Topological Insulator Sb2Te3 Research Articles

Interfacial reaction behavior between ferromagnetic CoFeB and the topological insulator Sb2Te3

The successful prevention of interfacial reactions between ferromagnetic materials and topological insulators (TIs) is crucial for the realization of reliable spintronic devices using TIs. To this end, we investigated the magnetic properties and interfacial reaction behavior of a bilayer structure composed of ferromagnetic CoFeB and TI Sb2Te3. The effects of including a MgO interlayer was also investigated. Ferromagnetic resonance (FMR) studies showed a remarkably weak resonance peak for the sample annealed at 400 °C for the 2-nm-thick interlayer, and finally, no resonance peak for a film below 1 nm in thickness was observed. X-ray diffraction (XRD) results demonstrated that the Sb2Te3 peak intensities started to decrease upon annealing at 200 °C and completely disappeared for annealing temperatures >400 °C. Hard X-ray photoelectron spectroscopy (HAXPES) results also support that the core-level peaks of Sb and Te split upon annealing at temperatures >200 °C, suggesting the dissociation of Sb2Te3. These results indicate that Sb2Te3 and CoFeB react at the interface during annealing, resulting in a loss of the ferromagnetic properties of the CoFeB layer. Meanwhile, a sample containing a 3-nm-thick MgO layer retained its original Sb2Te3/MgO/CoFeB structure even after annealing at 400 °C, as evidenced by its unchanged XRD peak intensity and FMR spectrum. We expect that our findings will be highly valuable in developing TI-based spintronic devices.

Electronic band structure of Sb2Te3

We report on Landau-level spectroscopy of an epitaxially grown thin film of the topological insulator Sb2Te3, complemented by ellipsometry and magnetotransport measurements. The observed response suggests that Sb2Te3 is a direct-gap semiconductor with the fundamental band gap located at the Γ point or along the trigonal axis, and its width reaches Eg=(190±10) meV at low temperatures. Our data also indicate the presence of other low-energy extrema with a higher multiplicity in both the conduction and valence bands. The conclusions based on our experimental data are confronted with and to a great extent corroborated by the electronic band structure calculated using the GW method. Published by the American Physical Society 2024

Thermoelectric response of textured Sb2Te3–BiSb and Sb2Te3–Bi2Te3 thin film junctions

Surface mounted thin film thermoelectric (TE) devices for localized cooling, power generation, and sensing are topics of immense current interest. Here, we establish the superior TE performance of thin film junctions made of topological insulators (TIs) Bi85Sb15 (BiSb) and Sb2Te3 by comparing their performance with those of Bi2Te3 and Sb2Te3. Thin films of these TIs were first evaluated for their carrier concentration, Hall mobility, resistivity, and thermopower. The Seebeck coefficient of BiSb, Sb2Te3, and Bi2Te3 measured against copper at ambient temperature is −100, +160, and −70 (±10) μV/K, while their power factors are 0.5, 0.5, and 0.45 (±0.05) 10−3 W m−1 K−2, respectively. Single TE junctions of BiSb–Sb2Te3 and Bi2Te3–Sb2Te3 yield a response of 272 and 240 (±10) μV/K, respectively. This comparative study shows that BiSb is a superior n-type counter electrode for Sb2Te3 compared to the n-type Bi2Te3. Moreover, Bi2Te3 is prone to tellurium antisite disorder, which affects its TE properties significantly.

High-performance, ultra-broadband Sb2Te3 photodetector assisted by multimechanism

Ultra-broadband spectral detection is critical for the application of several technologies, such as imaging, sensing, spectroscopy, and communication. However, the development of high-performance photodetectors with a wide spectral response remains a significant challenge. In this study, we present an ultra-broadband, responsive, dual-mechanism fusion photodetector based on the topological insulator Sb2Te3. By employing methods such as applying bias voltages, energy band structures, and photocurrent imaging, we have confirmed that the operating mechanism in the visible to the infrared light region is the photoconductive effect, exhibiting a room-temperature responsivity of 114.6 mA/W and a specific detectivity of 1.78 × 108 cm2 Hz1/2 W−1, especially under the working condition of 520 nm. Moreover, extending the spectral detection capability from infrared to terahertz is critically important. In the terahertz range, where photon energies are significantly lower than those in the visible to infrared spectrum, the detector achieves a room-temperature responsivity of 38.5 mA/W at 0.12 THz and a specific detectivity of 3.44 × 1010 cm2 Hz1/2 W−1, with a response time of 20 µs, primarily attributed to the asymmetric scattering behavior of the topological surface states. Notably, we have successfully demonstrated high-resolution imaging in the visible to infrared and terahertz spectral ranges at room temperature, providing strong evidence of practical applications for our photodetector. In conclusion, Sb2Te3, as an emerging broadband photodetection material, exhibits significant potential and lays a robust foundation for further research in the field of ultra-broadband photodetectors.

