Free-standing Bi Research Articles

Defect-induced photogating effect and its modulation in ultrathin free-standing Bi2O2Se nanosheets with a visible-to-near-infrared photoresponse

Defect induced persistent negative photoconductivity in free-standing Bi2O2Se nanosheets and its modulation with vacuum annealing.

Interface controlled solid-state lithium storage performance in free-standing bismuth nanosheets.

Bismuth (Bi) has recently been discovered as a potential lithium-ion anode material for batteries with high Li capacity and suitable equilibrium potential, and without dendrite formation. However, the reversible electrochemical stability remains insufficient for applications. Herein, it is demonstrated that two-dimensional free-standing Bi nanosheets (Bi-NSs) have superior anode performance using either liquid or solid electrolytes. The Bi-NSs with a uniform thickness of ∼40 nm prepared by aqueous methods exhibit a record high capacity of ∼287 mA h g-1 at a current density of 250 mA g-1 with the LiBH4 solid electrolyte even after 100 cycles. Fast and stable solid-state lithium plating and stripping occur without side reactions. The 2D layered nanostructure has more active sites and a shorter diffusion length, and forms stable interfaces with the electrolyte. The present work reveals a facile synthesis route of novel 2D materials and paves an efficient pathway for high-capacity and safe bismuth-based anodes for lithium batteries.

A high-volumetric-capacity bismuth nanosheet/graphene electrode for potassium ion batteries

Potassium ion batteries (PIBs) with high-volumetric energy densities are promising for next-generation low-cost energy storage devices. Metallic bismuth (Bi) with a structure similar to graphite, is a promising anode material for PIBs due to its high theoretical volumetric capacity (3763 mA h cm−3) and relatively low working potential (−2.93 V vs. standard hydrogen electrode). However, it experiences severe capacity decay caused by a huge volume expansion of Bi when alloying with potassium. This study reports a flexible and free-standing Bi nanosheet (BiNS)/reduced graphene oxide composite membrane with designed porosity close to the expansion ratio of BiNS after charging. The controlled pore structure improves the electron and ion transport during cycling, and strengthens the structural stability of the electrode during potassiation and depotassiation, leading to excellent electrochemical performance for potassium-ion storage. In particular, it delivers a high reversible volumetric capacity of 451 mA h cm−3 at the current density of 0.5 A g−1, which is much higher than the previously reported commercial graphite material.

Bismuthene for highly efficient carbon dioxide electroreduction reaction

Bismuth (Bi) has been known as a highly efficient electrocatalyst for CO2 reduction reaction. Stable free-standing two-dimensional Bi monolayer (Bismuthene) structures have been predicted theoretically, but never realized experimentally. Here, we show the first simple large-scale synthesis of free-standing Bismuthene, to our knowledge, and demonstrate its high electrocatalytic efficiency for formate (HCOO−) formation from CO2 reduction reaction. The catalytic performance is evident by the high Faradaic efficiency (99% at −580 mV vs. Reversible Hydrogen Electrode (RHE)), small onset overpotential (<90 mV) and high durability (no performance decay after 75 h and annealing at 400 °C). Density functional theory calculations show the structure-sensitivity of the CO2 reduction reaction over Bismuthene and thicker nanosheets, suggesting that selective formation of HCOO− indeed can proceed easily on Bismuthene (111) facet due to the unique compressive strain. This work paves the way for the extensive experimental investigation of Bismuthene in many different fields.

Epitaxial Growth of Free-Standing Bismuth Film on Graphene Embedded with Nontrivial Properties

Topological insulators (TIs) have become one of the intensely pursed topics during the past few years due to their robust gapless edge states, among which bismuth (Bi) enjoys a good reputation because of its rich quantum states. However, bulk Bi itself is topologically trivial. While thin Bi film has long been theoretically perceived to be a two-dimensional topological insulator, it still remains a challenge to construct free-standing Bi(111) film experimentally. Herein, we investigate the epitaxial growth of bismuth films on graphene with the film facet ranging from (110) to (111) with increasing coverage and demonstrate the topologically nontrivial property embedded in Bi(111) layers by angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. In contrast to the bulk Bi with semimetallicity, a metallic surface state is revealed in the 7-bilayer Bi(111) with a circle-shape photoemission intensity plot, while the stacking of free-standing bismuth bilayers on graphene is rationalized to be responsible for the nontrivial characteristics of Bi(111) with an odd crossing of the Fermi level. Our report herein proposes an intriguing approach toward fabricating topologically nontrivial bismuth film, as well as advancing the fundamental understanding of Bi(111) bilayers. Importantly, the combination of the nontrivial Bi(111) film and graphene might also shed useful insights into the development of nanotronics and spintronics.

