Bi Films Research Articles

Electrodeposited partially oxidized Bi & NiCo alloy based thin films for aqueous hybrid high energy microcapacitor

Electrochemical energy storage (EES) devices play an important role in meeting the demand for the electrical energy. Particularly, electrochemical hybrid capacitors (HCs) are considered as one of the potential candidates for fulfilling the energy requirement. In this work, aqueous electrolyte-based high voltage HC is fabricated and its electrochemical characteristics are analyzed. Here, electrochemically prepared NiCo and Bi thin films on stainless steel substrate were used as positive and negative electrodes, respectively. The phase structure and crystalline size of the electrodeposited films are confirmed from XRD and AFM. It is observed that the as prepared NiCo film showed an excellent specific capacitance value of 83.33 mF cm−2 at 0.5 mA cm−2; on the other side, Bi film also showed a good storage capacity of 32.2 μA h cm−2 at 1 mA cm−2; To realize the high energy density, Bi||NiCo combination of the thin film HC is fabricated and the HC devices showed excellent performance at a tested potential window of 1.5 V. The cell plateau voltage is observed at 0.8 V. The maximum specific energy density of 7.3 μW h was achieved at a specific power density is 0.3 mW cm−2. Maximum specific power of 7.5 mW cm−2 at an energy density of 1.875 μW h was obtained from HSC. This indicates the excellent performance of the HC at various conditions. Thus, the binder-free, electrodeposited active electrode materials of the present work will play a significant role in the various electronic applications by supplying the energy demand.

Electrodeposition conditions-dependent crystal structure, morphology and electronic properties of Bi films

The study is devoted to the electronic properties and structure of bismuth films obtained by electrodeposition from high-speed perchlorate electrolyte. Polycrystalline samples were synthesized in acidic perchlorate electrolyte under the (0.18–70.0) mA/cm2 cathode current density and annealed at 265 °C in the He gas atmosphere. The structure, microstructure and electron properties of Bi films were characterized using X-Ray diffraction analysis, scanning electron microscopy, and electron backscattered diffraction. Electrical resistance, magnetoresistance, and Hall coefficient were studied at the 5–300 K temperature range under magnetic field up to 8 T. A specific and unexpected behavior has been found concerning the average size and shape of Bi grains with increasing of the current density: the increase in deposition current density up to 70.0 mA/cm2 contributes to Bi films formation with more isotropic and reduced average grains size. Samples synthesized under 0.18 mA/cm2 current density after annealing showed electronic properties, including magnetoresistance, similar to those of flawless single-crystals. The differences in the electronic characteristics of Bi films electrodeposited under different currents and electrolyte composition were explained by changes in electronic mobility due to scattering on grain boundaries.

Epitaxial growth of Bi(110) and Bi2Se3 thin films on a ferromagnetic insulator substrate of Cr2Ge2Te6

When a topological insulator (TI) is brought to the proximity of a ferromagnetic insulator (FMI), the breaking of the time-reversal symmetry may give rise to quantum anomalous Hall effect (QAHE). The physical properties of such TI–FMI systems are greatly affected by the interfacial structures of the components. Here, we report the growth and structural properties of Bi(110) and Bi2Se3 thin films on a FMI of Cr2Ge2Te6 (CGT) substrate by scanning tunneling microscopy. We observed various defects and impurities on the CGT surfaces, which serve as the preferential sites for initial nucleation and epitaxial growth of Bi(110) thin films. The exposure of the as-grown Bi(110) thin films to Se vapor leads to the formation of polycrystalline Bi2Se3 thin films with randomly distributed holes. The structure and composition of the as-prepared Bi2Se3 thin films were further confirmed by Raman spectroscopy and x-ray photoelectron spectroscopy. Our work shows that the quality of the CGT crystals is vital for the growth of high-quality TIs on CGT substrates for QAHE.

Electronic properties of bismuth nanostructures

The passivation of thin Bi(1 1 1) films with hydrogen and oxide capping layers is investigated from first principles. Considering termination-related changes of the crystal structure, we show how the bands and density of states are affected. In the context of the much discussed semimetal-to-semiconductor transition and the band topology of the bulk material, we consider the effects of confinement in the whole Brillouin zone and go beyond standard density functional theory by including many-body interactions via the G$_0$W$_0$ approximation. The conductivity of unterminated films is calculated via the Boltzmann transport equation using the simple constant relaxation time approximation and compared to experimental observations that have suggested a two-channel model.

