Quantum-size effects in n-type bismuth thin films
TL;DR: In this paper, the authors attribute this oscillatory behavior to quantum-size effects, which are observable when the electron mean-free path and Fermi wave length exceed the film thickness d.
Abstract: Oscillatory thickness dependences of the electrical conductivity, Hall coefficient, charge carrier mobility, and Seebeck coefficient were obtained at room temperature for n-type thin Bi films (d=3–300 nm) fabricated by the thermal evaporation of a bismuth crystal in a vacuum and deposition on mica substrates at 380 K. We attribute this oscillatory behavior to quantum-size effects, which are observable when the electron mean-free path and Fermi wave length exceed the film thickness d.
Citations
More filters
TL;DR: It is theoretically predicted that two-dimensional bismuth will show the quantum spin-Hall effect, both by calculating the helical edge states, and by showing the nontriviality of the Z2 topological number.
Abstract: We show that the spin-Hall conductivity in insulators is related to a magnetic susceptibility representing the strength of the spin-orbit coupling. We use this relationship as a guiding principle to search real materials showing quantum spin-Hall effect. As a result, we theoretically predict that two-dimensional bismuth will show the quantum spin-Hall effect, both by calculating the helical edge states, and by showing the nontriviality of the ${Z}_{2}$ topological number, and propose possible experiments.
525 citations
TL;DR: In this paper, Nanocalorimetry was used to investigate the melting of Bi nanoparticles and small particles were found to exhibit size-dependent melting temperatures less than the bulk melting temperature (e.g., ΔT=67K for a 3-nm radius particle).
Abstract: Nanocalorimetry was used to investigate the melting of Bi nanoparticles. The particles were formed by evaporating Bi onto a silicon nitride substrate, which was then heated. The particles self-assemble into truncated spherical particles. Below 5-nm average film thickness, mean particle sizes increased linearly with deposition thickness but increased rapidly for 10-nm-thick films. As expected, small particles were found to exhibit size-dependent melting temperatures less than the bulk melting temperature (e.g., ΔT=67K for a 3-nm radius particle). The measured melting temperatures for particles below ∼7nm in radius, however, were ∼50K above the value predicted by the homogeneous melting model. We discuss this discrepancy in terms of a possible size-dependent crystal structure change and the superheating of the solid phase.
172 citations
TL;DR: In this paper, a review of the physical mechanisms that result in the superior thermoelectric performance of low-dimensional solids, compared to bulk thermolectric materials is presented.
Abstract: Thermoelectric materials are used as solid-state heat pumps and as power generators The low e‐ciency of devices based on conventional bulk thermoelectric materials conflnes their applications to niches in which their advantages in compactness and controllability outweigh that drawback Recent developments in nanotechnologies have led to the development of thermoelectric nano-materials with double the e‐ciency of the best bulk materials, opening several new classes of applications for thermoelectric energy conversion technology We review here flrst the physical mechanisms that result in the superior thermoelectric performance of low-dimensional solids, compared to bulk thermoelectric materials: they are a reduction of the lattice thermal conductivity, and an increase in the Seebeck coe‐cient S for a given carrier density The second part of this review summarizes experimental results obtained on macroscopic arrays of bismuth, antimony, and zinc nanowires with diameters ranging from 200 to 7 nm We show how size-quantization efiects greatly increase S for a given carrier concentration, as long as the diameter of the nanowires remains above 9 nm, below which localization efiects start dominating In a third part, we give data on PbTe nanocomposites, particularly bulk samples containing 30 nm diameter Pb inclusions These inclusions afiect the electron scattering in such a way as to again increase the Seebeck coe‐cient
147 citations
TL;DR: A rational supramolecular methodology that yielded the highest Seebeck coefficient around ambient temperatures is reported, based on the encapsulation of triiodide ions in α-cyclodextrin, whose equilibrium is shifted to the complexation at lower temperatures, whereas it is inverted at elevated temperatures.
