Bi2Se3 Thin Films Research Articles

Bi2Se3 Nanostructured Thin Films as Perspective Anodes for Aqueous Rechargeable Lithium-Ion Batteries

In recent years, aqueous rechargeable lithium-ion batteries (ARLIBs) have attracted attention as an alternative technology for electrical storage. One of the perspective battery anode materials for application in ARLIBs is Bi2Se3, which has already shown good perspectives in the application of conventional lithium-ion batteries (LIBs) that use organic electrolytes. In this study, the electrochemical properties of Bi2Se3 thin films with two different layers on the electrode surface—the solid electrolyte interphase (SEI) and the Bi2O3 layer—were investigated. The results of this work show that the formation of the SEI layer on the surface of Bi2Se3 thin films ensures high diffusivity of Li+, high electrochemical stability, and high capacity up to 100 cycles, demonstrating the perspectives of Bi2Se3 as anode material for ARLIBs.

Manipulating the magneto-resistance of Bi2Se3 thin films by strontium doping

We systematically studied the magneto-resistance of strontium (Sr) doped topological insulator (TI) Bi2Se3 films. For Bi2Se3 films with relatively large classic magneto-resistance, we found that slightly Sr doping can completely suppress the classic magneto-resistance and make the weak antilocalization effect dominate in the weak-magnetic-field region. Consequently, the value of the magneto-resistance is enhanced by more than 2.7 times at 0.2 T in slightly doped samples. In contrast, in the strong-magnetic-field region, the magneto-resistance is strongly reduced by doping but exhibits linear magneto-resistance behavior. Our results suggest that the linear magneto-resistance behavior originates from the mobility fluctuation induced by disorder described by the Parish–Littlewood model. Our findings not only gain insights into the doping effect on the topological insulator but also provide an effective way to manipulate the magneto-transport properties of TI for potential applications in future.

Controlled vapor phase epitaxial growth of high quality Bi2Se3 thin films and their photoresponse studies

Topological insulators are foreseen to offer promising applications in next generation electronic and optoelectronic devices. However, this is said to be tedious without understanding the underlying factors governing the fabrication and physicochemical properties of high-quality topological insulators. In this regard, this article reports on the two-dimensional (2D) growth of high quality Bi2Se3 thin films layered on sapphire by vapor phase epitaxy (VPE). The crystalline rhombohedral phase of the films was substantiated using X-ray diffraction results. The homogeneous distribution of Bi2Se3 thin films were approved through the microscopic images. The high quality in Bi2Se3 was examined through the selected area diffraction patterns and lattice fringes recorded using transmission electron microscopy. Finally, an infrared photo switching device was fabricated using the thin films processed in this work. The device responded linearly in accordance to the intensity of the laser irradiation subjected. The fabricated thin film based device also presented a high reliability through the lack of any degradation in its performance after consecutive switching cycles. Finally, we would like to augment with the fact that the findings observed this work could assist in opening up the possibility for novel optoelectronic applications using Bi2Se3 thin films.

Polarization of Bi2Se3 thin film toward non-volatile memory applications

In recent years, topological insulators have drawn growing interest as a unique electronic state of matter toward quantum information technology. Despite the logic devices with magnetization switching through spin–orbit torque or the topological magneto-electric effect, realizing memory devices based on topological insulators has been urged in quantum computing applications. In this work, we report the design and fabrication of a non-volatile memory device that employs polarization of Bi2Se3 thin films achieving fast memory speed, sufficient memory window, and good reliability. The Bi2Se3 film polarizes under an external electrical field with charges accumulated on the top and bottom surfaces separating the electrons and holes. Such polarization is much faster than the carrier tunneling in conventional floating-gate flash memory and ferroelectric-based memory devices. In addition, good memory retention and endurance properties have also been obtained, showing great potential in high-performance memory application in future topological insulator-involved information technology.