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.

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.

Spin‐Orbit Readout Using Thin Films of Topological Insulator Sb2Te3 Deposited by Industrial Magnetron Sputtering

AbstractDriving a spin‐logic circuit requires the production of a large output signal by spin‐charge interconversion in spin‐orbit readout devices. This should be possible by using topological insulators, which are known for their high spin‐charge interconversion efficiency. However, high‐quality topological insulators have so far only been obtained on a small scale, or with large scale deposition techniques that are not compatible with conventional industrial deposition processes. The nanopatterning and electrical spin injection into these materials have also proven difficult due to their fragile structure and low spin conductance. The fabrication of a spin‐orbit readout device from the topological insulator Sb2Te3 deposited by large‐scale industrial magnetron sputtering on SiO2 is presented. Despite a modification of the Sb2Te3 layer structural properties during the device nanofabrication, a sizeable output voltage is measured that can be unambiguously ascribed to a spin‐charge interconversion process. The results pave the way for the integration of layered van der Waals materials in spin‐logic devices.

Spatial distribution of photo-induced anomalous Hall effects of the top and bottom surface states in topological insulators Sb2Te3

Three-dimensional (3D) topological insulators (TIs) have attracted much attention due to their topologically protected surface states. Here, we use circularly polarized light to induce the photo-induced anomalous Hall effect (PAHE) in 3D TIs Sb2Te3 thin films with different thicknesses of 5, 7, 12 and 20 quintuple layers (QLs) at room temperature. The dependence of PAHE current on the light spot locations of Sb2Te3 thin films is investigated, which is found to show a completely different behavior from that observed in two-dimensional electron gas (2DEG) systems. This is due to the fact that the total PAHE current is the superposition of the PAHE of the top and bottom surface states. A theoretical model has been proposed to separate the PAHE current of the top and bottom surface states. In addition, as the thickness of the Sb2Te3 film increases from 5 QL to 20 QL, the PAHE currents of the top and bottom surface states first increase and then decrease. The photo-induced anomalous Hall, conductivity of the top surface states in the 7-QL Sb2Te3 film excited by 1064 nm light is as large as 592 nA⋅ V−1⋅ cm ⋅ W−1, which is much larger than that observed in InGaAs/AlGaAs quantum wells (4.45 nA⋅ V−1⋅ cm ⋅ W−1) and GaN/AlGaN heterostructures (1.43 nA⋅ V−1⋅ cm ⋅ W−1). The giant PAHE value observed in Sb2Te3 films suggests that 3D TIs may provide a good platform for spintronic devices.

Light-induced vacuum micromotors based on an antimony telluride microplate

Manipulating motion of microobjects with light is indispensable in various technologies. On solid interfaces, its realizations, however, are hampered by surface friction. To resolve this difficulty, light-induced elastic waves have been recently proposed to drive microobjects against friction. Despite its expected applicability for arbitrary optical-absorptive objects, the new principle has only been tested with microsized gold plates. Herein, we validate this principle using a new material and report directional and continuous movements of a two-dimensional topological insulator (Sb2Te3) plate on an untreated microfiber surface driven by nanosecond laser pulses. The motion performance of the Sb2Te3 plate is characterized by a scanning electron microscope. We observe that the motion velocity can be controlled by tuning the average power of laser pulses. Further, by intentionally increasing the pulse repetition rate and exploiting the low thermal conductivity of Sb2Te3, we examine the thermal effects on actuation and reveal the motion instability induced by formations of microbumps on Sb2Te3 surfaces due to the Marangoni effects. Moreover, as the formed microbumps are heated to viscoelasticity states, liquid-like motion featuring asymmetry in contact angles is observed and characterized, which expands the scope of light-induced actuation of microobjects.

