Bi Films Research Articles

Elemental topological ferroelectrics and polar metals of few-layer materials

Ferroelectricity can exist in elemental phases as a result of charge transfers between atoms occupying inequivalent Wyckoff positions. We investigate the emergence of ferroelectricity in two-dimensional elemental materials with buckled honeycomb lattices. Various multibilayer structures hosting ferroelectricity are designed by stacking engineering. Ferroelectric materials candidates formed by group IV and V elements are predicted theoretically. Ultrathin Bi films show layer-stacking-dependent physical properties of ferroelectricity, topology, and metallicity. The two-bilayer Bi film with a polar stacking sequence is found to be an elemental topological ferroelectric material. Three and four bilayers Bi films with polar structures are ferroelectriclike elemental polar metals with topological nontrivial edge states. For Ge and Sn, trivial elemental polar metals are predicted. Our work reveals the possibility of designing two-dimensional elemental topological ferroelectrics and polar metals by stacking engineering. Published by the American Physical Society 2024

Signal oscillations in helium scattering by bismuth atoms in the low energy range

Low Energy Ion Scattering (LEIS) analysis of bismuth selenide (Bi2Se3), a strong 3D topological insulator, revealed oscillations of the detected signal in dependence on primary ion beam energy. Bismuth is a part of the group of elements where oscillatory behaviour was already noticed, such as gallium, indium, or lead. Generally, it is ascribed to quasi-resonant charge exchange processes between primary ion and target atom. Moreover, clear differences exist between data for target atoms in elemental form and for atoms in compound form. A study of Bi signal yields in different material forms was performed for primary He+ projectiles within the energy range from 0.50 to 6.00 keV. A foil of pure Bi, Bi2Se3, Bi2O3 and thin Bi films deposited on several substrates (SiO2, CuOx) by Molecular Beam Epitaxy were analysed. The energy shift in the position of oscillations was observed when the oxygen atoms were present at the surface and bonded to Bi atoms. A description of the Bi ion yield variation is provided to facilitate the quantification process in LEIS.

Floquet engineering of axion and high-Chern number phases in a topological insulator under illumination

Quantum anomalous Hall, high-Chern number, and axion phases in topological insulators are characterized by its Chern invariant C (respectively, C=1, integer C>1, and C=0 with half-quantized Hall conductance of opposite signs on top and bottom surfaces). They are of recent interest because of novel fundamental physics and prospective applications, but identifying and controlling these phases has been challenging in practice. Here we show that these states can be created and switched between in thin films of Bi_22Se_33 by Floquet engineering, using irradiation by circularly polarized light. We present the calculated phase diagrams of encountered topological phases in Bi_22Se_33, as a function of wavelength and amplitude of light, as well as sample thickness, after properly taking into account the penetration depth of light and the variation of the gap in the surface states. These findings open pathways towards energy-efficient optoelectronics, advanced sensing, quantum information processing and metrology.

Ultrasensitive detection of L-phenylalanine on DC magnetron sputtered bismuth thin films using label-free SERS method

Quantitative imbalances in monomers such as amino acids, nucleotides etc. act as biomarkers for various health conditions like cancer, Alzheimer’s, genetic disorders etc. Different atmospheric annealed bismuth (Bi) thin films deposited by DC magnetron sputtering were compared to select a better Surface Enhanced Raman Spectroscopy (SERS) platform. The tailored morphology favoured confining of localized surface plasmon resonance (LSPR) between close-packed Bi nanoparticles. The Multiple SPRs (Mult. SPRs) from interband plasmonic transitions are majorly present in the ultraviolet (UV)-near-visible region making the film a promising UV- SERS platform. The varied annealing’s have tuned the SPR and are supported by the colour changes of Bi films. The Raman modes confirmed the chemical stability the films. XPS etching revealed the metallic state of the optimized Bi-D-PDV2 film. The preliminary SERS applications on Rhodamine 6 G (RH6G) selected a Bi-D-PDV2 film with an enhancement factor (EF) of ∼4 ×105. This optimized film used for L-phenylalanine (Phe) detection, resulted in enhanced sensitivity of ∼2 ×106 EF with a sub-micromolar (0.1 μM) limit of detection (LOD). The ultra-sensitive responses in the sub-μM level from the optimized Bi thin film for standard as well as biomolecule is reported for the first time to the best extent of knowledge. The potential responses from the optimized film gives promising response for its use in ultra-sensitive bio detections even down to single molecular level in future.