Quantum Size Effects, Multiple Dirac Cones, and Edge States in Ultrathin Bi(110) Films

The presence of inherently strong spin-orbit coupling in bismuth, its unique layer-dependent band topology and high carrier mobility make it an interesting system for both fundamental studies and applications. Theoretically, it has been suggested that strong quantum size effects should be present in the Bi(110) films, with the possibility of Dirac Fermion states in the odd-bilayer (BL) films, originating from dangling pz orbitals and quantum-spin hall (QSH) states in the even-bilayer films. However, the experimental verification of these claims has been lacking. Here, we study the electronic structure of Bi(110) films grown on a high-Tc superconductor, Bi2Sr2CaCu2O8+δ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES). We observe an oscillatory behavior of electronic structure with the film thickness and identify the Dirac-states in the odd-bilayer films, consistent with the theoretical predictions. In the even-bilayer films, we find another Dirac state that was predicted to play a crucial role in the QSH effect. In the low thickness limit, we observe several extremely one-dimensional states, probably originating from the edge-states of Bi(110) islands. Our results provide a much needed experimental insight into the electronic and structural properties of Bi(110) films.

Do equidistant energy levels necessitate a harmonic potential?

Experimental results from literature show equidistant energy levels in thin Bi films on surfaces, suggesting a harmonic oscillator description. Yet this conclusion is by no means imperative, especially considering that any measurement only yields energy levels in a finite range and with a nonzero uncertainty. Within this study we review isospectral potentials from the literature and investigate the applicability of the harmonic oscillator hypothesis to recent measurements. First, we describe experimental results from literature by a harmonic oscillator model, obtaining a realistic size and depth of the resulting quantum well. Second, we use the shift-operator approach to calculate anharmonic non-polynomial potentials producing (partly) equidistant spectra. We discuss different potential types and interpret the possible modeling applications. Finally, by applying nth order perturbation theory we show that exactly equidistant eigenenergies cannot be achieved by polynomial potentials, except by the harmonic oscillator potential. In summary, we aim to give an overview over which conclusions may be drawn from the experimental determination of energy levels and which may not.

Structural and optical properties of n-type and p-type GaAs(1−x)Bi x thin films grown by molecular beam epitaxy on (311)B GaAs substrates

In this paper, we report on the structural and optical properties of n-type Si-doped and p-type Be-doped GaAs(1−x)Bi x thin films grown by molecular beam epitaxy on (311)B GaAs substrates with nominal Bi content x = 5.4%. Similar samples without Bi were also grown for comparison purposes (n-type GaAs and p-type GaAs). X-ray diffraction (XRD), micro-Raman at room temperature, and photoluminescence measurements as a function of temperature and laser excitation power (P EXC) were performed to investigate their structural and optical properties. XRD results revealed that the Bi incorporation in both n-type and p-type doped GaAsBi was similar, despite that the samples present remarkable differences in the number of Bi related defects, non-radiative centers and alloy disorder. Particularly, our results evidence that the Bi-related defects in n- and p-doped GaAsBi alloys have important impact on the differences of their optical properties.

Terahertz Photoconductivity Spectra of Electrodeposited Thin Bi Films.

Electron dynamics in the polycrystalline bismuth films were investigated by measuring emitted terahertz (THz) radiation pulses after their photoexcitation by tunable wavelength femtosecond duration optical pulses. Bi films were grown on metallic Au, Pt, and Ag substrates by the electrodeposition method with the Triton X-100 electrolyte additive, which allowed us to obtain more uniform films with consistent grain sizes on any substrate. It was shown that THz pulses are generated due to the spatial separation of photoexcited electrons and holes diffusing from the illuminated surface at different rates. The THz photoconductivity spectra analysis has led to a conclusion that the thermalization of more mobile carriers (electrons) is dominated by the carrier–carrier scattering rather than by their interaction with the lattice.

