Impact of the Topological Surface State on the Thermoelectric Transport in Sb2Te3 Thin Films

Topological insulators for thermoelectrics: A perspective from beneath the surface

Thermoelectric properties of topological insulators have traditionally been examined in the context of their metallic surface states. However, recent studies have begun to unveil intriguing thermoelectric effects emerging from the bulk electronic states of topological insulators, which have largely been overlooked in the past. Charge transport phenomena through the bulk are especially important under typical operating conditions of thermoelectric devices, necessitating a comprehensive review of both surface and bulk transport in topological insulators. Here, we review thermoelectric properties that are uniquely observed in topological insulators, placing special emphasis on unconventional phenomena emerging from bulk states. We demonstrate that unusual thermoelectric effects arising from bulk states, such as band inversion-driven warping, can be discerned in experiments through a simple analysis of the weighted mobility. We believe that there is still plenty to uncover within the bulk of topological insulators, yet our current understanding can already inspire new strategies for designing and discovering new materials for next-generation thermoelectrics.

Thermal biasing for lattice symmetry breaking and topological edge state imaging

Marginally twisted bilayer graphene with large Bernal stacked domains involves symmetry-breaking features with domain boundaries that exhibit topological edge states normally obscured by trivial bands. A vertical electric field can activate these edge states through inversion symmetry breaking and opening a bandgap around the edge state energy. However, harnessing pristine topological states at the Fermi level without violent electric or magnetic bias remains challenging, particularly above room temperature. Here, we demonstrate that thermal biasing can break the vertically stacked lattice symmetry of twisted bilayer graphene via the interatomic Seebeck effect, enabling thermoelectric imaging of topological edge states at tunable Fermi levels above room temperature. The high spatial resolution in the imaging is achieved through atomic-scale thermopower generation between a metallic tip and the sample, reflecting the local electronic band structure and its derivative features of twisted bilayer graphene at the Fermi level. Our findings suggest that thermal biasing provides a sensitive, non-destructive method for symmetry breaking and topological state imaging above room temperature, making it a practical and accessible approach.

Spin-dependent thermoelectric properties of a hybrid ferromagnetic metal/quantum dot/topological insulator junction

The thermoelectric properties of hybrid system based on a single-level quantum dot coupled to a ferromagnetic metallic lead and attached to the surface states of a three-dimensional topological insulator are theoretically investigated. On the surface of a three-dimensional topological insulator, massless helical Dirac fermions emerge. We calculate the thermoelectric coefficients, including electrical conductance, Seebeck coefficient (thermopower), heat conductance, and the figure of merit, using the nonequilibrium Green's function technique. The results are analyzed in terms of the emergence of new effects. The calculations are performed within the Hubbard I approximation concerning the dot's Coulomb interactions. Additionally, the spin-dependent coupling of the quantum dot to the ferromagnetic lead lifts the spin degeneracy of the dot's level, which influences the transport properties of the system. We incorporate this effect perturbatively to obtain the spin-dependent renormalization of the dot's level. We also consider the case of finite spin accumulation in the ferromagnetic electrode, which leads to spin thermoelectric effects.

Are topological insulators promising thermoelectrics?

Some of the best thermoelectric (TE) materials to date are also topological insulators (TIs). While many studies have investigated the effects of topologically-protected TI surface states on TE properties, the conditions needed to realize such effects are quite different from typical operating conditions of TE devices for, e.g., power generation and room-temperature Peltier cooling. As a result, it is still unclear what properties of TIs, especially those related to the bulk band structure, are beneficial for TE performance, if any. Here, we perform high-throughput transport calculations using density functional theory to reveal that, within the same structure type, TIs tend to outperform normal insulators as TEs when properly optimized. The calculations also indicate that the TE performance is higher for TIs with strongly inverted bands. To explain these observations, we develop models based on Boltzmann transport theory which show that warping driven by band inversion, a key characteristic of TIs, is responsible for the high TE performance of TIs. We find that warping benefits the TE performance because of reduced transport mass and effectively higher valley degeneracy. Our results show that the band inversion strength is a critical property of a TI dictating the TE performance, and we suggest potential strategies to tune the inversion strength and enhance the TE performance in TIs, such as alloying and strain engineering. The study marks TIs as serious candidates for TE applications owing to band inversion-driven warping.