Structure, ferroelectric, and enhanced fatigue properties of sol–gel-processed new Bi-based perovskite thin films of Bi(Cu1/2Ti1/2)O3–PbTiO3

Bi-based perovskite ferroelectric thin films have wide applications in electronic devices due to their excellent ferroelectric properties. New Bi-based perovskite thin films Bi(Cu1/2Ti1/2)O3–PbTiO3 (BCT–PT) are deposited on Pt(111)/Ti/SiO2/Si substrates in the present study by the traditional sol–gel method. Their structures and related ferroelectric and fatigue characteristics are studied in-depth. The BCT–PT thin films exhibit good crystallization within the phase-pure perovskite structure, besides, they have a predominant (100) orientation together with a dense and homogeneous microstructure. The remnant polarization (2P r) values at 30 μC/cm2 and 16 μC/cm2 are observed in 0.1BCT–0.9PT and 0.2BCT–0.8PT thin films, respectively. More intriguingly, although the polarization values are not so high, 0.2BCT–0.8PT thin films show outstanding polarization fatigue properties, with a high switchable polarization of 93.6% of the starting values after 108 cycles, indicating promising applications in ferroelectric memories.

Fabrication and characterization of Sn-57 wt% Bi film by pulse DC current co-electrodeposition and reflow

The electrodeposited Sn-Bi eutectic alloys can satisfy the need for micro-bumps with low melting point in 3D packaging. We deposited Sn-Bi film by pulse DC current in electroplating solutions based on methane sulfonic acid (MSA). The effects of MSA concentrations and pulse frequencies on morphology and composition of deposits were investigated. As the MSA concentration increased from 120 ml/L to 160 ml/L, the hydrogen evolution reaction weakened resulting in smoother deposited surfaces, and the Bi content decreased. In the use of pulses allowing deposits to become more uniform and denser, the Sn-56.73 wt% Bi film with smooth surfaces were fabricated at a pulse frequency of 4 Hz and an MSA concentration of 120 ml/L, of which melting point was 134.35 °C. By using transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD), the structures were characterized more microscopically, which are essential for diffusion and electromigration of micro-bumps. The near-eutectic deposits, both before and after reflowing, consisted of uniformly distributed tetragonal β-Sn phases and hexagonal Bi phases, and the most preferred orientations after reflowing were the same as before reflowing, that were (220)//ND for β-Sn and (1̅012)//ND for Bi. The average grain sizes of Sn and Bi in the deposits after reflowing were 1.44 µm and 0.64 µm, respectively, which were much smaller than those of Sn58Bi alloy obtained by rapid solidification. Dislocations were observed in Sn grains and misorientation values in β-Sn phases were larger, which illustrated that the β-Sn phase carried larger strains. As evaluated by constant loading rate (CLR) nanoindentation tests, the hardness and modulus were significantly improved after reflowing, and creep stress indices of the deposits before and after reflowing were 51.74 and 7.07, respectively.

A modular table-top setup for ultrafast x-ray diffraction.

We present a table-top setup for femtosecond time-resolved x-ray diffraction based on a Cu Kα (8.05keV) laser driven plasma x-ray source. Due to its modular design, it provides high accessibility to its individual components (e.g., x-ray optics and sample environment). The Kα-yield of the source is optimized using a pre-pulse scheme. A magnifying multilayer x-ray mirror with Montel-Helios geometry is used to collect the emitted radiation, resulting in a quasi-collimated flux of more than 105 Cu Kα photons/pulse impinging on the sample under investigation at a repetition rate of 10Hz. A gas ionization chamber detector is placed right after the x-ray mirror and used for the normalization of the diffraction signals, enabling the measurement of relative signal changes of less than 1% even at the given low repetition rate. Time-resolved diffraction experiments on laser-excited epitaxial Bi films serve as an example to demonstrate the capabilities of the setup. The setup can also be used for Debye-Scherrer type measurements on poly-crystalline samples.