The scattering mechanism analysis of anisotropic (00l)-orientation bismuth films based on magnetoresistance measurement method

The magnetoresistance anisotropic effect of (00l)-oriented semimetal polycrystalline bismuth (Bi) films is proposed as a candidate for Ettingshausen device and magnetoresistance device. By tuning film samples, 29 nm thickness (00l)-oriented columnar bismuth films with different grain sizes were prepared. The electrical transport measurement results showed that due to the different grain sizes, the magnetoresistance anisotropy of the Bi films with the same thicknesses differed by 3 times at low temperatures (T = 2 K). The main collision mechanism controlling the Bi film resistance was discriminated by whether the surface scattering or grain boundary scattering, based upon whether the magnetoresistance depends on the thickness or the grain sizes. The analysis result indicated that the surface scattering of the Bi film directly affected the semimetal to semiconductor transition. The present study provides a pathway for the electronic device application operating at low temperature by controlling the grain sizes.

Surface Structure Control and Charge/Discharge Characteristics of Bismuth Anode Materials By Electrodeposition for Magnesium-Ion Batteries

The magnesium ion secondary battery (MIB) has attracted much attention because magnesium (Mg) metal has high theoretical capacity (3,755 mAh/cm3), high electrochemical stability, and high abundance in the earth. However, developing anode and cathode materials with superior capacity for conventional electrolytes has required considerable study. Bismuth (Bi) has high capacity as an anode and also resists oxidation in Mg-containing electrolyte, so is a promising candidate material. However, experimental studies suggest that the available capacity has not yet reached the theoretical limit, and that the cycle life is short. This is due to sluggish diffusion of Mg2+ into the anode material and the slow interfacial charge transfer. We aimed to improve the electrochemical properties by atomizing the internal and surface structure of the Bi anode and enhancing the diffusibility of Mg2+. In this study, we prepared Bi films by an electrodeposition method, in which the structure of the Bi electrodes could be controlled by changing some of the electrodeposition conditions. We also evaluated the electrochemical properties of Bi anode materials deposited with different deposition times and current densities, to elucidate the relationships between the structure of the Bi anode and its capacity and cycle life.We prepared an electrolytic bath by dissolving H2SO4, Na2SO4 and polyethylene glycol in pure water, and subsequently added H2SO4 to the electrolytic bath to control the pH at 1.2. Before electrodeposition, we removed organic species and native oxide from the Cu foil surface with NaOH and HCl. We employed a Cu plate with an exposed foil area of 2 × 2 cm2 and a carbon plate as the cathode and anode, respectively. The Bi electrodeposition was carried out, while changing the deposition time and current density (from 10 to 50 mA/cm2). The film thickness was controlled by the deposition time (from 1.5 to 3.0 μm), and was calculated based on the weight increase under the assumption that the film was uniform. The surface morphology and crystal structure of the Bi films were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. In addition, the characteristics were compared before and after discharge/charge. The cycle characteristics of the Bi anodes were studied in PhMgCl/THF electrolyte. The bipolar half-cells were assembled with Mg plate as the anode and Bi electrode as the cathode. We established representative galvanostatic charge/discharge profiles of the Bi electrodes, at a current density of 50 mA/g within 0–0.6 V, versus Mg2+/Mg. We evaluated the cyclic stability of the Bi electrodes under the same conditions until the 50th cycle. The elemental composition and chemical bonding state of the Bi films after charge/discharge were evaluated by energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS).Figure 1 shows the cycling performance of Bi films with thicknesses of 1.5, 2.0, and 3.0 μm until the 50th cycle. Each film showed a capacity of about 300 mAh/g at the early stage of cycling, which is the maximum capacity. The capacities were about 1.5 times those of the Bi anodes which Niu et al. prepared by powder metallurgy. The cycle performance differed among Bi films differing in thickness, and the 1.5-µm Bi film maintained the maximum capacity until the 25th cycle. Thus, film thickness had an impact on the cycle performance.Figure 2 shows the surface morphology of Bi films with different thicknesses before and after the charge/discharge reaction. The figure also shows the compositional ratio of Bi to Mg on the surface of the Bi films. As the film thickness increased, agglomerates involving a large number of particles, indicated by the circles marked by “A” in Fig. 2, tended to increase in diameter from 1 to 3 µm. After charging/discharging, Mg dendrites were observed on the surface of the Bi films with thicknesses of 2 and 3 µm (circles marked by “B”), which resulted in a low capacity maintenance rate. Thus, the localized formation of Mg dendrites on the Bi surface could explain the decreased cycle performance of the Bi films prepared in this study. Figure 1

