Topological Insulator Materials Research Articles

Electrochemical Platform Based on the Bi2Te3 Family of Topological Insulators for the Detection of SARS-CoV-2 Pathogenic Factors.

Accurate and rapid detection of the causative agent of a disease is of great importance in controlling the spread of the disease. This work developed a biosensor with the Bi2Te3 family of topological insulators for detection of the SARS-CoV-2 virulence factor. The Bi2Te3 family is a three-dimensional topological insulator material with topologically protected surface states; the presence of these surface states facilitates charge transfer between the electrode and electrolyte interface. Compared with the detection performance of Bi2Se3, BiSbTeSe2, and a trivial insulator like Sb2Se3, Bi2Te3 exhibits superior characteristics. A Bi2Te3 electrochemical detection platform is utilized to fabricate a sensor that can detect SARS-CoV-2 DNA, RNA, and antigen for label-free target detection. The concentration range of DNA detection by the biosensor using Bi2Te3 is between 1.0 × 10-15 and 1.0 × 10-10 M, and the detection limit can reach 1.41 × 10-16 M. Furthermore, it exhibits excellent selectivity and maintains good stability even after being stored for 14 days. This study provides a new way to apply topological insulator materials in the field of biosensors and use their unique electronic structure to improve the accuracy and speed of disease detection and diagnosis.

Plasmonic resonance and sensing based on Sb2Te3 topological insulator nanocolumn array

Topological insulators (TIs) are new kind of electronic materials with special energy band structures and topological properties. With topologically-protected metallic surface state and insulating bulk state, TIs have excellent electronic and optical characteristics containing the excitation of surface plasmons. Herein, we deposited the antimony telluride (Sb2Te3) TI layer using the magnetron sputtering method and employed focused ion beam (FIB) lithography to fabricate a nanocolumn array on the Sb2Te3 layer. The experimental measurements show that the localized surface plasmon resonance (LSPR) can be excited in the Sb2Te3 TI nanocolumn array in the visible region. The resonance wavelength is dependent on the size and period of the Sb2Te3 nanocolumn array. The experiment results are in good agreement with the finite-difference time-domain (FDTD) numerical simulations. Moreover, the LSPR enables to realize the microscale optical sensing of the surrounding refractive index with a high sensitivity of 450 nm/RIU. This work will pave a new pathway for the excitation of surface plasmons in TI materials and their applications in optical sensing at the microscale.

Enhancement of spin Hall angle in semimetallic materials α -Sn under voltage regulation

Utilizing current-induced spin–orbit torque (SOT) to control magnetization is essential for the advancement of spintronics. SOT offers high energy efficiency and rapid operation speed. The ideal SOT material should have a high charge-to-spin conversion efficiency and excellent electrical conductivity. Recently, there has been a focus on topological insulator materials with topological surface states in SOT research due to their controllability in spin–orbit coupling, conductivity, and energy band topology. While topological Dirac semimetallic materials show promise for SOT applications, research on voltage regulation of their spin Hall angle is still in its early stages. This paper investigates the multilayer structure of a Dirac semimetallic material. In an α-Sn/Ag bilayer, the voltage regulation effect can increase the spin Hall angle by five times by adjusting the strain on the Fermi level. Experiments explore the role of a silver layer as a transport layer in the electric field control of multilayer films. This material system can enhance its effects under electric field regulation and offer insight for achieving regulation in new spintronic devices.

Induced superconducting correlations in a quantum anomalous Hall insulator

Thin films of ferromagnetic topological insulator materials can host the quantum anomalous Hall effect without the need for an external magnetic field. Inducing Cooper pairing in such a material is a promising way to realize topological superconductivity with the associated chiral Majorana edge states. However, finding evidence of the superconducting proximity effect in such a state has remained a considerable challenge due to inherent experimental difficulties. Here we demonstrate crossed Andreev reflection across a narrow superconducting Nb electrode that is in contact with the chiral edge state of a quantum anomalous Hall insulator. In the crossed Andreev reflection process, an electron injected from one terminal is reflected out as a hole at the other terminal to form a Cooper pair in the superconductor. This is a compelling signature of induced superconducting pair correlation in the chiral edge state. The characteristic length of the crossed Andreev reflection process is found to be much longer than the superconducting coherence length in Nb, which suggests that the crossed Andreev reflection is, indeed, mediated by superconductivity induced on the quantum anomalous Hall insulator surface. Our results will invite future studies of topological superconductivity and Majorana physics, as well as for the search for non-abelian zero modes.