Emergence of quasi-1D spin-polarized states in ultrathin Bi films on InAs(111)A for spintronics applications.

The discovery, characterization, and control of heavy-fermion low-dimensional materials are central to nanoscience since quantum phenomena acquire an exotic and highly tunable character. In this work, through a variety of comprehensive experimental and theoretical techniques, it was observed and predicted that the synthesis of ultrathin Bi films on the InAs(111)A surface produces quasi-one-dimensional spin-polarized states, providing a platform for the realization of a unique spin-transport regime in the system. Scanning tunneling microscopy and low-energy electron diffraction measurements revealed that the InAs(111)A substrate facilitates the formation of the Bi-dimer phase of 2√3 × 3 periodicity with an admixture of the Bi-bilayer phase under submonolayer Bi deposition. X-ray photoelectron spectroscopy (XPS) measurements have shown the chemical stability of the Bi-induced phases, while spin and angle resolved photoemission spectroscopy (SARPES) observations combined with state-of-the-art DFT calculations have revealed that the electronic spectrum of the Bi-dimer phase holds a quasi-1D hole-like spin-split state at the Fermi level with advanced spin texture, whereas the Bi-bilayer phase demonstrates metallic states with large Rashba spin-splitting. The band structure of the Bi/InAs(111)A interface is discovered to hold great potential as a high-performance spintronics material fabricated in the ultimate two-dimensional limit.

Grain size effect on the transport properties of polycrystalline Bi(000l) films grown in MgO(001) and glass

The crystal growth and transport properties of the polycrystalline Bi(0001) films grown on MgO(001) and glass substrates were compared. The morphology and electron transport properties with different thickness have been analyzed. The stress-induced bandgap leads to the shift of transition temperature from semimetal to semiconductor transition. Compared with MgO(001) substrates, Bi(0001) thin films on glass substrates have the higher semimetal-semiconductor transition temperature. Furthermore, the electron transport performance is improved, which is attributed to the strain-free van-der-Waals epitaxial preferred orientation grown on the glass substrate. The strain effect on the transport performance is greater than that on in-plane grain boundaries.

(Invited) Thermal Atomic Layer Deposition of Elemental Antimony and Bismuth Films

The elements antimony (Sb) and bismuth (Bi) are important components of many thin film materials. In many applications, films need to be grown in high aspect ratio features with perfect conformality and Angstrom-level control of film thicknesses. Atomic layer deposition (ALD) is a method that can afford uniform thickness and conformal films, even in narrow and deep nanoscale features. To date, there have been several ALD processes reported for elemental Sb films using SbCl3 and Sb(SiR3)3 (R = Me, Et), but no reports of Bi films grown by ALD have been published.1,2 Herein, several families of thermal ALD processes will be described for the growth of elemental Sb and Bi films. Precursors that were explored for Sb and Bi include SbCl3, BiCl3, BiPh3, and Bi(NMe2)3. Reducing precursors encompassed 2-methyl-1,4-bis(trimethylsilyl)-2,5-cyclohexadiene,3 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine,3 and nitrogen sources such as ammonia, hydrazine, alkyl hydrazines, and alkyl amines. The processes employing the nitrogen co-reactants are proposed to occur through unstable SbN and BiN, which then decompose with loss of N2 to afford elemental Sb and Bi films. Self-limited Sb and Bi film growth was established between 25 and 175 °C. High purity Sb (>98%) and Bi (>94%) films were demonstrated by X-ray photoelectron spectroscopy analysis. X-ray diffraction analyses revealed the growth of crystalline Sb and Bi films. The results of additional film characterization will be presented. Al Hareri, M.; Emslie, D. J. H. Chem. Mater. 2022, 34, 2400-2409.Pore, V.; Knapas, K.; Hatanpää, T.; Sarnet, T.; Kemell, M.; Ritala, M.; Leskelä, M.; Mizohata, K. Chem. Mater. 2011, 23, 247– 254.Klesko, J. P.; Thrush, C. M.; Winter, C. H. Chem. Mater. 2015, 27, 4918-4921.