A new type of cyclotron resonance from charge-impurity scattering in the bulk-insulating Bi2Se3 thin films

This work focuses on the low frequency Drude response of bulk-insulating topological insulator (TI) Bi2Se3 films. The frequency and field dependence of the mobility and carrier density are measured simultaneously via time-domain terahertz spectroscopy. These films are grown on buffer layers, capped by Se, and have been exposed in air for months. Under a magnetic field up to 7 Tesla, we observe prominent cyclotron resonances (CRs). We attribute the sharp CR to two different topological surface states from both surfaces of the films. The CR sharpens at high fields due to an electron-impurity scattering. By using magneto-terahertz spectroscopy, we confirm that these films are bulk-insulating, which paves the way to use intrinsic TIs without bulk carriers for applications including topological spintronics and quantum computing.

Signature of weak-antilocalization in sputtered topological insulator Bi2Se3 thin films with varying thickness

We report a low-temperature magneto transport study of Bi2Se3 thin films of different thicknesses (40, 80 and 160 nm), deposited on sapphire (0001) substrates, using radio frequency magnetron sputtering technique. The high-resolution x-ray diffraction measurements revealed the growth of rhombohedral c-axis {0003n} oriented Bi2Se3 films on sapphire (0001). Vibrational modes of Bi2Se3 thin films were obtained in the low wavenumber region using Raman spectroscopy. The surface roughness of sputtered Bi2Se3 thin films on sapphire (0001) substrates were obtained to be ~ 2.26–6.45 nm. The chemical and electronic state of the deposited Bi2Se3 was confirmed by X-ray photoelectron spectroscopy and it showed the formation of Bi2Se3 compound. Resistivity versus temperature measurements show the metallic nature of Bi2Se3 films and a slight up-turn transition in resistivity at lower temperatures < 25 K. The positive magneto-resistance value of Bi2Se3 films measured at low temperatures (2–100 K) confirmed the gapless topological surface states in Bi2Se3 thin films. The quantum correction to the magnetoconductivity of thin films in low magnetic field is done by employing Hikami–Larkin–Nagaoka theory and the calculated value of coefficient ‘α’ (defining number of conduction channels) was found to be 0.65, 0.83 and 1.56 for film thickness of 40, 80 and 160 nm, respectively. These observations indicate that the top and bottom surface states are coupled with the bulk states and the conduction mechanism in Bi2Se3 thin films varied with the film thicknesses.

Proximity-Induced Magnetism in a Topological Insulator/Half-Metallic Ferromagnetic Thin Film Heterostructure

Topological insulator (TI) Bi2Se3 thin films were prepared on half-metallic ferromagnetic La0.7Sr0.3MnO3 thin film by magnetron sputtering, forming a TI/FM heterostructure. The conductivity of Bi2Se3was modified by La0.7Sr0.3MnO3 at high- and low-temperature regions via different mechanisms, which could be explained by the short-range interactions and long-range interaction between ferromagnetic insulator and Bi2Se3 due to the proximity effect. Magnetic and transport measurements prove that the ferromagnetic phase and extra magnetic moment are induced in Bi2Se3 films. The weak anti-localized (WAL) effect was suppressed in Bi2Se3 films, accounting for the magnetism of La0.7Sr0.3MnO3 layers. This work clarifies the special behavior in Bi2Se3/La0.7Sr0.3MnO3 heterojunctions, which provides an effective way to study the magnetic proximity effect of the ferromagnetic phase in topological insulators.

Contribution of both bulk and surface states on photothermoelectric transport in epitaxial Bi2Se3 thin films

Probing the surface states in Bi2Se3 via electronic transport measurements is difficult due to the significantly larger numbers of bulk carriers. Herein, we report the contribution of both bulk and surface states on the measured photocurrent at room temperature and the electrical transport measurements at low temperatures in an epitaxial thin film of intrinsic Bi2Se3 capped with a high K dielectric, Al2O3. The measured photocurrent continuously increases when the gate voltage is swept toward the negative side from 10 to −10 V, indicating the movement of the Fermi level below the conduction band edge. Moreover, the contribution of the surface state conduction increases when the Fermi level moves inside the bandgap toward the Dirac point. Furthermore, the measured Seebeck coefficient (S) continuously increases with sweeping gate voltage from 10 to −10 V. Similar correlations of the photocurrent and S value with the gate voltage illustrate the contribution of the photothermoelectric effect due to the conduction of both bulk and surface states. Additionally, the ambipolar charge transport was observed in the electrical measurement at 32 K for the four-probe configuration. The ambipolar charge transport is possibly indicative of surface state transport.