Topological Insulator Optoelectronic Synapses for High‐Accuracy Binary Image Recognition using Recurrent Neural Networks

AbstractArtificial optoelectronic synapses have drawn tremendous attention in neuromorphic computing due to their exceptional properties of incorporating optical‐sensing and synaptic functions. However, the complex fabrication processes and device architectures greatly limit their applications. More importantly, artificial neural networks (ANNs) commonly implemented with optoelectronic synapses cannot take full advantage of the time‐dependent data of synaptic devices, resulting in defective accuracies. Here, facile two‐terminal optoelectronic synapses based on topological insulator Sb2Te3 films are fabricated, which exhibit significant photocurrent responses, owing to the efficient light‐matter interaction in bulk and the topological surface state of Sb2Te3. The performance of Sb2Te3 devices can be tuned both optically and electrically. Typical characteristics of synapses, such as paired‐pulse facilitation, short‐term memory, long‐term memory, and learning behavior, have been demonstrated. With the establishment of recurrent neural networks (RNNs) that are committed to processing temporal data, the as‐fabricated synapse devices are employed for binary image recognition of handwritten numbers “0” and “1”. The recognition accuracy of RNNs can reach as high as 100%, which is dramatically higher than those of ANNs. The effective employment of temporal data with RNNs ensured high recognition accuracy. These Sb2Te3 optoelectronic synapses with RNNs indicate the great potential for developing high‐performance brain‐inspired neuromorphic computing.

Magnetotransport and ARPES studies of the topological insulators Sb2Te3 and Bi2Te3 grown by MOCVD on large-area Si substrates

Recently, the topological insulators (TIs) antimony telluride (Sb2Te3) and bismuth telluride (Bi2Te3) are attracting high interest for applications based on spin-charge interconversion mechanisms. Aiming to make a step toward the technology transfer, it is of major importance to achieve and investigate epitaxial quality-TIs on large area Si-based substrates. In view of that, we report here magnetotransport and angle-resolved photoemission spectroscopy (ARPES) studies on Sb2Te3 and Bi2Te3 thin films grown by metal organic chemical vapor deposition (MOCVD) on top of 4″ Si(111) substrates. Clear weak antilocalization (WAL) effects are observed in both TIs, proving the existence of quantum transport mechanism, and the data are successfully interpreted in the framework of the Hikami–Larkin–Nagaoka model. Further, by dedicated magnetotransport experiments, it has been confirmed that the investigated WAL originates from two-dimensional (2D) topological states. ARPES has been performed ex-situ, and in both TIs the gapless Dirac cones have been observed and attributed to the topological surface states. Combining the proofs of the existence of quantum 2D transport as deduced from the analysis of the magnetoconductance curve with the direct observation of the Dirac-like band structure revealed by the ARPES spectra, it is possible to unambiguously confirm the topological nature of our Sb2Te3 and Bi2Te3 thin films. The results obtained on thin films grown by MOCVD on 4’’ Si(111) substrate mark an important step towards the technology transfer of the topological insulators studied in this work.

Tuning of circular photogalvanic effect of surface states in the topological insulator Sb2Te3 via structural deformation

Strain is a useful method to manipulate properties of three-dimensional (3D) topological insulators (TIs). In this study, we demonstrate the possibility to tune the circular photogalvanic effect (CPGE) of surface states of 3D TI Sb2Te3 films by applying external strain. The CPGE of 3D TI Sb2Te3 grown on SrTiO3 (STO) with different thicknesses has been systematically investigated. It is found that as the thickness of Sb2Te3 films increases from 7-quintuple layer (QL) to 27-QL, the CPGE current first increases and then decreases. Additionally, the CPGE currents demonstrate remarkable temperature dependence, which even reverse sign when the temperature is increased from 77 to 300 K. This phenomenon is due to the vertical thermoelectric effect and inverse spin Hall effect. Finally, the CPGE measurements of Sb2Te3 films under different mechanical strains are performed, and it is found that the CPGE current linearly decreases with the increase in the external strain. The variation in the CPGE current can be tuned up to 11% and 44% in the 18- and 12-QL Sb2Te3 grown on STO substrates under a tensile strain of 0.0225 and 0.0066, respectively. In particular, it can even reach 100% in the 30-QL Sb2Te3 film grown on an InP substrate under a tensile strain of 0.0033, which is due to the combined effect of mechanical deformation and spin injection from substrates. Our work provides a method to effectively manipulate the CPGE in 3D TIs by the combined effect of mechanical strain and spin injection from substrates, which paves the way for novel opto-spintronic devices.