High thermoelectric performance of n-type Mg3Bi2 films deposited by magnetron sputtering

Mg3Bi2 are of great interest for thermoelectric applications near room temperature. In this paper, A metastable cubic phase(c-Mg3Bi2) films were prepared on glass substrate using Mg and Bi elemental targets by magnetron sputtering. Investigations were done on the phase composition, microstructure and thermoelectric properties of films that were deposited with various atomic ratios of Bi to Mg. When the atomic ratio of Mg/Bi exceeded the stoichiometry of 3:2, the films displayed a metastable cubic phase (c-Mg3Bi2) with a highly (111) preferred orientation. The deposited (c-Mg3Bi2) films possess n-type conductivity characteristics with high carrier mobility. In the temperature range of 300–490 K, a state-of-the-art average PF of 3.11 μW cm−1K−2 is attained in a 23 at.% Bi film. A fully Mg3Bi2 thermoelectric device consisting of both n-type and p-type Mg3Bi2 thin films is fabricated. It demonstrates a maximum output power density of 239 μW cm−2 at a temperature difference of 20 K. This work offers an effective approach to development of thermoelectric devices based on Mg3Bi2 materials.

Strain and atomic stacking of bismuth thin film in its quasi-van der Waals epitaxy on (111) Si substrate.

We report on the structural properties of Bi thin films grown on (111) Si substrates with a thickness of 22-30 BL. HRXRD and EBSD measurements show that these Bi films are mainly composed of twinning grains in the (0003) direction. The grain size can be as large as tens of microns. From a double-peak (01[Formula: see text]4) φ-scan, we found two pairs of twinning phases coexisting with a rotation angle of ~ 3.6°. We proposed a coincidence site lattice model based on preferential close-packed sites for Bi atoms on Si (111) surface to explain the coexistence of the rotation phases in the quasi-van der Waals epitaxy. From the measured lattice constants c and a of our samples, along with the data from the literature, we derived a c-a relation: (c-c0) = - 2.038(a-a0), where c0 and a0 are the values of bulk Bi. The normalized position of the second basis atom in the unit cell x, in these strained Bi films is found very close to that of bulk Bi, indicating that the strain does not disturb the Peierls distortion of the lattice. The fixed ratio of bilayer thickness to lattice constant c, reveals that the elastic properties of covalent-bonded bilayer dominate those of Bi crystal.

Giant Spin-Orbit Torque in Sputter-Deposited Bi Films.

Bismuth (Bi) has the strongest spin-orbit coupling among non-radioactive elements and is thus a promising material for efficient charge-to-spin conversion. However, previous electrical detections have reported controversial results for the conversion efficiency. In this study, an optical detection of a spin-orbit torque is reported in a Bi/CoFeB bilayer with a polycrystalline texture of (012) and (003). Taking advantage of the optical detection, spin-orbit torque is accurately separated from the Oersted field and achieves a giant damping-like torque efficiency of +0.5, verifying efficient charge-to-spin conversion. This study also demonstrates a field-like torque efficiency of -0.1. For the mechanism of the charge-to-spin conversion, the bulk spin Hall effect and the interface Rashba-Edelstein effect are considered.

