-1)) in the same device, which unambiguously proves that the charged impurity scattering is the prevailing mechanism for graphene mobility. In addition, the charge neutral point or the Dirac point can also be independently tuned over a wide gate voltage range. The reversible tuning is useful for fabricating large-area graphene devices such as nonvolatile memory with enhanced sensitivity.

Reported is the controllable selectivity syntheses of four distinct products from the same starting materials by visible-light photoredox catalysis. By employing a dicyanopyrazine-derived chromophore (DPZ) as photoredox catalyst, an aerobic radical mechanism has been developed, and allows the reactions of N-tetrahydroisoquinolines (THIQs) with N-itaconimides to through four different pathways, including addition-cyclization, addition-elimination, addition-coupling, and addition-protonation, with satisfactory chemoselectivity. The current strategy provide straightforward access to four different but valuable N-heterocyclic adducts in moderate to excellent yields.

NM 87545With a method to systematically tune the mobility of the same graphene devices, we have investigated the dependence of magneto-thermoelectric transport properties of graphene on the carrier mobility. In zero magnetic field, we find that as the mobility increases, the Seebeck coefficient S xx exhibits a more pronounced diverging trend near the Dirac point. In an external magnetic field, regular oscillations in S xx are identified corresponding to quantized Landau levels (LLs). Only in high-mobility states, an extra pair of peak and dip in S xx emerges near the Dirac point that persists at least to 150 K and the sign of the peak/dip is reversed as the mobility increases. Based on the signatures in the electrical conductivity and the Hall conductance near the Dirac point, we argue that the extra peak/dip in S xx is associated with an insulating behavior. Furthermore, the main Nernst coefficient peak increases linearly as the mobility increases. Our magneto-thermoelectric transport results reflect the contrast in the electronic properties of graphene between low and high carrier mobility states.