Superconductivity of topological insulator Sb2Te3− y Se y under pressure

The crystal structures of Sb2Te3−y Se y (y = 0.6 and y = 1.2) at 0–24 GPa were investigated by synchrotron x-ray diffraction. The stoichiometry of Sb2Te3−y Se y used in this study was determined to be Sb2Te2.19(9)Se0.7(2) for y = 0.6 and Sb2Te1.7(1)Se1.3(3) for y = 1.2, on the basis of energy-dispersive x-ray spectroscopy. The sample of Sb2Te2.19(9)Se0.7(2) showed a structural phase transition from a rhombohedral structure (space group No. 166, R m) (phase I) to a monoclinic structure (space group No. 12, C2/m) (phase II), with increasing pressure up to ∼9 GPa. A new structural phase (phase II′) emerged at 17.7 GPa, a monoclinic structure with the space group C2/c (No. 15). Finally, a 9/10-fold monoclinic structure (space group No. 12, C2/m) (phase III) was observed at 21.8 GPa. In contrast, the sample of Sb2Te1.7(1)Se1.3(3) provided only phase I (space group No. 166, R m) and phase II (space group No. 12, C2/m), showing one structural phase transition from 0–19.5 GPa. These samples were not superconductors at ambient pressure, but superconductivity suddenly appeared with increasing pressure. Superconductivity with superconducting transition temperatures (T c’s) of 2 and 4 K was observed above 6 GPa in phase I of Sb2Te2.19(9)Se0.7(2). In this sample, the T c values of 6 and 9 K were observed in phase II and phase II′ or III of Sb2Te2.19(9)Se0.7(2), respectively. Superconductivity with T c’s of 4 and 5 K suddenly emerged in Sb2Te1.7(1)Se1.3(3) at 13.6 GPa, which corresponds to phase II, and it evolved to 6.0 K under further increased pressure. A T c value of 9 K was finally found above 15 GPa. The magnetic field dependence of T c in phase II of Sb2Te2.19(9)Se0.7(2) and Sb2Te1.7(1)Se1.3(3) followed a p-wave polar model, suggesting topologically nontrivial superconductivity.

Visualizing the in-Gap States in Domain Boundaries of Ultra-Thin Topological Insulator FilmsSupported by the National Natural Science Foundation of China (Grant Nos. 61804056 and 92065102), and the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University.

Ultra-thin topological insulators provide a platform for realizing many exotic phenomena such as the quantum spin Hall effect, and quantum anomalous Hall effect. These effects or states are characterized by quantized transport behavior of edge states. Experimentally, although these states have been realized in various systems, the temperature for the edge states to be the dominating channel in transport is extremely low, contrary to the fact that the bulk gap is usually in the order of a few tens of milli-electron volts. There must be other in-gap conduction channels that do not freeze out until a much lower temperature. Here we grow ultra-thin topological insulator Bi2Te3 and Sb2Te3 films by molecular beam epitaxy and investigate the structures of domain boundaries in these films. By scanning tunneling microscopy and spectroscopy we find that the domain boundaries with large rotation angles have pronounced in-gap bound states, through which one-dimensional conduction channels are suggested to form, as visualized by spatially resolved spectroscopy. Our work indicates the critical role played by domain boundaries in degrading the transport properties.

The AiiDA-KKR plugin and its application to high-throughput impurity embedding into a topological insulator

The ever increasing availability of supercomputing resources led computer-based materials science into a new era of high-throughput calculations. Recently, Pizzi et al. introduced the AiiDA framework that provides a way to automate calculations while allowing to store the full provenance of complex workflows in a database. We present the development of the AiiDA-KKR plugin that allows to perform a large number of ab initio impurity embedding calculations based on the relativistic full-potential Korringa-Kohn-Rostoker Green function method. The capabilities of the AiiDA-KKR plugin are demonstrated with the calculation of several thousand impurities embedded into the prototypical topological insulator Sb2Te3. The results are collected in the JuDiT database which we use to investigate chemical trends as well as Fermi level and layer dependence of physical properties of impurities. This includes the study of spin moments, the impurity’s tendency to form in-gap states or its effect on the charge doping of the host-crystal. These properties depend on the detailed electronic structure of the impurity embedded into the host crystal which highlights the need for ab initio calculations in order to get accurate predictions.