Phase transitions in intrinsic magnetic topological insulator with high-frequency pumping

In this work, we investigate the topological phase transitions in an effective model for a topological thin film with high-frequency pumping. In particular, our results show that the circularly polarized light can break the time-reversal symmetry and induce the quantum anomalous Hall insulator (QAHI) phase. Meanwhile, the bulk magnetic moment can also break the time-reversal symmetry. Therefore, it shows rich phase diagram by tuning the intensity of the light and the thickness of the thin film. Using the parameters fitted by experimental data, we give the topological phase diagram of the Cr-doped Bi2Se3 thin film, showing that by modulating the strength of the polarized optical field in an experimentally accessible range, there are four different phases: the normal insulator phase, the time-reversal-symmetry-broken quantum spin Hall insulator phase, and two different QAHI phases with opposite Chern numbers. Comparing with the non-doped Bi2Se3, it is found that the interplay between the light and bulk magnetic moment separates the two different QAHI phases with opposite Chern numbers. The results show that an intrinsic magnetic topological insulator with high-frequency pumping is an ideal platform for further exploring various topological phenomena with a spontaneously broken time-reversal symmetry.

The Electrochemical Characterization of Nanostructured Bi2Se3 Thin Films in an Aqueous Na Electrolyte

Due to their layered structure and high theoretical capacity, bismuth chalcogenides have been proposed as anodes in organic electrolyte Li- and Na-ion batteries. On the other hand, their electrochemical properties in aqueous systems have not been reported. Here, the electrochemical performance of Bi2Se3 thin films in 1 M NaNO3 aqueous electrolyte is presented. This aqueous Bi2Se3 system was found to have up to two orders of magnitude increased diffusion coefficients, compared to other anode materials in Na electrolyte-based systems, as well as limited anode electrode degradation over 5 CVs and significant changes in the anode after 30 CVs.

EIS characterization of aging and humidity-related behavior of Bi2Se3 films of different morphologies

Bi2Se3 thin films are widely studied as topological insulators and as prospective materials for thermoelectric and sensor applications. Storage of films under ambient conditions, especially at different humidities, may influence the physical properties of thin films.In the present work, electrochemical impedance spectroscopy is used to investigate the dependence of properties of continuous planar and partly disordered Bi2Se3 thin films, as well as nanoplate networks, deposited on mica, quartz and glass substrates by physical vapor deposition on the humidity in oxygen and nitrogen atmospheres. It has been shown that electrochemical impedance spectroscopy (EIS) can be used as an effective tool for the determination of the oxidation level of Bi2Se3 surfaces, the quality of the surface covering and for characterization of the type and parameters of the nanopores in Bi2Se3 thin films. Equivalent circuit models developed in this work characterize the interaction of gaseous water with non-oxidized and oxidized surfaces of Bi2Se3 nanostructures, as well as and with the substrate areas located between the Bi2Se3 nanoplates in the Bi2Se3 networks and open to the surrounding environment, in oxygen and nitrogen atmospheres.

Structural, Electronic and Thermoelectric Properties of Bi2Se3 Thin Films Deposited by RF Magnetron Sputtering

Structural, Electronic and Thermoelectric Properties of Bi2Se3 Thin Films Deposited by RF Magnetron Sputtering

Epitaxial Growth of Bi2Se3 Infrared Transparent Conductive Film and Heterojunction Diode by Molecular Beam Epitaxy.

