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Register a new userp-type Bi2Se3 for topological insulator and low temperature thermoelectric applications
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The growth and elementary properties of p-type Bi2Se3 single crystals are reported. Based on a hypothesis about the defect chemistry of Bi2Se3, the p-type behavior has been induced through low level substitutions (1 percent or less) of Ca for Bi. Scanning tunneling microscopy is employed to image the defects and establish their charge. Tunneling and angle resolved photoemission spectra show that the Fermi level has been lowered into the valence band by about 400 meV in Bi1.98Ca0.02Se3 relative to the n-type material. p-type single crystals with ab plane Seebeck coefficients of +180 microVK-1 at room temperature are reported. These crystals show a giant anomalous peak in the Seebeck coefficient at low temperatures, reaching +120 microVK-1 at 7 K, giving them a high thermoelectric power factor at low temperatures. In addition to its interesting thermoelectric properties, p-type Bi2Se3 is of substantial interest for studies of technologies and phenomena proposed for topological insulators.
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Solid-state thermoelectric cooling is expected to be widely used in various cryogenic applications such as local cooling of superconducting devices. At present, however, thermoelectric cooling using p- and n-type Bi2Te3-based materials has been put to practical use only at room temperature. Recently, M4SiTe4 (M = Ta, Nb) has been found to show excellent n-type thermoelectric properties down to 50 K. This paper reports on the synthesis of high-performance p-type M4SiTe4 by Ti doping, which can be combined with n-type M4SiTe4 in a cooling device at low temperatures. The thermoelectric power factor of p-type M4SiTe4 reaches a maximum value of approximately 60 uW cm-1 K-2 at 210 K and exceeds the practical level in a wide temperature range of 130-270 K. A finite temperature drop by Peltier cooling was also achieved in a cooling device made of p- and n-type Ta4SiTe4 whisker crystals. These results clearly indicate that M4SiTe4 is promising to realize a practical thermoelectric cooler for use at low temperatures, which are not covered by Bi2Te3-based materials.
We investigate the ultrafast transient absorption spectrum of Bi2Se3 topological insulator. Bi2Se3 single crystal is grown through conventional solid-state reaction routevia self-flux method. The structural properties have been studied in terms of high-resolution Powder X-ray Diffraction (PXRD). Detailed Rietveld analysis of PXRD of the crystal showed that sample is crystallized in the rhombohedral crystal structure with a space group of R-3m, and the lattice parameters are a=b=4.14A and c=28.7010A. Scanning Electron Microscopy (SEM) result shows perfectly crystalline structure with layered type morphology which evidenced from surface XRD. Energy Dispersive Spectroscopy (EDS) analysis determined quantitative amounts of the constituent atoms, found to be very close to their stoichiometric ratio. Further the fluence dependent nonlinear behaviour is studied by means of ultrafast transient absorption spectroscopy. The ultrafast spectroscopy also predicts the capability of this single crystal to generate Terahertz (THz) radiations (T-rays).
We measure gate-tuned thermoelectric power of mechanically exfoliated Bi2Se3 thin films in the topological insulator regime. The sign of the thermoelectric power changes across the charge neutrality point as the majority carrier type switches from electron to hole, consistent with the ambipolar electric field effect observed in conductivity and Hall effect measurements. Near charge neutrality point and at low temperatures, the gate dependent thermoelectric power follows the semiclassical Mott relation using the expected surface state density of states, but is larger than expected at high electron doping, possibly reflecting a large density of states in the bulk gap. The thermoelectric power factor shows significant enhancement near the electron-hole puddle carrier density ~ 0.5 x 1012 cm-2 per surface at all temperatures. Together with the expected reduction of lattice thermal conductivity in low dimensional structures, the results demonstrate that nanostructuring and Fermi level tuning of three dimensional topological insulators can be promising routes to realize efficient thermoelectric devices.
We present a magneto-infrared spectroscopic study of thin Bi2Se3 single crystal flakes. Magneto-infrared transmittance and reflectance measurements are performed in the Faraday geometry at 4.2K in a magnetic field up to 17.5T. Thin Bi2Se3 flakes (much less than 1{mu}m thick) are stabilized on the Scotch tape, and the reduced thickness enables us to obtain appreciable far-infrared transmission through the highly reflective Bi2Se3 single crystals. A pronounced electron-phonon coupling is manifested as a Fano resonance at the {alpha} optical phonon mode in Bi2Se3, resulting from the quantum interference between the optical phonon mode and the continuum of the electronic states. However, the Fano resonance exhibits no systematic line broadening, in contrast to the earlier observation of a similar Fano resonance in Bi2Se3 using magneto-infrared reflectance spectroscopy.
Combining high resolution scanning tunneling microscopy and first principle calculations, we identified the major native defects, in particular the Se vacancies and Se interstitial defects that are responsible for the bulk conduction and nanoscale potential fluctuation in single crystals of archetypal topological insulator Bi2Se3. Here it is established that the defect concentrations in Bi2Se3 are far above the thermodynamic limit, and that the growth kinetics dominate the observed defect concentrations. Furthermore, through careful control of the synthesis, our tunneling spectroscopy suggests that our best samples are approaching the intrinsic limit with the Fermi level inside the band gap without introducing extrinsic dopants.
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