Highly Efficient Spin Current Transport Properties in Spintronic Devices Based on Topological Insulator

AbstractRecently, topological insulators (TIs) have regained extensive attention in spintronics due to their potential applications in new‐generation spintronic devices, following the discovery of the quantum Hall effect and quantum anomalous Hall effect, which introduce the topological concept. In this review, the exotic spin transport phenomena are explored in TIs. The review offers a concise overview of the fundamental principles of TIs, followed by an exploration of diverse fabrication methods for TI materials. Characterization techniques of the topological surface states are also presented. The review delves into the intriguing spin current transport phenomena, focusing on spin‐to‐charge and charge‐to‐spin conversions in TI/ferromagnet bilayers, respectively. The review culminates summarizing key insights and project future directions for research on spin transport phenomena in TIs, emphasizing practical implications.

Double ferromagnetic barriers resonant tunneling diodes (RTD) based on the surface of a topological insulator

Resonant tunneling diodes (RTDs), which are based on double barrier quantum well structures, are typically achieved by combining different materials with varying band gap sizes. However, this approach often poses challenges such as material mismatching and dislocations. In this study, we present a novel resonant tunneling diode scheme utilizing the unique properties of topological insulator materials. Specifically, we exploit the gap opening in the band structure of the topological insulator by employing perpendicular magnetization. In this proposed RTD platform, the barrier regions are formed from a ferromagnetic topological insulator through the proximity effect. By adjusting the thickness and spacing of the ferromagnetic barriers, a well region with confined states emerges between the barrier regions. Theoretical analysis reveals that by tuning the back gate voltage, the I-V characteristics exhibit two significant behaviors: negative differential resistance (NDR) and step-like behavior for Fermi energy values of EF = −3 and EF = 3, respectively. Furthermore, we observe an increase in the peak-to-valley ratio (PVR) with higher magnetization values. Notably, the PVR reaches a value of 7.13 for a magnetization value of m = 9. Additionally, we investigate the influence of the well width and barrier thickness on the transport properties of the device.

Application of femtosecond mode-locked SnTe thin films and generation of bound-state solitons

In the realm of ultrafast laser technology, the exploration of two-dimensional materials as saturable absorbers (SA) has garnered significant research interest. Our research investigates the characteristics of SnTe thin films, a topological crystalline insulator material, as a potential saturable absorber for ultrafast lasers. Using the molecular beam epitaxy (MBE) technique, we analyze the films’ morphology and composition through atomic force microscopy (AFM) and successfully deposit SnTe epilayers on Au(111)/mica substrates. Through the utilization of SnTe-SA, an erbium-doped fiber laser is fabricated, demonstrating a pulse output with a width of 276 fs and a center wavelength of 1560 nm, highlighting the potential of SnTe films in manufacturing ultrafast optical devices. Additionally, tightly bound solitons with a soliton interval of 1.01 ps are observed, contributing to the exploration of soliton nonlinear dynamics.

Transition metals of Pt and Pd on the surface of topological insulator Bi2Se3.

Transition metal catalysts supported on topological insulators are predicted to show improved catalytic properties due to the presence of topological surface states, which may float up to the catalysts and provide robust electron transfer. However, experimental studies of surface structures and corresponding catalytic properties of transition metal/topological insulator heterostructures have not been demonstrated so far. Here, we report the structures, chemical states, and adsorption behaviors of two conventional transition metal catalysts, Pt and Pd, on the surface of Bi2Se3, a common topological insulator material. We reveal that Pt forms nanoparticles on the Bi2Se3 surface. Moreover, the interaction between Pt and surface Se is observed. Furthermore, thermal dosing of O2 onto the Pt/Bi2Se3 heterostructure leads to no oxygen adsorption. Detailed scanning tunneling microscopy study indicates that Pt transforms into PtSe2 after the thermal process, thus preventing O2 from adsorption. For another transition metal Pd, it exhibits approximate layer-island growth on Bi2Se3, and Pd-Se interaction is also observed. Our work provides significant insights into the behaviors of transition metals on top of a common topological insulator material and will assist in the future design of catalysts built with topological materials.

