L. Konopko

Confinement effects and surface-induced charge carriers in Bi quantum wires

We present measurements of Shubnikov–de Haas oscillations in arrays of bismuth nanowires. For 80 nm wires, the hole concentration is less than 30% that of bulk Bi, a finding that is consistent with current models of quantum confinement effects. However, 30-nm-diam nanowires which are predicted to be semiconductors show a nearly isotropic short period of 0.025 T−1, consistent with a heavy carrier concentration five times that of bulk Bi. These results are discussed in terms of surface-induced charge carriers in a spherical Fermi surface pocket that are uniformly distributed in the 30 nm nanowire volume and that inhibit the semimetal-to-semiconductor transition.

Surface state band mobility and thermopower in semiconducting bismuth nanowires

Many thermoelectrics like Bi exhibit Rashba spin-orbit surface bands for which topological insulator behavior consisting of ultrahigh mobilities and enhanced thermopower has been predicted. Bi nanowires realize surface-only electronic transport since they become bulk insulators when they undergo the bulk semimetal-semiconductor transition as a result of quantum confinement for diameters close to 50 nm. We studied 20-, 30-, 50- and 200-nm trigonal Bi wires. Shubnikov-de Haas magnetoresistance oscillations caused by surface electrons and bulklike holes enable the determination of their densities and mobilities. Surface electrons have high mobilities exceeding 2(m^2)/(Vsec) and contribute strongly to the thermopower, dominating for temperatures T< 100 K. The surface thermopower is - 1.2 T microvolt/(K^2), a value that is consistent with theory, raising the prospect of developing nanoscale thermoelectrics based on surface bands.

Role of boundary roughness in the electronic transport of Bi nanowires

We present a study of electronic transport in 200 nm diameter bismuth nanowire arrays embedded in an alumina matrix where the nanowires are oriented preferentially with the trigonal crystalline axis parallel to the wire length. The study is based on measurements of the resistance and thermopower over a wide range of temperatures (4–300 K) as well as of magnetoresistance for fields of up to 9 T. The Fermi energies are obtained from the Landau level spectrum; results show that the wires have the intrinsic electron and hole concentrations. At high temperatures, the mobilities are temperature dependent and the electron mobility is several orders of magnitude larger than that of holes. This nanowire mobility behavior, which is also observed in the bulk, is attributed to carrier-phonon scattering. At low temperatures, the mobilities are temperature independent and roughly the same for electrons and holes. An interpretation in terms of boundary roughness scattering is proposed.

Thermoelectric Properties of Films and Monocrystalline Whiskers of Tellurium

Tellurium is an interesting semiconductor for thermoelectric applications because its thermopower is high (500 muV/K). Its figure of merit is low in the bulk because it has high thermal conductivity. However, in confined geometries, the phonon conductivity decreases due to phonon scattering. Therefore we have investigated the thermoelectric properties of Te films and whiskers. We report a study of the thermoelectric properties of Te films obtained by vacuum-condensation method on glass, polyamide, mica at various temperatures of condensation. The crystallographic structure of the films is characterized by the distribution function of the grain major axis. It is shown that the films condensed at 433 K possess the highest mobility. Te whiskers were grown from the paragas phase and they were single crystals in the form of hexahedral cylinder having cavities inside. When deposited on cold substrate, the cylinders had no cavities. The direction of the whisker crystal growth was always [0001] with respect to the whiskers length. It is shown that in films with the thickness of ~1 mum, the room temperature thermopower S is 330 muV/K and increases with temperature increasing (S = 450 muV/K at 450 K). The resistivity decreases with the temperature growth, as a result the power factor alpha2sigma increases by almost a factor of two. The 77 K electric conductivity of Te whiskers is 2-to-3 order-of-magnitude higher than in bulk crystals, presumably due to high structural perfection of whisker crystals. The thermopower in the range of 300 K is S = 400-750 muV/K

Spiral Modes and the Observation of Quantized Conductance in the Surface Bands of Bismuth Nanowires

When electrons are confined in two-dimensional materials, quantum-mechanical transport phenomena and high mobility can be observed. Few demonstrations of these behaviours in surface spin-orbit bands exist. Here, we report the observation of quantized conductance in the surface bands of 50-nm Bi nanowires. With increasing magnetic fields oriented along the wire axis, the wires exhibit a stepwise increase in conductance and oscillatory thermopower, possibly due to an increased number of high-mobility spiral surface modes based on spin-split bands. Surface high mobility is unexpected since bismuth is not a topological insulator and the surface is not suspended but in contact with the bulk. The oscillations enable us to probe the surface structure. We observe that mobility increases dramatically with magnetic fields because, owing to Lorentz forces, spiral modes orbit decreases in diameter pulling the charge carriers away from the surface. Our mobility estimates at high magnetic fields are comparable, within order of magnitude, to the mobility values reported for suspended graphene. Our findings represent a key step in understanding surface spin-orbit band electronic transport.

