Growth and characterization of epitaxial bismuth films

Effects of the thickness and laser irradiation on the electrical properties of e-beam evaporated 2D bismuth

Two-dimensional (2D) bismuth is expected to yield exotic electrical properties for various nanoelectronics, despite the difficulty in large-area preparation and property tuning directly on a device substrate. This work reports electron beam (e-beam) evaporation of large-area 2D bismuth directly on SiO2/Si with an electrical conductivity of ∼105 S m-1 and a field effect carrier mobility of ∼235 cm2 V-1 s-1 at room temperature, comparable to those of the molecular beam epitaxy (MBE) counterparts with a similar thickness. Interestingly, the electrical conductivity of 2D bismuth changes when exposed to laser irradiation that possibly induced an increase of the defect concentration, indicating a potential photo-sensor application. The electrical response of 2D bismuth can be modified either by laser irradiation or by varying the layer thickness. Due to the dimension and surface state effects in 2D bismuth, the layer thickness has a strong influence on the carrier concentration and mobility. Inspiringly, a simultaneous increase of the electrical conductivity and the Seebeck coefficient was achieved in 2D bismuth, which is preferred for thermoelectric performance but rarely reported. Our results provided a more accessible platform than MBE to produce decent quality 2D bismuth and similar Xenes with tunable electrical properties for various nanoelectronics.

Anisotropic thermoelectric effect and field-effect devices in epitaxial bismuthene on Si (111)

This experimental study reveals intriguing thermoelectric effects and devices in epitaxial bismuthene, two-dimensional (2D) bismuth with thickness ⩽30 nm, on Si (111). Bismuthene exhibits interesting anisotropic Seebeck coefficients varying 2-5 times along different crystal orientations, implying the existence of a puckered atomic structure like black phosphorus. An absolute value of Seebeck coefficient up to 237 μV K^-1 sets a record for elemental Bi ever measured to the best of our knowledge. Electrical conductivity of bismuthene can reach up to 4.6 × 10⁴ S m^-1, which is sensitive to thickness and magnetic field. Along with a desired low thermal conductivity ∼1.97 W m^-1 K that is 20% of its bulk form, the first experimental zT value at room temperature for bismuthene was measured ∼10^-2, which is much higher than many other VA Xenes and comparable to its bulk compounds. Above results suggest a mixed buckled and puckered Bi atomic structure for epitaxial 2D bismuth on Si (111). Our work paves the way to explore potential applications, such as heat flux sensor, energy converting devices and so on for bismuthene.

Enhanced Thermoelectric Performance of Novel Reaction Condition-Induced Bi2S3-Bi Nanocomposites

This is the first report on the enhanced thermoelectric (TE) properties of novel reaction-temperature (T_Re) and duration-induced Bi₂S₃-Bi nanocomposites synthesized using a facile one-step polyol method. They are well characterized as nanorod composites of orthorhombic Bi₂S₃ and rhombohedral Bi phases in which the latter coats the former forming Bi₂S₃-Bi core-shell-like structures along with independent Bi nanoparticles. A very significant observation is the systematic reduction in electrical resistivity ρ with a whopping 7 orders of magnitude (∼10⁷) with just reaction temperature and duration increase, revealing a promising approach for the reduction of ρ of this highly resistive chalcogenide and hence resolving the earlier obstacles for its thermoelectric application potentials in the past few decades. Most astonishingly, a TE power factor at 300 K of the highest Bi content nanocomposite pellet, made at 27 °C using ∼900 MPa pressure, is 3 orders of magnitude greater than that of hot-pressed Bi₂S₃. Its highest ZT at 325 K of 0.006 is over twice of that of similarly prepared CuS or Ag₂S-based nanocomposites. A significantly improved TE performance potential near 300 K is demonstrated for these toxic-free and rare-earth element-free TE nanocomposites, making the present synthesis method as a pioneering approach for developing enhanced thermoelectric properties of Bi₂S₃-based materials without extra sintering steps.

Phosphomolybdic Acid-Assisted Growth of Ultrathin Bismuth Nanosheets for Enhanced Electrocatalytic Reduction of CO2 to Formate

Oxides containing two-dimensional metallic catalysts have shown enhanced catalytic activity, stability, and product selectivity. Porous three-dimensional structures maximize the accessibility of the active sites, thus enhancing the catalytic performance of the catalysts. By integrating these desirable features in a single catalyst, further improvement in catalytic activity and selectivity is expected. In this study, oxide-containing bismuth (Bi) nanosheets of about 4 nm thickness interconnected to form a porous three-dimensional structure were synthesized by electrodeposition in the presence of phosphomolybdic acid under hydrogen evolution conditions. These Bi nanosheets catalyze CO₂ reduction in a CO₂ -saturated 0.5 m NaHCO₃ solution to formate with a faradaic efficiency of 93±2 % at -0.86 V vs. RHE with a formate partial current density as high as 30 mA cm^-2 . The Tafel slope of about 78 mV dec^-1 suggests that the protonation of the adsorbed CO₂ ^.- is the rate-limiting step.

Large-Area Dry Transfer of Single-Crystalline Epitaxial Bismuth Thin Films

We report the first direct dry transfer of a single-crystalline thin film grown by molecular beam epitaxy. A double cantilever beam fracture technique was used to transfer epitaxial bismuth thin films grown on silicon (111) to silicon strips coated with epoxy. The transferred bismuth films retained electrical, optical, and structural properties comparable to the as-grown epitaxial films. Additionally, we isolated the bismuth thin films on freestanding flexible cured-epoxy post-transfer. The adhesion energy at the bismuth/silicon interface was measured to be ∼1 J/m², comparable to that of exfoliated and wet transferred graphene. This low adhesion energy and ease of transfer is unexpected for an epitaxially grown film and may enable the study of bismuth's unique electronic and spintronic properties on arbitrary substrates. Moreover, this method suggests a route to integrate other group-V epitaxial films (i.e., phosphorus) with arbitrary substrates, as well as potentially to isolate bismuthene, the atomic thin-film limit of bismuth.

Room-temperature electrochemical reduction of epitaxial Bi2O3 films to epitaxial Bi films

A new facile approach to fabricate high-quality epitaxial Bi thin films at room-temperature with enhanced magnetotransport properties has been reported.

Constructing anisotropic single-Dirac-cones in Bi(1-x)Sb(x) thin films

The electronic band structures of Bi(1-x)Sb(x) thin films can be varied as a function of temperature, pressure, stoichiometry, film thickness, and growth orientation. We here show how different anisotropic single-Dirac-cones can be constructed in a Bi(1-x)Sb(x) thin film for different applications or research purposes. For predicting anisotropic single-Dirac-cones, we have developed an iterative-two-dimensional-two-band model to get a consistent inverse-effective-mass-tensor and band gap, which can be used in a general two-dimensional system that has a nonparabolic dispersion relation as in the Bi(1-x)Sb(x) thin film system.

Electrodeposition of mesoporous semimetal and magnetic metal films from lyotropic liquid crystalline phases

Mesoporous semimetal bismuth film and magnetic metal nickel and cobalt thin films have been electrodeposited from hexagonal or lamellar structured lyotropic liquid crystalline phases with polyoxyethylene surfactant. The liquid crystalline templates are characterized by low-angle X-ray diffraction (XRD) and polarized-light optical microscopy (POM). The metal films are characterized by low-angle and wide-angle XRD, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The magnetic measurements on the mesoporous nickel and cobalt films are shown to have higher coercivity (Hc) than the nonporous polycrystalline films.