The synthesis and tunable topological properties of Sm-doped (Bi, Sb)2Te2S crystal

Topological insulators exhibit Dirac points in their topological surface states, but often such Dirac points are far from the Fermi level and sometimes even reside outside the bulk band gap. Here, we report the synthesis of a new topological insulator material Sm-doped (Bi, Sb)2Te2S, which has its surface tunable Dirac point exposed within the bulk gap and close to the Fermi level at a minimal distance of 73 meV with a large tunability of 157 meV, as evidenced by angle-resolved photoemission spectroscopy. The compensation doping by Sm creates a nearly insulating bulk and triggers an interesting phenomenon of all-configuration negative magnetoresistance. This signals a nearly compensated state where the bulk transport is mainly via percolation through charge puddles. Our work offers a new material platform suitable for regulating the position of surface Dirac points and exploring unique physical properties of topological insulators.

Enhancement of spin to charge conversion efficiency at the topological surface state by inserting normal metal spacer layer in the topological insulator based heterostructure

In this study, we report efficient spin to charge conversion (SCC) in the topological insulator (TI) based heterostructure (BiSbTe1.5Se1.5/Cu/Ni80Fe20) by using spin-pumping technique, where BiSbTe1.5Se1.5 is the TI and Ni80Fe20 is the ferromagnetic (FM) layer. The SCC, characterized by inverse Edelstein effect length (λIEE) in the TI material, gets altered with an intervening Copper (Cu) layer, and it depends on the interlayer thickness. The introduction of Cu layer at the interface of TI and FM metal provides a new degree of freedom for tuning the SCC efficiency of the topological surface states. The significant enhancement of the measured spin-pumping voltage and the increased linewidth of ferromagnetic resonance absorption spectra due to the insertion of Cu layer at the interface indicate a reduction in spin memory loss at the interface that resulted from the presence of exchange coupling between the surface states of TI and the local moments of FM metal. The temperature dependence (from 8 to 300 K) of the evaluated λIEE data for all the trilayer systems, TI/Cu/FM with different Cu thicknesses, confirms the effect of exchange coupling between the TI and FM layer on the SCC efficiency of the topological surface state. This study offers promising ways for designing more efficient spin-charge conversion devices of the TI-based heterostructure by controlling the interface effects.

Pressure-driven dome-shaped superconductivity in topological insulator GeBi2Te4

The discovery of new superconductors based on topological insulators always captures special attention due to their unique structural and electronic properties. High pressure is an effective way to regulate the lattice as well as electronic states in the topological insulators, thus altering their electronic properties. Herein, we report the structural and electrical transport properties of the topological insulator GeBi2Te4 by using high-pressure techniques. The synchrotron x-ray diffraction revealed that GeBi2Te4 underwent two structural phase transitions from R-3m (phase I) to C2/m (phase II) and then into Im-3m (phase III). Superconductivity was observed at 6.6 GPa to be associated with the first structural phase transition. The superconducting transition temperature Tc reached a maximum value of 8.4 K, accompanied by the RH sign changing from negative to positive at 14.6 GPa, then gradually decreased with increasing pressure in phase III, showing a dome-shaped phase diagram. The present results provide a platform for understanding the interplay between the crystal structure and superconductivity by the regulation of pressure in the topological insulator materials.

Study on Bulk-Surface Transport Separation and Dielectric Polarization of Topological Insulator Bi1.2Sb0.8Te0.4Se2.6.

This study successfully fabricated the quaternary topological insulator thin films of Bi1.2Sb0.8Te0.4Se2.6 (BSTS) with a thickness of 25 nm, improving the intrinsic defects in binary topological materials through doping methods and achieving the separation of transport characteristics between the bulk and surface of topological insulator materials by utilizing a comprehensive Physical Properties Measurement System (PPMS) and Terahertz Time-Domain Spectroscopy (THz-TDS) to extract electronic transport information for both bulk and surface states. Additionally, the dielectric polarization behavior of BSTS in the low-frequency (10-107 Hz) and high-frequency (0.5-2.0 THz) ranges was investigated. These research findings provide crucial experimental groundwork and theoretical guidance for the development of novel low-energy electronic devices, spintronic devices, and quantum computing technology based on topological insulators.

The effect of charged particle irradiation on the transport properties of bismuth chalcogenide topological insulators: a brief review.