Epitaxial n-type infrared transparent conductive Bi2Se3 thin film was cultivated by molecular beam epitaxy (MBE) method on Al2O3 (001) substrate. The orientation between Bi2Se3 and the substrate is Bi2Se3(001)//Al2O3(1 10). Conducting mechanism ensued the small-polaron hopping mechanism, with an activation energy of 34 meV. The film demonstrates conductivity of n-type, and the resistivity is 7 × 10−4 Ωcm at room temperature. The Film exhibits an excellent carrier mobility of 1,015 cm2/Vs at room temperature and retains optical transparency in the near-infrared (>70%) and far-infrared (>85%) ranges. To the best of our knowledge, the Bi2Se3 film yields the best result in the realm of n-type Infrared transparent conductive thin films generated through either physical or chemical methods. To demonstrate the application of such films, we produced N-Bi2Se3/P-CuScO2 heterojunction diode device, the ∼3.3 V threshold voltage of which conformed fairly well with the CuScO2 bandgap value. The high optical transparency and conductivity of Bi2Se3 film make it very promising for optoelectronic applications, where a wide wavelength range from near-infrared to far-infrared is required.

Growth and characterization of crystalline Bi2Se3 thin films on flexible metal foils by magnetron sputtering system

We report the large area thin film deposition of Bi2Se3 on flexible thin molybdenum (Mo) and titanium (Ti) metal foils using magnetron sputtering technique at temperature of 400 °C. The crystallinity of post-selenized thin film was investigated using x-ray diffraction and high intense peaks of Bi2Se3 (0006) along other planes was obtained which showed the preferred orientation along c-axis on these polycrystalline substrates. Raman spectroscopy showed three main characteristic Raman peaks i.e. A11g, E2g, and A21g which revealed the formation of pure hexagonal phase of the Bi2Se3. Field emission scanning electron microscopy studied showed hexagonal nanoflakes shape Bi2Se3 films with large granular sizes ranging from 200 to 450 nm and stoichiometry of Bi2Se3 thin film was confirmed by energy dispersive x-ray analysis. The growth of crystalline hexagonal phase of Bi2Se3 thin films on large area polycrystalline low cost flexible metal foils using magnetron sputtering system pave the way for futuristics flexible energy devices.

Thickness dependent optical properties of sputtered Bi2Se3 films on mica

The Bi2Se3 thin films of different thicknesses (100, 200 and 400 nm) were deposited on mica substrates by magnetron sputtering technique and their structural and optical properties have been studied. The X-ray diffraction analysis revealed the crystalline nature of Bi2Se3 films with preferred orientation along the c-axis. The presence of three well-pronounced Raman modes i.e. A11g, E2g, and A21g confirms the formation of pure Bi2Se3 compound on mica substrates. The field emission scanning electron microscopy and elemental analysis measurements showed the growth of Bi2Se3 film with varying grain sizes with thickness while maintaining the stoichiometric ratio of 2:3 of Bi and Se. Surface roughness was analysed using atomic force microscopy and it revealed that the roughness increases from 2.16 to 28.3 nm when the film thickness increases from 100 to 400 nm. The optical bandgap analysed with broadband absorption spectroscopy showed a decrease in bandgap from 1.3 to 1.08 eV with increasing film thickness. The sputtered Bi2Se3 films on mica possess good crystalline and structural quality with tunable bandgap that can be used for futuristic optoelectronic applications.

Anomalous Superconducting Proximity Effect in Bi2 Se3 /FeSe0.5 Te0.5 Thin-Film Heterojunctions.

The superconducting proximity effect (SPE) induces a superconductivity transition in otherwise non-superconducting thin films in proximity with a superconductor. The SPE usually occurs in real space and decays exponentially with film thickness. Herein, an abnormal SPE in a topological insulator (TI)/superconductor heterostructure is unveiled, which is attributed to the topologically protected surface state. Surprisingly, such abnormal SPE occurs in momentum space regardless of the TI film thickness, as long as the topological surface states are robust and form a continuous conduction loop. Combining transport measurements and scanning tunneling microscopy/spectroscopy techniques, the SPE in Bi2 Se3 /FeSe0.5 Te0.5 heterostructures is explored, where Bi2 Se3 is an ideal 3D topological insulator and FeSe0.5 Te0.5 a typical iron-based superconductor. As the thickness of the Bi2 Se3 thin film exceeds 400 nm, there still exists SPE-induced superconductivity on the surface of Bi2 Se3 thin film with a transition temperature Tc not less than 10 K. Such an extraordinary behavior is induced by the unique properties of topologically protected surface states of Bi2 Se3 . This research deepens the understanding of the important role of topologically protected surface states in the SPE.