Surface state effects on the thermopower of 30- to 200-nm diameter bismuth nanowires

Nanostructured composites and nanowire arrays of traditional thermoelectrics like Bi, Bi1-xSbx and Bi2Te3 have metallic Rashba surface spin-orbit bands featuring high mobilities rivaling that of the bulk, for which topological insulator behavior has been proposed. Nearly pure surface electronic transport has been observed at low temperatures in Bi nanowires with diameter around the critical diameter, 50 nm, for the semimetal-to semiconductor transition. The surface contributes strongly to the thermopower, actually dominating for temperatures T < 100 K in these nanowires. The surface thermopower was found to be −1 T μV/K2, a value that is consistent with theory. We show that surface electronic transport together with boundary phonon scattering leads to enhanced thermoelectric performance at low temperatures of Bi nanowire arrays. We compare with bulk n-BiSb alloys, optimized CsBi4Te6 and optimized Bi2Te3. Surface dominated electronic transport can be expected in nanomaterials of the other traditional therm...

Temperature dependencies of the electric field effect resistivity and thermoelectric properties of thin Bi-alloy wires

The temperature dependencies (4.2 - 300 K) of electrical field effect (EFE) in bismuth monocrystal wires were investigated. From our experiments on EFE the temperature ranges where /spl mu//sub n/ > /spl mu//sub p/ and /spl mu//sub n/ < /spl mu//sub p/ were defined. The influence of EFE on the thermoelectric power was studied. On the thin Bi wires have been detected essential altering of the thermoelectric power.

Galvano-thermomagnetic properties of Bi/sub 1-x/Sb/sub x/ nanowires at semiconductor-semimetal transition induced by elastic elongation

In the work thin single crystal (0.3 < d < 5 /spl mu/m) wires Bi/sub 1-x/Sb/sub x/ obtained by the liquid phase casting in a glass coating were investigated under elastic deformations up to 2-3% relative elongation in the temperature range 4.2-300 K. In the nondeformed state the wires are semiconductor with minimal indirect gap and have a characteristic semiconductor dependence on T. Under elastic stretch, the semiconductor-semimetal transition was realized, accompanied by the sample metallization and appearance of the Shubnikov-de Haas oscillations from L charge carriers. The decrease of the resistance here 40-50% is observed, and of the thermopower absolute value near 15-20% decreases. Both values depend on wire diameter d. The longitudinal magneto-Seebeck coefficient in the magnetic field up to 0.3 T was measured. It is shown that elastic stretch and longitudinal magnetic field lead to 30-40% increase of the power factor /spl alpha//sup 2//spl sigma/ in the temperature range 150-200 K and at 77 K in the longitudinal magnetic field up to 0.07 T. The temperature region wherein the power factor increase takes place, depends significantly on wire diameter d.

Lifshitz topological transitions, induced by doping and deformation in single-crystal bismuth wires

The features associated with the manifestation of Lifshitz electron topological transitions (ETT) in glass-insulated bismuth wires upon qualitative changes to the topology of the Fermi surface are investigated. The variation of the energy spectrum parameters was implemented by doping Bi with an acceptor impurity Sn and using elastic strain of up to 2%, relative to the elongation in the weakly-doped p-type Bi wires. Pure and doped glass-insulated single-crystal bismuth with different diameters and (1011) orientations along the axis were prepared by the Ulitovsky liquid phase casting method. For the first time, ETT-induced anomalies are observed along the temperature dependences of the thermoemf α(T) as triple-changes of the α sign (given heavy doping of Bi wires with an acceptor impurity Sn). The concentration and energy position of the Σ-band given a high degree of bismuth doping with Sn was assessed using the Shubnikov-de Haas effect oscillations, which were detected both from L-electrons and from T-hole...

Anisotropic Thermoelectric Generator Made from Single Crystal Bi Microwire

Currently, for thermoelectric conversion of heat most widely used is the approach based on the Seebeck and Peltier effects created at the interface of two materials with different values S of the Seebeck coefficient. Another type of thermoelectric converter is the anisotropic thermoelement (AT) using anisotropy of thermoelectric power. AT has some advantages: - the transverse thermopower, unlike a conventional thermocouple, is proportional to the temperature gradient (T1- T2)/h instead of the temperature difference T1-T2; - Voltage V is proportional to the length l of AT. To increase the AT output voltage we need either increase the length l of AT or decrease its thickness h. According to our experimental data, to obtain a thermoelectric voltage of 1 V at a transverse temperature gradient of 5 K, the microwire with a diameter of 2 μm and a length of 8 m must be used. In our experimental sample the long wire in glass coating was wound into a flat spiral.