Localization and Directionality of Surface Transport in Bi2Te3 Ordered 3D Nanonetworks

The resistance of an ordered 3D-Bi2Te3 nanowire nanonetwork was studied at low temperatures. Below 50 K the increase in resistance was found to be compatible with the Anderson model for localization, considering that conduction takes place in individual parallel channels across the whole sample. Angle-dependent magnetoresistance measurements showed a distinctive weak antilocalization characteristic with a double feature that we could associate with transport along two perpendicular directions, dictated by the spatial arrangement of the nanowires. The coherence length obtained from the Hikami-Larkin-Nagaoka model was about 700 nm across transversal nanowires, which corresponded to approximately 10 nanowire junctions. Along the individual nanowires, the coherence length was greatly reduced to about 100 nm. The observed localization effects could be the reason for the enhancement of the Seebeck coefficient observed in the 3D-Bi2Te3 nanowire nanonetwork compared to individual nanowires.

Wafer-Scale Growth of Sb2Te3 Films via Low-Temperature Atomic Layer Deposition for Self-Powered Photodetectors

In this work, we demonstrate the performance of a silicon-compatible, high-performance, and self-powered photodetector. A wide detection range from visible (405 nm) to near-infrared (1550 nm) light was enabled by the vertical p-n heterojunction between the p-type antimony telluride (Sb₂Te₃) thin film and the n-type silicon (Si) substrates. A Sb₂Te₃ film with a good crystal quality, low density of extended defects, proper stoichiometry, p-type nature, and excellent uniformity across a 4 in. wafer was achieved by atomic layer deposition at 80 °C using (Et₃Si)₂Te and SbCl₃ as precursors. The processed photodetectors have a low dark current (∼20 pA), a high responsivity of (∼4.3 A/W at 405 nm and ∼150 mA/W at 1550 nm), a peak detectivity of ∼1.65 × 10¹⁴ Jones, and a quick rise time of ∼98 μs under zero bias voltage. Density functional theory calculations reveal a narrow, near-direct, type-II band gap at the heterointerface that supports a strong built-in electric field leading to efficient separation of the photogenerated carriers. The devices have long-term air stability and efficient switching behavior even at elevated temperatures. These high-performance and self-powered p-Sb₂Te₃/n-Si heterojunction photodetectors have immense potential to become reliable technological building blocks for a plethora of innovative applications in next-generation optoelectronics, silicon-photonics, chip-level sensing, and detection.

Two-Dimensional Van Der Waals Topological Materials: Preparation, Properties, and Device Applications

Over the past decade, 2D van der Waals (vdW) topological materials (TMs), including topological insulators and topological semimetals, which combine atomically flat 2D layers and topologically nontrivial band structures, have attracted increasing attention in condensed-matter physics and materials science. These easily cleavable and integrated TMs provide the ideal platform for exploring topological physics in the 2D limit, where new physical phenomena may emerge, and represent a potential to control and investigate exotic properties and device applications in nanoscale topological phases. However, multifaced efforts are still necessary, which is the prerequisite for the practical application of 2D vdW TMs. Herein, this review focuses on the preparation, properties, and device applications of 2D vdW TMs. First, three common preparation strategies for 2D vdW TMs are summarized, including single crystal exfoliation, chemical vapor deposition, and molecular beam epitaxy. Second, the origin and regulation of various properties of 2D vdW TMs are introduced, involving electronic properties, transport properties, optoelectronic properties, thermoelectricity, ferroelectricity, and magnetism. Third, some device applications of 2D vdW TMs are presented, including field-effect transistors, memories, spintronic devices, and photodetectors. Finally, some significant challenges and opportunities for the practical application of 2D vdW TMs in 2D topological electronics are briefly addressed.