Abstract: Thermo-electrochemical cells have potential to generate thermoelectric voltage 1 order higher than that given by semiconductor materials. To overcome the current issues in thermoelectric energy conversion, it is of paramount importance to grow and fulfill the full potential of thermo-electrochemical cells. Here we report a rational supramolecular methodology that yielded the highest Seebeck coefficient of ca. 2.0 mV K–1 around ambient temperatures. This is based on the encapsulation of triiodide ions in α-cyclodextrin, whose equilibrium is shifted to the complexation at lower temperatures, whereas it is inverted at elevated temperatures. This temperature-dependent host–guest interaction provides a concentration gradient of redox ion pairs between two electrodes, leading to the eminent performance of the thermo-electrochemical cells. The figure of merit for this system, zT reached a high value of 5 × 10–3. The introduction of host–guest chemistry to thermoelectric cells thus provides a new perspective in t...
110 citations
TL;DR: In this article, the peak shift and broadening of the Raman bands (Eg and A1g modes) observed with an increase in substrate temperature can be attributed to the phonon confinement and compressive stress effects.
Abstract: Bismuth (Bi) thin films of various microstructures were synthesized by thermal evaporation at varying substrate temperatures. The substrate temperature strongly affects the surface morphology and crystalline orientation of the Bi thin films. Peak shift and broadening of the Raman bands (Eg and A1g modes) observed with an increase in substrate temperature can be attributed to the phonon confinement and compressive stress effects. The Bi thin film depicts a laser-induced oxidation and phase transition as a function of varying laser power. Photoluminescence spectra show visible–near infrared broadband emission for polycrystalline Bi thin film prepared at high substrate temperature. This result indicates that polycrystalline Bi thin film can be a promising candidate for broadband optical fibre amplifiers and tunable lasers.
83 citations
References
More filters
TL;DR: In this paper, two-dimensional quantum well superlattices were used to separate the two bands and transform the material to an effective one-carrier system, and the effect of such an approach was investigated theoretically.
Abstract: Currently, the materials with the highest thermoelectric figure of merit (ZT) are one‐band materials. The presence of both electrons and holes lowers ZT, so two‐band materials such as semimetals are not useful thermoelectric materials. However, by preparing these materials in the form of two‐dimensional quantum‐well superlattices, it is possible to separate the two bands and transform the material to an effective one‐carrier system. We have investigated theoretically the effect of such an approach and our results indicate that a significant increase in ZT may be achieved. This result allows the possibility of using a new class of materials as thermoelectric refrigeration elements.
508 citations
TL;DR: In this article, the authors studied the thickness dependence of the resistivity, the Hall coefficient, and the transverse magneto-resistance, by gradually varying the thickness of a single film which was kept under high vacuum during the entire experiment.
Abstract: Bismuth films (200-1400 \AA{}) were grown epitaxially on freshly cleaved mica substrates. These films consisted of a mosaic of equally oriented crystallites averaging several microns in diameter. The plane of the films coincided with the trigonal plane of Bi. We have studied the thickness dependence of the resistivity, the Hall coefficient, and the transverse magneto-resistance, by gradually varying the thickness of a single film which was kept under high vacuum during the entire experiment. The resistivity at 360 and 77 \ifmmode^\circ\else\textdegree\fi{}K is a smooth monotonic function of the thickness. At 12 \ifmmode^\circ\else\textdegree\fi{}K, we observed small oscillations in the resistivity and in the magnetoresistance. These oscillations are regarded as probable manifestations of the quantum size effect (QSE). The thickness dependence of the Hall coefficient is in striking disagreement with the predictions of the infinite-potential-well model. Better agreement between the theory and experimental results is obtained when we assume a less rigid boundary condition. Also for several films we have investigated the temperature dependence of these three transport coefficients and found it to be quite different from that of bulk bismuth. We have attempted to explain these results in terms of the behavior of the carrier concentration and of the different scattering mechanisms that can come into play in these films.
165 citations
TL;DR: In this article, the authors describe the growth of the first thin (0.1 -2 ) epitaxial films of pure bismuth using molecular-beam-epitaxy techniques.