Terahertz probe of nonequilibrium carrier dynamics and ultrafast photocurrents in the topological insulator Sb2Te3

In this study, we report the ultrafast excitation and dynamics of the carrier and photocurrent in a 3D typical topological insulator Sb2Te3. We utilize time-resolved optical pump-THz probe spectroscopy to explore the nonequilibrium carrier dynamics of Sb2Te3. The electron system undergoes an ultrafast relaxation and releases through intra-band and inter-band scattering. Additionally, THz emission spectroscopy is employed to investigate the different ultrafast photocurrents in Sb2Te3 through tuning the polarization of excitation pulses and rotating the sample's azimuthal angle. We distinguish the different ultrafast photocurrents driven by the linear photogalvanic effect, circular photogalvanic effect, and thermoelectric effect. Our results potentially enable an all-optical modulation of THz emission without any external bias field, which could play an important role in the development of topological insulator-based high-speed THz optoelectronic and opto-spintronic devices.

First-principles calculation of Sb2Te3 topological insulator under pressure

The theoretical density of states (DOS) along with band structure (BS) has been evaluated for Topological insulator Sb2Te3 upon pressure. The density functional theory (DFT) calculation shows electronic topological transition (ETT) under the application of pressure. When pressure is applied conduction band and valence shifts significantly which is indicated as electronic transition of the Fermi surfaces. The band gap between conduction band and valence band reduced at Z-high symmetry point whereas it is observed to enhance at Γ-point with increase in pressure. Interestingly, we found semiconductor to metal transition near 2 GPa with LDA approximation. The overall change in the Fermi surfaces is found to follow other topological materials like Bi2Te3, Sb2Se3 and Bi2Se3. The topologically non-trivial surface state remains unchanged under pressure; however, the bulk band gap reduces.

Inelastic helium atom scattering from Sb2Te3(111): phonon dispersion, focusing effects and surfing.

We present an experimental study of inelastic scattering processes on the (111) surface of the topological insulator Sb2Te3 using helium atom scattering. In contrast to other binary topological insulators such as Bi2Se3 and Bi2Te3, Sb2Te3 is much less studied and the as-grown Sb2Te3 sample turns out to be p-doped, with the Fermi-level located below the Dirac-point as confirmed by angle-resolved photoemission spectroscopy. We report the surface phonon dispersion along both high symmetry directions in the energy region below 11 meV, where the Rayleigh mode exhibits the strongest intensity. The experimental data is compared with a study based on density functional perturbation theory calculations, providing good agreement except for a set of additional peculiar inelastic events below the Rayleigh mode. In addition, an analysis of angular scans with respect to a number of additional inelastic events is presented, including resonance enhancement, kinematical focusing, focused inelastic resonance and surfing. In the latter case, phonon-assisted adsorption of the incident helium atom gives rise to a bound state where the helium atom rides the created Rayleigh wave.

Heterostructures of Graphene and Topological Insulators Bi2Se3, Bi2Te3, and Sb2Te3

Prototypical 3D topological insulators of the Bi2Se3 family provide a beautiful example of the appearance of the surface states inside the bulk bandgap caused by spin‐orbit coupling‐induced topology. The surface states are protected against backscattering by time reversal symmetry, and exhibit spin‐momentum locking whereby the electron spin is polarized perpendicular to the momentum, typically in the plane of the surface. In contrast, graphene is a prototypical 2D material, with negligible spin‐orbit coupling. When graphene is placed on the surface of a topological insulator, giant spin‐orbit coupling is induced by the proximity effect, enabling interesting novel electronic properties of its Dirac electrons. A detailed theoretical study of the proximity effects of monolayer graphene and topological insulators Bi2Se3, Bi2Te3, and Sb2Te3 is presented, and the appearance of the qualitatively new spin‐orbit splittings, well described by a phenomenological Hamiltonian, is elucidated by analyzing the orbital decomposition of the involved band structures. This should be useful for building microscopic models of the proximity effects between the surfaces of the topological insulators and graphene.

Creating thin magnetic layers at the surface of Sb2Te3 topological insulators using a low-energy chromium ion beam

The surfaces of Sb2Te3 topological insulator crystals were implanted using a 40 keV chromium ion beam. To facilitate uniform doping, the Sb2Te3 was passivated with a thin TiO2 film before the implantation step. The resulting chemical structure was studied using atomic-resolution transmission electron microscopy. A fluence of 7 × 1015 ions/cm2 at 40 keV lead to amorphization of the Sb2Te3 surface, with chromium predominantly incorporated in the amorphous layer. Heating to 200 °C caused the amorphous region to recrystallize and led to the formation of a thin chromium-rich interfacial layer. Near-edge x-ray absorption spectroscopy indicates a uniform valence state of Cr3+ throughout, with no evidence of metallic clustering. High-temperature superparamagnetic behavior was detected up to 300 K, with an increased magnetic moment below 50 K.