Toward Reduced Interface Contact Resistance: Controllable Surface Energy of Sb2Te3 Films via Tuning the Crystallization and Orientation

The electrical contact resistance between a metal and semiconductor is one of the keys to improving the output performance of thin-film thermoelectric devices. Herein, we reduced the interface contact resistance by controlling the surface energy of a Sb₂Te₃ semiconductor via tuning of the crystallization and orientation, preparing an intrinsically compact and flat Sb₂Te₃ film with high surface energy and low roughness, which can give rise to a low average specific contact resistivity (8.2 × 10^-6 Ω cm²) with a Ni/Cu metal. The improvement in interface electrical properties is due to the increase in the surface energy and decrease in the surface roughness of the semiconductor surface, which lead to a transformation from three-dimensional island-shaped nucleation to two-dimensional layered nucleation for surface-attached metal films, forming a longitudinally tight connection contact with a low resistance. This approach allows the resistivity to become close to the fundamental theoretically calculated limit. Our work provides a new idea for reducing the contact resistivity of thin-film thermoelectric devices, which is conducive to supporting the development of thermoelectric semiconductor planarization.

3D Bi2Te3 Interconnected Nanowire Networks to Increase Thermoelectric Efficiency

3D interconnected nanowire scaffoldings are shown to increase the thermoelectric efficiency in comparison to similar diameter 1D nanowires and films grown under similar electrodeposition conditions. Bi₂Te₃ 3D nanonetworks offer a reduction in thermal conductivity (κ_T) while preserving the high electrical conductivity of the films. The reduction in κ_T is modeled using the hydrodynamic heat transport equation, and it can be understood as a heat viscosity effect due to the 3D nanostructuration. In addition, the Seebeck coefficient is twice that of nanowires and films, and up to 50% higher than in a single crystal. This increase is interpreted as a nonequilibrium effect that the geometry of the structure induces on the distribution function of the phonons, producing an enhanced phonon drag. These thermoelectric metamaterials have higher performance and are fabricated with large areas by a cost-effective method, which makes them suitable for up-scale production.

Interface-Dominated Topological Transport in Nanograined Bulk Bi2 Te3

3D topological insulators (TI) host surface carriers with extremely high mobility. However, their transport properties are typically dominated by bulk carriers that outnumber the surface carriers by orders of magnitude. A strategy is herein presented to overcome the problem of bulk carrier domination by using 3D TI nanoparticles, which are compacted by hot pressing to macroscopic nanograined bulk samples. Bi₂ Te₃ nanoparticles well known for their excellent thermoelectric and 3D TI properties serve as the model system. As key enabler for this approach, a specific synthesis is applied that creates nanoparticles with a low level of impurities and surface contamination. The compacted nanograined bulk contains a high number of interfaces and grain boundaries. Here it is shown that these samples exhibit metallic-like electrical transport properties and a distinct weak antilocalization. A downward trend in the electrical resistivity at temperatures below 5 K is attributed to an increase in the coherence length by applying the Hikami-Larkin-Nagaoka model. THz time-domain spectroscopy reveals a dominance of the surface transport at low frequencies with a mobility of above 10³ cm² V^-1 s^-1 even at room temperature. These findings clearly demonstrate that nanograined bulk Bi₂ Te₃ features surface carrier properties that are of importance for technical applications.

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.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Phonon anharmonicities and ultrafast dynamics in epitaxial Sb2Te3

In this study we report on the investigation of epitaxially grown Sb₂Te₃ by employing Fourier-Transform transmission Spectroscopy (FTS) with laser-induced Coherent Synchrotron Radiation (CSR) in the Terahertz (THz) spectral range. Static spectra in the range between 20 and 120 cm⁻¹ highlight a peculiar softening of an in-plane IR-active phonon mode upon temperature decrease, as opposed to all Raman active modes which instead show a hardening upon temperature decrease in the same energy range. The phonon mode softening is found to be accompanied by an increase of free carrier concentration. A strong coupling of the two systems (free carriers and phonons) is observed and further evidenced by exciting the same phonon mode at 62 cm⁻¹ within an ultrafast pump-probe scheme employing a femtosecond laser as pump and a CSR single cycle THz pulse as probe. Separation of the free carrier contribution and the phonon resonance in the investigated THz range reveals that, both damping of the phonon mode and relaxation of hot carriers in the time domain happen on the same time scale of 5 ps. This relaxation is about a factor of 10 slower than expected from the Lorentz time-bandwidth limit. The results are discussed in the framework of phonon scattering at thermal and laser induced transient free carriers.