Quantum interference phenomena and electron – electron interaction in topological insulator Bi2Se3 thin polycrystalline films

The temperature (T = 2.0 - 300 K) and magnetic field (B = 0 - 14 T) dependences of the transport coefficients were obtained for the topological insulator (TI) polycrystalline Bi2Se3 films with thicknesses d = 40 and 200 nm grown on glass substrates by thermal evaporation in vacuum from a single source. In the region of weak magnetic fields, a sharp increase in magnetoresistance (MR) with increasing B was observed, indicating the presence of the weak antilocalization (WAL) effect, which is typical for 3D-TIs. However, the longitudinal resistance Rxx increases logarithmically with decreasing temperature below certain temperature Tc, which is not characteristic of the WAL effect in 2D-structures and indicates possible existence of the so-called “transport paradox”. In the temperature dependences of the Hall resistance, Hall coefficient and MR we also registered clearly pronounced kinks near Tc. The Rxx(T) dependence for a 40 nm film, with decreasing temperature in addition to the Rxx minimum near Tc, exhibits a maximum near 2.1 K. Estimates of the number of independent transport channels m and the dephasing length lφ show that as d increases from 40 nm to 200 nm, m increases from 1 to 3, while lφ changes insignificantly. The nonmonotonic behavior of the T- and B- dependences of the kinetic coefficients, the occurrence of the “transport paradox” are associated with a change in the contributions of WAL and electron-electron interaction effects when the nature and magnitude of the external influence and film thickness change. It follows from the data obtained that not only in very thin monocrystalline films, but also in rather thick polycrystalline Bi2Se3 films prepared by the simple method of the thermal evaporation in vacuum on amorphous substrates, quantum interference effects and “transport paradox” can be manifested.

Flexible N-Type Thermoelectric Composites Based on Non-Conductive Polymer with Innovative Bi2Se3-CNT Hybrid Nanostructured Filler.

This research is devoted to the fabrication of polyvinyl alcohol (PVOH) based n-type thermoelectric composites with innovative hybrid bismuth selenide-multiwalled carbon nanotube (Bi2Se3-MWCNT) fillers for application in flexible thermoelectric devices. Hybrid fillers were synthesized by direct deposition of Bi2Se3 on multiwalled carbon nanotubes using a physical vapor deposition method, thus ensuring direct electrical contact between the carbon nanotubes and Bi2Se3. The Seebeck coefficient of prepared PVOH/Bi2Se3-MWCNT composites was found to be comparable with that for the Bi2Se3 thin films, reaching −100 µV·K−1 for the composite with 30 wt.% filler, and fluctuations of the resistance of these composites did not exceed 1% during 100 repetitive bending cycles down to 10 mm radius, indicating the good mechanical durability of these composites and proving their high potential for application in flexible thermoelectrics. In addition, other properties of the fabricated composites that are important for the use of polymer-based materials such as thermal stability, storage modulus and linear coefficient of thermal expansion were found to be improved in comparison with the neat PVOH.

First-principles calculations of electronic and optical properties of orthorhombic Bi2Se3 nano thin film

This study explored the electronic and optical properties of nano-thin film orthorhombic bismuth selenide (Bi2Se3) using first-principles approach. The thin films were modelled in the [001] direction using slab geometry supercell with vacuum along z-direction so that periodic images and slab can be managed independently. We obtained band gap of 0.730 eV, 0.566 eV and 0.690 eV for 1, 2 and 3 slabs respectively. It shows dependence of band structure to the thickness of the films. The reduced band gap in Bi2Se3 films is due to the effect of quantum confinement. The electron contributions were increased as the thickness of the thin film increased. We also found that the thickness of Bi2Se3 thin film influences its optical properties. Higher thickness provides higher static dielectric function, refractive index, extinction coefficient, absorption coefficient and loss function. This study demonstrates Bi2Se3 thin film has good optical properties that are useful for optoelectronic applications.

Morphology Optimization of Bi2Se3 Thin Films for Enhanced Thermoelectric Performance

Morphology Optimization of Bi₂Se₃ Thin Films for Enhanced Thermoelectric Performance