Topological insulators (TIs) offer a novel platform for achieving exciting applications, such as low-power electronics, spintronics, and quantum computation. Hence, the spin-momentum locked and topologically nontrivial surface state of TIs is highly coveted by the research and development industry. Particle irradiation in TIs is a fast-growing field of research owing to the industrial scalability of the particle irradiation technique. Unfortunately, real three-dimensional TI materials, such as bismuth selenide, invariably host a significant population of charged native defects, which cause the ideally insulating bulk to behave like a metal, masking the relatively weak signatures of metallic topological surface states. Particle irradiation has emerged as an effective technique for Fermi energy tuning to achieve an insulating bulk in TI along with other popularly practiced methods, such as substitution doping and electrical gating. Irradiation methods have been used for many years to enhance the thermoelectric properties of bismuth chalcogenides, predominantly by increasing carrier density. In contrast, uncovering the surface states in bismuth-based TI requires the suppression of carrier density via particle irradiation. Hence, the literature on the effect of irradiation on bismuth chalcogenides extends widely to both ends of the spectrum (thermoelectric and topological properties). This review attempts to collate the available literature on particle irradiation-driven Fermi energy tuning and the modification of topological surface states in TI. Recent studies on particle irradiation in TI have focused on precise local modifications in the TI system to induce magnetic topological ordering and surface selective topological superconductivity. Promising proposals for TI-integrated circuits have also been put forth. The eclectic range of irradiation-based studies on TI has been reviewed in this manuscript.

Brief discussion on quantum anomaly Hall effect: From theory to application

The investigation of topological insulator materials plays a crucial role in the exploration of the quantum anomalous Hall effect. A topological insulator is a distinct type of insulator characterized by its band structure with non-trivial topological features. Topological insulators are characterized by the occurrence of topological phase transitions in the electron energy bands at the Fermi level, which can be attributed to the combined effects of spin-orbit coupling and an external magnetic field. These transitions give rise to the emergence of Hall conductive boundary states, facilitating the manifestation of quantum Hall conductance even in the absence of magnetic fields. The quantum anomalous Hall effect exhibits promising prospects for various applications. For instance, it can serve as a viable means of current transmission in low-power electronic devices, or alternatively, as a medium for constructing qubits in topological quantum computing systems. Furthermore, the utilization of the quantum anomalous Hall effect extends to the development of magnetic sensors with superior performance characteristics and the creation of energy-efficient spintronic devices. This work endeavors to conduct a comprehensive examination and evaluation of the theory and practical implementation of the quantum Anomaly Hall effect by the analysis and review of relevant literature. In addition, it intends to provide potential avenues for future applications in this field.

High-performance photothermal effect in MOCVD grown topological insulator Sb2Te3 nanograting

Photothermal energy has been widely used in high-tech applications, such as heating/cooling systems, bio-imaging, bio-sensing, and medical therapies. However, conventional photothermal materials have narrow photo-absorption bandwidth and low photothermal conversion efficiency. Innovative materials that can more efficiently harvest photothermal energy are highly demanded. Topological insulator materials with excellent optical properties hold great potential in photo-absorption and photothermal conversion. This work investigated and engineered photo-absorption and photothermal effect in Sb2Te3 topological insulator nanograting. The TI material was grown by metal-organic chemical vapor deposition to exploit the benefits of the process, yielding high material quality and large deposition areas. Through a meticulous process encompassing material synthesis, engineering, and characterization, highly absorptive Sb2Te3 topological insulator nanograting and efficient photothermal conversion have been achieved. This research contributes to the advancement of the fundamental knowledge of light–matter interaction and photothermal effects in topological insulator materials. The outcomes of this study can benefit the development of efficient photothermal materials for high-performance nano-energy and biomedical technologies.

Orietation-controlled synthesis and Raman study of 2D SnTe

Tin telluride (SnTe), as a narrow bandgap semiconductor material, has great potential for developing photodetectors with wide spectra and ultra-fast response. At the same time, it is also an important topological crystal insulator material, with different topological surface states on several common surfaces. Here, we introduce different Sn sources and control the growth of regular SnTe nanosheets along the (100) and (111) planes through the atmospheric pressure chemical vapor deposition method. It has been proven through various characterizations that the synthesized SnTe is a high-quality single crystal. In addition, the angular resolved Raman spectra of SnTe nanosheets grown on different crystal planes are first demonstrated. The experimental results showed that square SnTe nanosheets grown along the (100) plane exhibit in-plane anisotropy. At the same time, we use micro-nanofabrication technology to manufacture SnTe-based field effect transistors and photodetectors to explore their electrical and optoelectronic properties. It has been confirmed that transistors based on grown SnTe nanosheets exhibit p-type semiconductor characteristics and have a high response to infrared light. This work provides a new approach for the controllable synthesis of SnTe and adds new content to the research of SnTe-based infrared detectors.