Investigation of Calcium Alloying with Sb, Sn and Bi As Negative Electrode Materials for Rechargeable Calcium Battery in Non-Aqueous Electrolytes

Alloys were suggested as alternative anode materials for Ca metal anode because they have a high specific capacity and low overpotential. [1] A DFT study of CaxM has suggested a variety of alloy-type anode materials and considered the alloying voltage, volume expansion, and specific capacity. The metals such as Si, Sb, Bi, Al, Pt, Pb, and so on were predicted to be promising inexpensive anode candidates. [1] An Sn Nano-particular with a thickness of 25 μm was investigated for both the anode and current collector using 0.8 M Ca(PF6)2 in EC:PC:DMC:EMC (2:2:3:3) electrolytes. [2] It’s showed a high and wide voltage window (up to 4.8 V) and the cell could be cycled more than 300 cycles. The alloying compounds were found to be different phases with finally Ca7Sn6.In this work, we study the fundamentals of the alloying process alloying of Ca thin film of bulk Sb, Sn, and Bi on polycrystalline Au using non-aqueous electrolyte (1.5 M Ca(BH4)2 in THF) at room temperature. Calcium stripping and plating using a Ca(BH4)2 in THF electrolyte shows high Coulombic efficiency (95 %) at room temperature on an Au substrate. [3] The cycling reversibility of alloying/de-alloying with Sb, Sn, and Bi have been demonstrated. The cyclic voltammogram for Ca insertion/de-insertion into Sb, Sn, and Bi-modified electrode shows a positive shift of the onset potential of Ca deposition compared to that at bare Au electrode. The ratio of moles of Ca to Sb, Sn, and Bi agrees with the stoichiometry of CaSb2, CaSn2, and CaBi3 alloy. Potential step were used to study the kinetics of insertion. The diffusion coefficient of Ca into the Sb, Sn, and Bi layers is calculated to be 10−14 \U0001d461\U0001d45c 10−15 cm2s-1. Acknowledgments: The authors acknowledge funding by the Federal Ministry of Education and Research of Germany (BMBF, FKZ; 03EK3027A).[1] Z. Yao, V. I. Hegde, A. Aspuru-Guzik, and C. Wolverton, Advanced Energy Materials 9:1802994 (2019).[2] N. Z. Wu, W. J. Yao, X. H. Song, G. Zhang, B. J. Chen, J. H. Yang, and Y. B. Tang, Advanced Energy Materials 9:7 (2019).[3] D. Wang, X. Gao, Y. Chen, L. Jin, C. Kuss, and P. G. Bruce, Nature Materials 17:16 (2017).

Morphological influence of BiVO4 nanostructures on peroxymonosulfate activation for highly efficient catalytic degradation of rhodamine B.

Bismuth vanadate (BiVO4) nanostructured films were prepared and successfully applied for peroxymonosulfate (PMS) activation for the degradation of rhodamine B (RhB) in aqueous solution. The BiVO4 thin films were obtained by thermal reaction between electrodeposited bismuth (Bi) films and vanadium precursor. The as-prepared BiVO4 porous, nanoflowers, and cluster nanostructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and BET analysis. The catalytic performance of BiVO4 nanostructures has been carefully evaluated in activating PMS for the degradation of RhB. The nanoflower-like BiVO4 nanostructures exhibit the best catalytic activity. Under optimized conditions, the complete catalytic degradation of RhB using BiVO4 nanoflowers/PMS system was achieved in 17 min at room temperature as revealed by high-performance liquid chromatography (HPLC) analysis. Quenching experiments suggested that sulfate radicals are the main active species in the degradation process. Additionally, BiVO4 catalyst remained stable without any apparent activity loss after five cycling runs.