Visualizing topological edge states of single and double bilayer Bi supported on multibilayer Bi(111) films

Freestanding single-bilayer Bi(111) is a two-dimensional topological insulator with edge states propagating along its perimeter. Given the interlayer coupling experimentally, the topological nature of Bi(111) thin films and the impact of the supporting substrate on the topmost Bi bilayer are still under debate. Here, combined with scanning tunneling microscopy and first-principles calculations, we systematically study the electronic properties of Bi(111) thin films grown on a NbSe2 substrate. Two types of non-magnetic edge structures, i.e., a conventional zigzag edge and a 2x1 reconstructed edge, coexist alternately at the boundaries of single bilayer islands, the topological edge states of which exhibit remarkably different energy and spatial distributions. Prominent edge states are persistently visualized at the edges of both single and double bilayer Bi islands, regardless of the underlying thickness of Bi(111) thin films. We provide an explanation for the topological origin of the observed edge states that is verified with first-principles calculations. Our paper clarifies the long-standing controversy regarding the topology of Bi(111) thin films and reveals the tunability of topological edge states via edge modifications.

Band Structures of Ultrathin Bi(110) Films on Black Phosphorus Substrates Using Angle-Resolved Photoemission Spectroscopy**Supported by National Key R&D Program of China under Grant Nos 2017YFA0402901, 2016YFA0401004 and 2016YFB0901600, the National Natural Science Foundation of China under Grant Nos 11534010, 11404172, U1532136, U1632102, U1632272, 11574201, 11674296 and 11190022, the National Basic Research Program of China under Grant No 2014CB921102, the Key

The band structures of two-monolayer Bi(110) films on black phosphorus substrates are studied using angle-resolved photoemission spectroscopy. Within the band gap of bulk black phosphorus, the electronic states near the Fermi level are dominated by the Bi(110) film. The band dispersions revealed by our data suggest that the orientation of the Bi(110) film is aligned with the black phosphorus substrate. The electronic structures of the Bi(110) film strongly deviate from the band calculations of the free-standing Bi(110) film, suggesting that the substrate can significantly affect the electronic states in the Bi(110) film. Our data show that there are no non-trivial electronic states in Bi(110) films grown on black phosphorus substrates.

Ultrathin Bi Nanosheets with Superior Photoluminescence.

Despite substantial progress in the science and technology of 2D nanomaterials, facile fabrication of ultrathin 2D metals remains challenging. Herein, an efficient hot-pressing method is developed to fabricate free-standing ultrathin Bi nanosheets from Bi nanoparticles. Highly crystalline Bi nanosheets with thickness as low as ≈2 nm and area of more than several micrometers are successfully fabricated on silicon substrates. The ultrathin Bi nanosheets exhibit morphology and structural dependent enhanced broad range photoemission in visible region of spectrum. Our cost-effective hot-pressing strategy may open an insight for production, application, and deficient fundamental understanding of other 2D semimetals/metalloids and noble metals.

Size-dependent structural and electronic properties of Bi(111) ultrathin nanofilms from first principles

Few layer bismuth nanofilms with (111) orientation have shown striking electronic properties, especially as building blocks of novel two-dimensional heterostructures. In this paper we present state-of-the-art first principles calculations, based on both density functional theory and maximally localized Wannier functions, that encompass electronic and structural properties of free-standing Bi(111) nanofilms. We accurately evaluate both the in-plane lattice constant and, by including the van der Waals interaction between bismuth bilayers, the intra/interlayer distances. Interestingly and somehow unexpectedly, the in-plane lattice constant is predicted to shrink by about 5% going from the thickest investigated nanofilm $(\ensuremath{\sim}80\phantom{\rule{0.16em}{0ex}}\AA{})$ to single bilayer Bi(111), entailing a thickness dependent lattice mismatch in complex heterostructures involving ultrathin Bi(111). Moreover, quantum confinement effects, that would be expected to rule the electronic structure at this size range, compete with surface states that appear close to and across the Fermi level. The implication is that not only all but the thinnest films have a metallic band structure but also that such surface states might play a role in either the formation of interfaces with other materials or for sensing applications. Finally, the calculated electronic structure compares extremely well with ARPES measurements.