Ultrathin Topological Insulator Absorber: Unique Dielectric Behavior of Bi2Te3 Nanosheets Based on Conducting Surface States

Topological insulators exhibit great potential in the fields of electronics and semiconductors for their gapless surface states. Intriguingly, most topological insulators are possibly excellent microwave-absorbing materials because of easy adjustment of electrical transport based on conducting surface states in the nanostructure. Herein, topological insulator Bi₂Te₃ nanosheets are synthesized by a simple solvothermal method. The material demonstrates a unique dielectric behavior based on conducting surface states, resulting in excellent microwave-absorbing performance. Benefiting from the outstanding impedance matching, Bi₂Te₃ nanosheets exhibit an ultrathin microwave absorption with the qualified frequency bandwidth of 3.0 GHz at only 0.77 mm thickness, which is thinner than other absorbers in reported references. Moreover, a strong reflection loss of -41 dB at 0.8 mm is achieved. The result provides a new approach for developing ultrathin microwave absorption materials at the submillimeter scale.

Origin of high thermoelectric performance with a wide range of compositions for BixSb2−xTe3 single quintuple layers

The high power factor of a p-type BST single QL is ensured by the robust multi-valley character of valence bands.

Extracting the Energy Sensitivity of Charge Carrier Transport and Scattering

It is a challenge to extract the energy sensitivity of charge carriers' transport and scattering from experimental data, although a theoretical estimation in which the existing scattering mechanism(s) are preliminarily assumed can be easily done. To tackle this problem, we have developed a method to experimentally determine the energy sensitivities, which can then serve as an important statistical measurement to further understand the collective behaviors of multi-carrier transport systems. This method is validated using a graphene system at different temperatures. Further, we demonstrate the application of this method to other two-dimensional (2D) materials as a guide for future experimental work on the optimization of materials performance for electronic components, Peltier coolers, thermoelectricity generators, thermocouples, thermopiles, electrical converters and other conductivity and/or Seebeck-effect-related sensors.

Effect of impurity resonant states on optical and thermoelectric properties on the surface of a topological insulator

We investigate the thermoelectric effect on a topological insulator surface with particular interest in impurity-induced resonant states. To clarify the role of the resonant states, we calculate the dc and ac conductivities and the thermoelectric coefficients along the longitudinal direction within the full Born approximation. It is found that at low temperatures, the impurity resonant state with strong energy de-pendence can lead to a zero-energy peak in the dc conductivity, whose height is sensitively dependent on the strength of scattering potential, and even can reverse the sign of the thermopower, implying the switching from n- to p-type carriers. Also, we exhibit the thermoelectric signatures for the filling process of a magnetic band gap by the resonant state. We further study the impurity effect on the dynamic optical conductivity, and find that the resonant state also generates an optical conductivity peak at the absorption edge for the interband transition. These results provide new perspectives for understanding the doping effect on topological insulator materials.

Efficient Photothermoelectric Conversion in Lateral Topological Insulator Heterojunctions

Tuning the electron and phonon transport properties of thermoelectric materials by nanostructuring has enabled improving their thermopower figure of merit. Three-dimensional topological insulators, including many bismuth chalcogenides, attract increasing attention for this purpose, as their topologically protected surface states are promising to further enhance the thermoelectric performance. While individual bismuth chalcogenide nanostructures have been studied with respect to their photothermoelectric properties, nanostructured p-n junctions of these compounds have not yet been explored. Here, we experimentally investigate the room temperature thermoelectric conversion capability of lateral heterostructures consisting of two different three-dimensional topological insulators, namely, the n-type doped Bi₂Te₂Se and the p-type doped Sb₂Te₃. Scanning photocurrent microscopy of the nanoplatelets reveals efficient thermoelectric conversion at the p-n heterojunction, exploiting hot carriers of opposite sign in the two materials. From the photocurrent data, a Seebeck coefficient difference of ΔS = 200 μV/K was extracted, in accordance with the best values reported for the corresponding bulk materials. Furthermore, it is in very good agreement with the value of ΔS = 185 μV/K obtained by DFT calculation taking into account the specific doping levels of the two nanostructured components.