Comparative growth study of ultrathin Bi films on clean and oxygen-reconstructed Nb(110)

We present a detailed study of the growth of Bi films on superconducting Nb(110) substrates in dependence on the Bi coverages and the Nb surface quality. We find that Bi grows in a (110) orientation at low coverage equivalent to about five pseudomorphic monolayers (${\mathrm{ML}}^{\mathrm{ps}}$) on clean Nb(110), but then undergoes a structural transition to Bi(111) below about 8 ${\mathrm{ML}}^{\mathrm{ps}}$. Comparison with two oxygen-reconstructed Nb(110) surfaces, the $\mathrm{Nb}{\mathrm{O}}_{x}$ phases I and II, reveals that the film thickness at which the (110)-to-(111) transition takes place depends on the surface quality. Whereas it is observed at lower coverage for the $\mathrm{Nb}{\mathrm{O}}_{x}$ phase I, our results indicate that Bi(110) remains stable on $\mathrm{Nb}{\mathrm{O}}_{x}$ phase II up to the largest film thickness studied here, i.e., 18 ML. The quality and smoothness of the thin Bi films considerably depends on the cleanliness of the Nb substrate, revealing the most flat and defect-free Bi films grown on the oxygen-free clean Nb(110) surface.

Negligible spin–charge conversion in Bi films and Bi/Ag(Cu) bilayers

Spin pumping experiments using ferromagnetic metals have reported highly efficient spin–charge conversion in Bi and at the Bi/Ag interface, possibly due to the inverse Rashba–Edelstein effect. However, longitudinal spin Seebeck effect experiments using the yttrium iron garnet ferrimagnetic insulator in Bi films and Bi/Ag bilayers do not show evidence of appreciable spin-to-charge conversion except the large Nernst signal inherent to Bi. These contrasting conclusions highlight the differences between magnetic metals and magnetic insulators as spin current injectors. Only the detected voltages that adhere to the inverse spin Hall effect of jC = (2e/ℏ)θSH jS × σ are due to spin currents.

Mapping the evolution of Bi/Ge(111) empty states: From the wetting layer to pseudo-cubic islands

Semiconductors interfaced with heavy elements possessing a strong atomic spin–orbit coupling are important building blocks for the development of new spintronic devices. Here, we present a microscopic and spin-resolved spectroscopic investigation of ultrathin Bi films grown onto a Ge(111) substrate. At monolayer coverage, a Bi wetting layer is formed, characterized by a semiconducting behavior and a (3×3)R30° superstructure. The wetting layer supports the subsequent growth of Bi islands with a pseudo-cubic structure similar to that of Bi(110), showing a well-defined orientation with respect to the substrate high-symmetry directions. We performed photoemission and spin-resolved inverse photoemission experiments at off-normal electron emission and incidence, respectively, along the substrate Γ¯K¯ direction. Inverse photoemission, in particular, highlights the presence of a spin-polarized empty Bi state, not reported so far, due to the strong spin–orbit effects characteristic of the Bi surface and thin layers. Finally, scanning tunneling spectroscopy is employed to link the observed spectroscopic features to either the wetting layer or the Bi islands.

Films of immiscible systems obtained by three-electrode ion-plasma sputtering

The data on the ion-plasma sputtering of composite targets of the systems (Fe, Co, Ni) – (Ag, Bi) whose components do not mix in the liquid state is presented. The lattice parameters and sizes of crystals of metastable phases were determined. The films deposited show the formation of supersaturated solid solutions ore nanocrystalline and amorphous phase. The decomposition of metastable phase is accompanied by precipitation of equilibrium solid solutions based on incoming elements ore an intermediate phase. We studied the thermal stability of metastable states in (Fe, Co, Ni) – (Ag, Bi) films, the electrical and hysteresis magnetic characteristics of as-deposited and heat-treated films and established the optimum film compositions with precise values of temperature coefficient of resistivity (∼ 10−5 K−1) and high coercive force (H c ∼ 150 kA/m). Such films can be promising for use as thin-film precision resistors and magnetic information carriers with an increased recording density.