Elastic interactions between interface dislocations and internal stresses in finite-thickness nanolayered materials

Anisotropic elasticity theory is used to analyze elastic strain interactions between heterophase interface dislocations and four types of internal stresses in finite-thickness nanolayered face-centered cubic materials. The first interaction is related to the mechanical forces acting on semicoherent interfaces, which arise from heterogeneous elastic fields between the neighboring materials with free surfaces. The second interaction is associated with the Peach-Koehler force exerted on lattice dislocations in free-standing bi- and tri-nanolayers, and is compared to the limiting case of infinite bicrystals. The third case yields to the interaction energy balance and the corresponding equilibrium distance between two Shockley partial dislocations. The fourth and last case is dedicated to the elastic interaction forces between vacancies and interfaces using the force dipole moment approximation.For such cases, the elastic interaction problems are treated by involving the effects of (i) the dislocation characters, (ii) the anisotropic (versus isotropic) elasticity calculations, and (iii) the additional third coherent MgO layer on bilayered Au/Al systems.

Phase-coherent transport in catalyst-free vapor phase depositedBi2Se3crystals

Free-standing Bi$_2$Se$_3$ single crystal flakes of variable thickness are grown using a catalyst-free vapor-solid synthesis and are subsequently transferred onto a clean Si$^{++}$/SiO$_2$ substrate where the flakes are contacted in Hall bar geometry. Low temperature magneto-resistance measurements are presented which show a linear magneto-resistance for high magnetic fields and weak anti-localization (WAL) at low fields. Despite an overall strong charge carrier tunability for thinner devices, we find that electron transport is dominated by bulk contributions for all devices. Phase coherence lengths $\l_\phi$ as extracted from WAL measurements increase linearly with increasing electron density exceeding $1 \mu $m at 1.7 K. While $\l_\phi$ is in qualitative agreement with electron electron interaction-induced dephasing, we find that spin flip scattering processes limit $\l_\phi$ at low temperatures.

Free-standing Bi–Sb–Te films derived from thermal annealing of sputter-deposited Sb2Te3/Bi2Te3multilayer films for thermoelectric applications

We introduce an easily acceptable method to produce free-standing Bi–Sb–Te films from Sb2Te3/Bi2Te3 multilayer films based on solid-state reactions. When sputter-deposited Sb2Te3/Bi2Te3 multilayer films were annealed at 400 °C under a N2 gas atmosphere, they were transformed into single layers composed of Bi–Sb–Te films. At the same time, they were spontaneously stripped on a large scale from the substrates (1 cm × 1 cm) without chemical etching. The as-fabricated free-standing Bi–Sb–Te films were very flexible, with a thickness of about 400 nm. The findings confirmed that the formation of voids and the strong flow rate of ambient gas play important roles in fabricating free-standing Bi–Sb–Te films during the thermal annealing process. The temperature-dependent crystallization behaviors of each of the Sb2Te3 and Bi2Te3 films were also investigated to understand the mechanism. Moreover, the thermoelectric properties of the free-standing Bi–Sb–Te films were examined with different thicknesses of the films. We believe that this method can contribute to the development of low-dimensional nanostructures that can be useful in various applications such as energy conversion, energy storage and wearable electronics.

Recrystallized Arrays of Bismuth Nanowires with Trigonal Orientation

We demonstrate methods to improve the crystalline-quality of free-standing Bi nanowires arrays on a Si substrate and enhance the preferred trigonal orientation for thermoelectric performance by annealing the arrays above the 271.4 °C Bi melting point. The nanowires maintain their geometry during melting due to the formation of a thin Bi-oxide protective shell that contains the molten Bi. Recrystallizing nanowires from the melt improves crystallinity; those cooled rapidly demonstrate a strong trigonal orientation preference.