Enhancement of carrier lifetime by spin-orbit coupling in a topological insulator of an Sb2Te3 thin film

Electrons and phonons in chalcogenide-based materials are important factors in the performance of optical data-storage media and thermoelectric devices. However, the fundamental kinetics of carriers in chalcogenide materials remains controversial, and active debate continues over the mechanism responsible for carrier relaxation. In this study, we used optical-pump terahertz-probe spectroscopy, which permits the relationship between structural phase transition and optical property transitions to be examined, to investigate the ultrafast carrier dynamics in a multilayered [Sb(3 Å)/Te(9 Å)]_n thin film during the transition from the disordered to crystalline phase. Using terahertz time-domain spectroscopy and a contact-free optical technique, we demonstrated that the optical conductance and carrier concentration vary as functions of annealing temperature. Moreover, we observed that the topological surface state (TSS) affects the enhancement of the carrier lifetime, which is closely related to the degree of spin-orbit coupling (SOC). The combination of the optical technique and proposed carrier relaxation mechanism provides a powerful tool for monitoring TSS and SOC. Consequently, it was determined that the response of the disordered phase is dominated by an electron-phonon coupling effect, while that of the crystalline structure is controlled by a Dirac surface state and SOC effects. These results are important for understanding the fundamental physics of phase change materials and for optimizing and designing materials with better performance in optoelectronic devices.

Robustness of a Topologically Protected Surface State in a Sb2Te2Se Single Crystal

A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb₂Te₂Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb₂Te₂Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb₂Te₂Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb₂Te₂Se compound has a great potential for future atmospheric applications.

Yttrium-Doped Sb2Te3: A Promising Material for Phase-Change Memory

Sb₂Te₃ exhibits outstanding performance among the candidate materials for phase-change memory; nevertheless, its low electrical resistivity and thermal stability hinder its practical application. Hence, numerous studies have been carried out to search suitable dopants to improve the performance; however, the explored dopants always cause phase separation and thus drastically reduce the reliability of phase-change memory. In this work, on the basis of ab initio calculations, we have identified yttrium (Y) as an optimal dopant for Sb₂Te₃, which overcomes the phase separation problem and significantly increases the resistivity of crystalline state by at least double that of Sb₂Te₃. The good phase stability of crystalline Y-doped Sb₂Te₃ (YST) is attributed to the same crystal structure between Y₂Te₃ and Sb₂Te₃ as well as their tiny lattice mismatch of only ∼1.1%. The significant increase in resistivity of c-YST is understood by our findings that Y can dramatically increase the carrier's effective mass by regulating the band structure and can also reduce the intrinsic carrier density by suppressing the formation of Sb_Te antisite defects. Y doping can also improve the thermal stability of amorphous YST based on our ab initio molecular dynamics simulations, which is attributed to the stronger interactions between Y and Te than that of Sb and Te.

The surface-to-volume ratio: a key parameter in the thermoelectric transport of topological insulator Bi2Se3 nanowires

We systematically investigated the role of topological surface states on thermoelectric transport by varying the surface-to-volume ratio (s/v) of Bi2Se3 nanowires. The thermoelectric coefficients of Bi2Se3 nanowires were significantly influenced by the topological surface states with increasing the s/v. The Seebeck coefficient of Bi2Se3 nanowires decreased with increasing the s/v, while the electrical conductivity increased with increasing the s/v. This implies that the influence of metallic surface states become dominant in thermoelectric transport in thin nanowires, and the s/v is a key parameter which determines the total thermoelectric properties. Our measurements were corroborated by using a two-channel Boltzmann transport model.

Solvothermal synthesis of wire-like SnxSb2Te3+x with an enhanced thermoelectric performance

Nanostructured tellurides have attracted increasing attention in thermoelectric applications for waste heat recovery and cooling devices. Here, we report on the synthesis of wire-like SnxSb2Te3+x (x = 0, 0.02 and 0.05) nanoparticles using elemental precursors in EG. The enhanced thermoelectric performance was achieved in alloyed samples due to the increase of carrier population in heavy valence band valleys by incorporating Sn(2+) at the Sb(3+) sublattice, enabling the simultaneous realization of low electrical resistivity along with a high Seebeck coefficient as well as the decline of thermal conductivity. Thus a boosted power factor and low thermal conductivity lead to the highest ZT value of 0.58 at 150 °C in the Sn0.02Sb2Te3.02 sample. Our research offers a general wet-chemical route for the preparation of one-dimensional nanomaterials and probably promotes the practical thermoelectric applications of Sb2Te3-based materials at low temperatures.