A novel method to improve the morphology of Bi2212 film by PVA-assisted Pechini sol-gel method

Bi2212 high-temperature superconducting thin films (HTSFs) have emerged as the most promising material for terahertz (THz) devices due to their intrinsic Josephson effect. Enhancing the surface smoothness of these films is crucial for their practical application. In this study, metal nitrates were utilized as the raw material to prepare Bi2212 HTSFs on SrTiO3 (100) substrates using the Pechini sol-gel method. The chelating agent ethylenediamine tetraacetic acid (EDTA) and the surfactant polyvinyl alcohol (PVA) were employed to improve the dissolution characteristics of Bi2212 solution. The influence of various parameters such as sintering temperatures, surfactant concentrations, and metal ion ratios was systematically investigated to assess the phase purity, crystallinity, surface morphology, and electrical properties of the Bi2212 superconducting thin films. The results demonstrated that a higher concentration of PVA led to reduced solubility of the Bi2212 solution. Notably, when the PVA concentration was set at 1 %, the fabricated Bi2212 thin film exhibited exceptional epitaxial properties, enhanced Bi2212 phase purity, reduced surface roughness, and higher Tc. These outcomes provide valuable insights for the development of Bi2212 HTSFs in superconducting devices, as they serve as a reference point for achieving optimal film characteristics.

Remarkable average thermoelectric performance of the highly oriented Bi(Te, Se)-based thin films and devices

Bi(Te, Se)-based compounds have attracted lots of attention for nearly two centuries as one of the most successful commercial thermoelectric (TE) materials due to their high performance at near room temperature. Compared with 3D bulks, 2D thin films are more compatible with modern semiconductor technology and have unique advantages in the construction of micro- and nano-devices. For device applications, high average TE performance over the entire operating temperature range is critical. Herein, highly c-axis-oriented N-type Bi(Te, Se) epitaxial thin films have been successfully prepared using the pulsed laser deposition technology by adjusting the deposition temperature. The film deposited at ∼260 °C demonstrate a remarkable average power factor (PFave) of ∼24.4 μW·cm−1·K−2 over the temperature range of 305–470 K, higher than most of the state-of-the-art Bi(Te, Se)-based films. Moreover, the estimated average zT value of the film is as high as ∼0.81. We then constructed thin-film TE devices by using the above oriented Bi(Te, Se) films, and the maximum output power density of the device can reach up to ∼30.1 W/m2 under the temperature difference of 40 K. Predictably, the outstanding average TE performance of the highly oriented Bi(Te, Se) thin films will have an excellent panorama of applications in semiconductor cooling and power generation.

Phase purity and surface morphology of high-Jc superconducting Bi2Sr2Ca1Cu2O8+δ thin films

Bi2Sr2Ca1Cu2O8+δ (Bi-2212) thin films with thicknesses less than 50 nm (<20 unit cells) are grown by pulsed-laser deposition (PLD) onto (001) LaAlO3 (LAO) single crystal substrates. Phase-pure and smooth c-axis oriented Bi-2212 films with optimal oxygen doping, critical temperature Tc0 up to 86 K, and critical current density Jc(60 K) above 1 MA/cm2 are obtained for samples that are annealed in situ at temperatures below 700 °C. At higher temperature Bi-2212 films on LAO substrates partially decompose to non-superconducting impurity phases, while films on MgO and SrTiO3 substrates are stable. The broadening ΔTc of the metal-to-superconductor resistive phase transition in magnetic fields is much larger for thin films of Bi-2212 as compared to YBa2Cu3O7. The magnetic field-induced suppression of Tc0 is stronger for Bi-2212 films containing impurity phases as compared to the phase-pure Bi-2212 films. The degradation of LAO substrate crystals after several steps of deposition and chemical removal of the Bi-2212 layer is investigated. New, commercially prepared substrates provide Bi-2212 films with smallest surface roughness (3 nm) and strong out-of-plane texture. However, thin films of almost the same quality are obtained on re-used LAO substrates that are mechanically polished after the chemical etching.