Nontrivial topological electronic structures in a single Bi(111) bilayer on different substrates: A first-principles study

Electronic structures, minimum energy configurations, and band topology of strained Bi(111) single bilayers placed on a variety of semiconducting and insulating substrates are investigated using first-principles calculations. A topological phase diagram of a free-standing Bi bilayer is presented to help guide the selection of suitable substrates. The insulating hexagonal-BN is identified as the best candidate substrate material for supporting nontrivial topological insulating phase of Bi bilayer thin films. A planar hexagonal Bi layer is predicted under tensile strain, which we show could be realized on a SiC substrate. The Bi bilayer becomes metallic under the compressive strain induced by Si and Ge substrates.

Femtosecond electron diffraction: Preparation and characterization of (110)-oriented bismuth films

Here, we present a new approach to synthesize (110)-oriented ultrathin membranes of bismuth (Bi). This rather exotic orientation was achieved by directing the growth through rationale control of lattice matching. Bi films were hetero-epitaxially grown on the (100)-surface of freshly cleaved potassium chloride crystals. The sample orientation was characterized by x-ray and electron diffraction. In addition, high quality free-standing films were obtained after dissolution of the substrate in water and controlled evaporation. Femtosecond electron diffraction (FED) was, therefore, used to monitor the coherent shear acoustic phonons in (110)-oriented free-standing Bi films produced by impulsive femtosecond optical excitation. The small de Broglie wavelength (flat Ewald sphere) of keV-electrons combined with an off-Bragg detection scheme provided a magnified view of shear atomic motions, i.e., lattice distortions in the transverse direction. All-optical pump-probe experiments are usually insensitive to shear displacements, a fact that makes FED a unique non-contact method to achieve the complete characterization of elastic properties of nanoscale materials.

Quantum well states in ultrathin Bi films: Angle-resolved photoemission spectroscopy and first-principles calculations study

The features of the QWSs are well-reproduced by ab initio calculations for free-standing Bi slabs. The standard method of the phase-shift accumulation model is applied to the QWSs and the bulk band dispersion perpendicular to the surface at finite parallel momentum is experimentally obtained for the first time. The phase shifts at the film interfaces are discussed in detail. The QWSs have little contribution to the electronic structure near the Fermi level and this suggests that the macroscopic physical properties of the films in the thickness of several atomic layers are likely determined by the highly metallic surface states.

Processing and Characterization of Single-Crystalline Ultrafine Bismuth Nanowires

By pressure injecting Bi liquid melt into the nanochannels of an anodic alumina template, we have successfully fabricated Bi nanowire arrays with ultrafine wire diameters and extremely high wire packing densities. Free-standing Bi nanowires with controlled wire diameters and large aspect ratios (length/diameter) were also obtained by subsequent etching of the alumina template. Various techniques such as SEM, TEM, AFM, EFM, HREM, and XRD have been used to investigate the physical characteristics of these nanowires. The Bi nanowires were found to be dense and continuous and had a uniform diameter throughout the length of the wires. Individual Bi nanowires were shown to be single crystals, and all the wires in an array were highly oriented. An interesting metastable phase of Bi was also observed, which can be attributed to a lattice stress-induced high-pressure phase of Bi formed inside the porous anodic alumina template.

Fabrication, Characterization and Electronic Properties of Bismuth Nanowire Systems

AbstractWe have fabricated ultra-fine Bi nanowire (10–120 nm) arrays with packing densities as high as 7.1 × 1010/cm2 by pressure injecting its liquid melt into the evacuated nano-channels of an anodic alumina template. Using this fabrication technique, we have also prepared Te-doped n-type Bi nanowires. Free-standing Bi nanowires with an aspect ratio (length/diameter) higher than 500 are obtained by etching away the anodic alumina matrix without attacking the Bi. The resulting Bi nanowires are shown to be single crystals (with the same crystal structure as bulk Bi) and all the wires of a nanowire array are similarly oriented along the wire axis. The small electron effective mass of Bi, the high anisotropy of its Fermi surface, and the large aspect ratio of the Bi nanowires make this a very promising material for low-dimensional thermoelectric applications and an excellent system for studying quantum confinement effects in a quasi-one-dimensional (1D) electron gas. A theoretical model based on the basic band structure of bulk Bi, suitably modified for the 1D situation, is constructed to explore the electrical transport properties of Bi nanowires, which are expected to be very different from those of bulk Bi.