Gapless linear dispersion in Bi2Se3 nanoparticles for high-performance broadband photodetectors

Topological insulators have demonstrated novel optoelectronic properties owing to their gapless linear dispersion and robust surface states. In this study, we implemented high-performance broadband photodetectors based on mixed-dimensional heterostructure of the topological insulator Bi2Se3 nanoparticles (NPs) with the semiconductor WSe2. Prominently, the Bi2Se3 NPs/WSe2 structure exhibited photoresponsivity of 22.17 A/W in the visible (Vis) wavelengths, and 5 A/W in the near-infrared (NIR) wavelengths and a fast response time of 40 μs. In comparison with typical semiconducting PbS NPs/WSe2 structure, we confirmed that the enhancement in the photoresponsivity, response time, and broadband photoresponse of the Bi2Se3 NPs/WSe2 structure can be attributed to the gapless behavior in surface states of topological insulator materials. Numerical simulations also confirmed a dramatic increase in the interfacial electric field by two orders of magnitude, resulting from the surface states in topological insulator NPs. The observed experimental results and the proposed mechanism pave the way for the emergence of efficient optoelectronic devices based on topological insulator materials.

Edge-induced excitations in Bi[formula omitted]Te[formula omitted] from spatially-resolved electron energy-gain spectroscopy

Among the many potential applications of topological insulator materials, their broad potential for the development of novel tunable plasmonics at THz and mid-infrared frequencies for quantum computing, terahertz detectors, and spintronic devices is particularly attractive. The required understanding of the intricate relationship between nanoscale crystal structure and the properties of the resulting plasmonic resonances remains, however, elusive for these materials. Specifically, edge- and surface-induced plasmonic resonances, and other collective excitations, are often buried beneath the continuum of electronic transitions, making it difficult to isolate and interpret these signals using techniques such as electron energy-loss spectroscopy (EELS). Here we focus on the experimentally clean energy-gain EELS region to characterise collective excitations in the topologically insulating material Bi2Te3 and correlate them with the underlying crystalline structure with nanoscale resolution. We identify with high significance the presence of a distinct energy-gain peak around −0.8eV, with spatially-resolved maps revealing that its intensity is markedly enhanced at the edge regions of the specimen. Our findings illustrate the reach of energy-gain EELS analyses to accurately map collective excitations in quantum materials, a key asset in the quest towards new tunable plasmonic devices.

Photo-electrochemical effects in topological insulator Sb2Te3 thin films

Topological insulators have attracted increased attention owing to their fascinating properties and important applications, such as spintronic devices, quantum computers, and optoelectronic devices. In this study, we demonstrated the photo-electrochemical properties of a typical topological insulator material, Sb2Te3 thin film. Sb2Te3 thin films were grown on a glass substrate using atomic layer deposition subsequently, samples or thin films were characterized using Raman spectroscopy, scanning electron microscopy, X-ray diffraction, and atomic force microscopy. The as-fabricated Sb2Te3 thin films have continuous and coarse surfaces with high crystallization. The photo-electrochemical effects of Sb2Te3 electrodes were observed. We show that the Sb2Te3 films have potential in photocatalytic water splitting application.

Sensitivity of actuation dynamics of Casimir oscillators on finite temperature with topological insulator materials: Response of repulsive vs attractive interactions

We explore the sensitivity of repulsive and attractive Casimir interactions on thermal fluctuations in topological insulator materials, and the associated effects on non-linear actuation towards chaotic motion. For system at low temperature and applied magnetization in the same direction, by increasing the temperature the sensitivity of the Casimir force to the magnetization, at large separations, weakens. Moreover, by increasing the temperature the repulsive force vanishes for weak magnetization. Furthermore, for conservative systems the stable actuation area for high temperature does not show strong sensitivity to changes of the magnetization and the magnetoelectric effect. For non-conservative system with weak magnetization in the same direction leading to attractive Casimir forces, increment of the temperature increases the possibility for occurrence of chaotic motion.