Preparation and electrical properties of ultra-thin Bi2212 films by sol-gel method

In order to achieve flexible applications of superconducting thin film materials in devices, the surface quality of the film is critical. The thickness of the film is one of the important factors affecting its surface quality. In this paper, Bi2212 superconducting thin films of different thicknesses were prepared on LaAlO3(100) and SrTiO3(100) single crystal substrates by sol-gel method and the effects of film thickness and different heterogeneous structures on their crystallographic, morphological and superconducting properties were investigated. The results showed that the roughness gradually increased with increasing Bi2212 thickness at the initial stage of film growth and stabilized after reaching a certain thickness, while the superconducting properties were destroyed at very small sample thickness, which was consistent in both single-crystal substrate films. In the Bi2212/LaAlO3 structure, the surface quality of the sample was excellent with a roughness of 5.4 nm at a thickness of about 18 nm for Bi2212, and the growth of the precipitated phase was inhibited to a greater extent. The film also had good superconductivity. This work provided a further research basis for the preparation of high quality Bi2212 films and offers further possibilities for their practical application.

Epitaxial growth of Bi, Sb, and Sn

Graphene analogs composed of Bi, Sb, and Sn, respectively, are predicted to be great candidates to realize the quantum spin Hall effect at high temperatures and have attracted intensive research interest in recent years. However, their structural and electronic properties are greatly affected by substrates. Here, we epitaxially grow Bi, Sb, and Sn overlayers on various substrates. We observed the formation of Au–Bi alloy on Au(111) substrates, while α-Bi was formed on the TaIrTe4, TiSe2and Cr2Ge2Te6 substrates. Large-scale thin films of α-Bi, α-Sb and β-Sn can be prepared on the TiSe2 substrates due to the high quality of the substrates with very few defects. The lattice of the Sb films is slightly compressed on the TiSe2 substrates, due to the interfacial interaction. α-Sn transitions to β-Sn on the TiSe2 substrates with increasing Sn coverages. Our work is very helpful for tuning the structural and electronic properties of epitaxial Bi, Sb, and Sn films via proper substrates.

Epitaxial growth of BP-like Bi(1 1 0) with moiré pattern and potential topological edge state

Black-phosphorus-like (BP-like) Bi(110) thin film, a potential large band gap two-dimensional (2D) topological insulator (TI) candidate that hosts a non-dissipative topological edge state, has attracted a lot of attention in recent years. However, the Bi(110) thin film with a large bulk band gap and topological edge states has not been definitely confirmed experimentally. Here, high-quality BP-like Bi(110) thin film is epitaxially grown on a semiconductor SnSe substrate. Employing scanning tunneling microscopy/spectroscopy (STM/S), the BP-like structure was determined, and the corresponding electronic structure was also investigated. In addition, a distinct quasi-squared moiré pattern was observed, and the electronic states far away from Fermi energy in the 1BL Bi(110) film are significantly modulated by the moiré pattern spatially; instead, the electronic states near the Fermi energy are less influenced by the moiré pattern. Importantly, we also observed the enhanced states at the edge of 1BL and 2BL Bi(110) films, especially for 2BL film, with a clear sign of edge state peak extending to 2 nm depth, suggesting that 2BL BP-like Bi(110) on SnSe is a potential 2D TI. As a result, our research provides a platform for the further topological investigation of the BP-like Bi(110) thin film.

Optimization of large magnetoresistance of polycrystalline Bi film

The oriented growth and morphology of polycrystalline bismuth (Bi) films deposited on insulating substrates is investigated based on the molecular beam epitaxy method. The film thickness effect on the resistance-temperature characteristics and magnetoresistance properties is analyzed. The Bi film resistivity with thickness range [25, 65] nm is investigated using the four-probe method at 2–300 K. The magnetic field contributes to opening the energy gap of Bi thin films and a more obvious opening of the energy gap is observed in the thinner films. The Bi film has the weak antilocalization (WAL) effect and the thickness increase affects the WAL effect. The large magnetic field is beneficial to the observation of the WAL effect. The temperature-dependent resistivity of Bi thin films is described based on the Ziman-Faber theory. The competition between the metal surface state and the bulk state explains the semimetal-semiconductor transition and the magnetoresistance effect in the thickness range [25, 65] nm. In addition, the thickness effect on the magnetoresistance property is researched, which provides a new approach to specify the Bi thin film magnetoresistance.