Abstract: The present work describes the growth of the first thin (0.1--2 \ensuremath{\mu}m) epitaxial films of pure bismuth using molecular-beam-epitaxy techniques. These structures were grown at elevated temperatures on single-crystal barium fluoride substrates of 〈111〉 orientation. Electron-microscope observations show the films to be featureless and defect free on the scale of 0.1 \ensuremath{\mu}m. The films grow with their trigonal axis parallel to the 〈111〉 axis of the substrate, and Laue-backscattering pictures show they are epitaxial. Mobilities at room temperature are on the order of 2 ${\mathrm{m}}^{2}$ ${\mathit{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$, and increase to over 10 at 20 K and 100 at liquid-helium temperatures. These values are far superior to those of other bismuth films grown to date, and approach mobilities observed in single-crystal bismuth. Further evidence of their single-crystal nature is given by the temperature-dependent resistivity below 6K, which is more akin to that of a bulk single crystal, rather than polycrystal, bismuth, and by the thickness dependence of the film mobilities, which are limited by scattering on film boundaries. The carrier density, as deduced from Hall measurements, is in the range (4--8)\ifmmode\times\else\texttimes\fi{}${10}^{24}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ at room temperature and decreases as the temperature is lowered, becoming constant below about 50 K at approximately 5\ifmmode\times\else\texttimes\fi{}${10}^{23}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$. We also observe Shubnikov--de Haas oscillations in the resistivity and Hall coefficient at 4.2 and 0.4 K. The carrier density calculated from the period of these oscillations correlates well with that found from Hall measurements.
105 citations
TL;DR: In this paper, the authors measured the resistivity, Hall coefficient, and magnetoresistance coefficient of twinned bismuth films between 1.15 and 300 K and found that the surface scattering in these films is not specular, contrary to the findings of some other workers.
Abstract: The resistivity, Hall coefficient, and magnetoresistance coefficient of well ordered but twinned bismuth films were measured between 1. 15 and 300 K. It was found that the surface scattering in these films is not specular, contrary to the findings of some other workers. At 300 K the thickness dependence of the resistivity can be roughly fitted by the Fuchs-Sondheimer boundary-scattering theory with a surface reflection coefficient of 0.6, indicating partially diffuse scattering. It was also observed that the apparent scattering becomes more diffuse with decreasing temperature until at low temperatures the data can no longer be explained by the Fuchs-Sondheimer theory. This indicates that an additional size-dependent temperature-dependent scattering mechanism exists in thin-film transport. It was observed that at low temperatures the temperature dependence of the conductivity could be explained on the basis of a constant mean free path for the thicker samples. For thinner samples, the temperature dependence of the conductivity again indicates that there is an additional scattering mechanism that becomes stronger with decreasing temperature and decreasing sample thickness. Values of the mobility and mean free path, calculated from the data, were also observed to vary consistently with the sample thickness. The conclusions, drawn from the thickness dependence of the resistivity, concerning the diffuseness of the surface scattering of the charge carriers were confirmed by the dependence of the mean free path upon the sample thickness. Finally, quantum size-effect oscillations were observed in all of the transport properties of the thin bismuth films at low temperatures. The period (about 400 \AA{}) and phase of the oscillations are in reasonable agreement with the theory and in good agreement with other values reported in the literature.
88 citations
TL;DR: In PbSe epitaxial thin films grown by thermal evaporation on KCl(001) substrates and covered with an EuS protective layer, oscillatory dependences of the galvanomagnetic and thermoelectric properties (electrical conductivity σ, the Hall coefficient RH, charge carrier mobility μ, and the Seebeck coefficient S) were observed at room temperature as mentioned in this paper.
Abstract: In PbSe epitaxial thin films grown by thermal evaporation on KCl(001) substrates and covered with an EuS protective layer, oscillatory dependences of the galvanomagnetic and thermoelectric properties (electrical conductivity σ, the Hall coefficient RH, charge carrier mobility μ, and the Seebeck coefficient S) on the PbSe layer thickness d (3
70 citations