In-gap states induced by distortion in α-bismuthene

Ultrathin Bi(110) films have attracted significant attention recently as potential two-dimensional topological insulators. Their topological band structure induces one-dimensional (1D) helical edge states inside the bulk band gap; however, modification of the density of states resulting from Bi atomic distortion has also been suggested. Here we report the distortion-modified electronic states of α-bismuthene films grown on graphene/6H-SiC(0001) by using low-temperature scanning tunneling microscopy/spectroscopy. We found that the edges along the zigzag boundary between α-bismuthene and graphene are reconstructed and distorted and that the edge states are topographically dependent. The possible trivial origin of edge states induced by distortion complicates the investigation of topological nature in α-bismuthene. Steps from the substrate, stripes, and domain boundaries also induce additional site-dependent in-gap states. Additionally, we located an inversion domain boundary across which atoms are configured in the opposite order. This defect has extended spatial distribution and 1D parabolic dispersion, and hence we suggest it to be a very good platform for further topological exploration in α-bismuthene.

Silicon dots films deposited by spin-coating as a generated carrier addition layer of third generation photovoltaics

In this work, silicon ink composing of silicon powder and zinc oxide solution was formulated and spin-coated on quartz and n/p-Si substrates followed by drying the films under atmosphere at the temperature of 550 ​°C. The results showed that this top-addition layer could be the highly promising layer for photo-generating carriers in third-generation photovoltaics to enhance blue-light absorption. X-ray diffraction and scanning electron microscopy techniques were used to study the presence of silicon and zinc oxide nano-crystallites. The thin films consisting of different energy bandgap of Si nanocrystals (~100 ​nm) with narrow bandgap and spherical ZnO:Bi nanocrystal (~20 ​nm) with wider bandgap could be obtained from the evidence of bandgap enlargement. The band gaps of the thin films were tunable by adjusting silicon dots density in ZnO:Bi film. Energy upshift of light absorption edge depended on the silicon dots density was observed in the range 1.6–3.3 ​eV related band gap enlargement by Tauc plot. Under illumination, a high photocurrent gain of the thin film comprised of low Si dots density coated on a quartz substrate was about 103 times higher compared with its dark current. This result is agreeably explained in terms of its lower superficial trap states at the interface between silicon and zinc oxide matrix. The composite layer can be applied to a third-generation solar cell with the efficiency 1.50% higher than that with a typical crystalline-Si solar cell.

Electrical conductivity of a thin film in the case of an arbitrarily oriented ellipsoidal isoenergetic surface of a conductor

The static electrical conductivity of a thin conducting film is calculated using the kinetic approach. The model of diffuse boundary conditions is used as boundary conditions for the charge carrier distribution function. The isoenergetic surface of the conducting film material is an ellipsoid of rotation that is arbitrarily oriented in the pulse space. The volume of the conductor is dominated by scattering on impurity atoms and on dislocations of electrons (holes). The electrical conductivity of the film is analyzed depending on the angles that determine the direction of the ellipsoid axis of rotation, and a comparison with the results for the case of a macroscopic sample is presented. A comparison is made with experimental data for the electrical conductivity of Bi and Si thin films.

Self-driving CO2-to-formate electro-conversion on Bi film electrode in novel microbial reverse-electrodialysis CO2 reduction cell

Sustainable energy recovery from microbial electrochemical system provided a promising way for the electrochemical reduction of CO2 into value-added products, which potentially tackled the problems regarding to the higher electrical energy input. Herein, a microbial reverse-electrodialysis CO2 reduction cell (MRECC) was developed to achieve self-driving CO2-to-formate conversion by extracting the energy from salinity gradient and wastewater simultaneously. Results confirmed that assembling 11 cell pairs in reverse electrodialysis (RED) stack of MERCC could meet the potential requirement of Bi film cathode for driving CO2 reduction. The cell pairs, flow rate of the salt solution (sodium chloride) and the external resistance showed obvious effects on the MRECC performance. Under the optimal condition, the maximum formate concentration of 185.4 ± 6.1 mg L−1 and faradaic efficiency of 88.9 ± 3.2% were yielded with anodic COD removal up to 87.0 ± 4.3%. Energy efficiency indicated nearly 5.8% of extracted energy from salinity gradient and wastewater had been utilized for CO2-to-formate conversion. Overall, MRECC exhibited a promising platform for CO2 reduction and value-added chemicals production by using sustainable energy from wastewater.