Surface State-Dominated Photoconduction and THz Generation in Topological Bi2Te2Se Nanowires

Unusual Janus Bi2TeSe2 Topological Insulators Displaying Second-Harmonic Generation, Linear-in-Temperature Resistivity, and Weak Antilocalization

Well-established knowledge about inversion-symmetric Bi2TexSe3-x topological insulators characterizes the promising new-generation quantum device. Noticeably, the inversion asymmetric phase containing different surface electronic structures may create an extra topological phenomenon pointing to a new device paradigm. Herein, Janus Bi2TeSe2 single-crystal nanosheets with an unconventional stacking sequence of Se-Bi-Se-Bi-Te are realized via chemical vapor deposition growth, which is clarified by atomically resolved AC-STEM and elemental mapping. An obvious polarization-dependent second-harmonic generation with a representative 6-fold rotational symmetry is detected due to the broken out-of-plane mirror symmetry in this system. Low-temperature transport measurements display a strange metal-like linear-in-temperature resistivity. Anomalous conductance peaks under low magnetic fields induced by the weak antilocalization effect of topological surface states and the two-dimensional transport-dominated anisotropic magnetoresistance are revealed. These findings correlate the Janus Bi2TeSe2 phase with emerging physics topics, which would inspire fresh thoughts in well-developed Bi3TexSe3-x topological insulators and open up opportunities for exploring hybrid nonlinear optoelectronic topological devices.

Surface terminations control charge transfer from bulk to surface states in topological insulators

Topological insulators (TI) hold significant potential for various electronic and optoelectronic devices that rely on the Dirac surface state (DSS), including spintronic and thermoelectric devices, as well as terahertz detectors. The behavior of electrons within the DSS plays a pivotal role in the performance of such devices. It is expected that DSS appear on a surface of three dimensional(3D) TI by mechanical exfoliation. However, it is not always the case that the surface terminating atomic configuration and corresponding band structures are homogeneous. In order to investigate the impact of surface terminating atomic configurations on electron dynamics, we meticulously examined the electron dynamics at the exfoliated surface of a crystalline 3D TI (Bi2 Se3 ) with time, space, and energy resolutions. Based on our comprehensive band structure calculations, we found that on one of the Se-terminated surfaces, DSS is located within the bulk band gap, with no other surface states manifesting within this region. On this particular surface, photoexcited electrons within the conduction band effectively relax towards DSS and tend to linger at the Dirac point for extended periods of time. It is worth emphasizing that these distinct characteristics of DSS are exclusively observed on this particular surface.

Optically Gated Electrostatic Field-Effect Thermal Transistor

Dynamic tuning of thermal transport in solids is scientifically intriguing with wide applications for thermal transport control in electronic devices. In this work, we demonstrate a thermal transistor, a device in which heat flow can be regulated using external control, realized in a topological insulator (TI) through the topological surface states. The tuning of thermal transport is achieved by using optical gating of a thin dielectric layer deposited on the TI film. The gate-dependent thermal conductivity is measured using micro-Raman thermometry. The transistor has a large ON/OFF ratio of 2.8 at room temperature and can be continuously and repetitively switched in tens of seconds by optical gating and potentially much faster by electrical gating. Such thermal transistors with a large ON/OFF ratio and fast switching times offer the possibilities of smart thermal devices for active thermal management and control in future electronic systems.

Modification of metallic and non-metallic sites in pentasupertetrahedral chalcogenidometalate clusters for third-order nonlinear optical response

Four isomorphic P2 chalcogenide clusters named [Sn₁₁In₉Cu₆S₄₄]·11(H⁺DBU) (1) (DBU = 1,8-diazabicyclo[5.4.0] undec-7-ene), [Sn₁₀In₁₀Cu₆Se₄₄]·6(H₂²⁺DMAPA)·2(DMAPA)·9EG (2) (DMAPA = 3-dimethylaminopropylamine, EG = ethylene glycol), [Sn₁₀In₁₀Cu₆S₄₀O₄]·6[H₂²⁺PMDETA]·10EG (3) (PMDETA = pentamethyldiethylenetriamine), [Sn₁₀Ga₁₀Cu₆S₄₀O₄]·6(H₂²⁺DMAPA)·7EG (4) have been isolated via organotin precursor and mixed-metal strategy. These clusters exhibit excellent solubility in organic solvents. The continuous-regulation of optical band and optical limiting performance have been realized through precise controlled substituting engineering of cationic and anionic elements.

Enhanced terahertz emission from mushroom-shaped InAs nanowire network induced by linear and nonlinear optical effects

The development of powerful terahertz (THz) emitters is the cornerstone for future THz applications, such as communication, medical biology, non-destructive inspection, and scientific research. Here, we report the THz emission properties and mechanisms of mushroom-shaped InAs nanowire (NW) network using linearly polarized laser excitation. By investigating the dependence of THz signal to the incidence pump light properties (e.g. incident angle, direction, fluence, and polarization angle), we conclude that the THz wave emission from the InAs NW network is induced by the combination of linear and nonlinear optical effects. The former is a transient photocurrent accelerated by the photo-Dember field, while the latter is related to the resonant optical rectification effect. Moreover, thep-polarized THz wave emission component is governed by the linear optical effect with a proportion of ∼85% and the nonlinear optical effect of ∼15%. In comparison, thes-polarized THz wave emission component is mainly decided by the nonlinear optical effect. The THz emission is speculated to be enhanced by the localized surface plasmon resonance absorption of the In droplets on top of the NWs. This work verifies the nonlinear optical mechanism in the THz generation of semiconductor NWs and provides an enlightening reference for the structural design of powerful and flexible THz surface and interface emitters in transmission geometry.

Dirac surface plasmons in photoexcited bismuth telluride nanowires: optical pump-terahertz probe spectroscopy

Collective excitation of Dirac plasmons in graphene and topological insulators has opened new possibilities of tunable plasmonic materials ranging from THz to mid-infrared regions. Using time resolved Optical Pump-Terahertz Probe (OPTP) spectroscopy, we demonstrate the presence of plasmonic oscillations in bismuth telluride nanowires (Bi₂Te₃ NWs) after photoexcitation using an 800 nm pump pulse. In the frequency domain, the differential conductivity (Δσ = σ_{pump on}-σ_{pump off}) spectrum shows a Lorentzian response where the resonance frequency (ω_p), attributed to surface plasmon oscillations, shifts with photogenerated carrier density (n) as . This dependence establishes the absorption of THz radiation by the Dirac surface plasmon oscillations of the charge carriers in the Topological Surface States (TSS) of Bi₂Te₃ NWs. Moreover, we obtain a modulation depth, tunable by pump fluence, of ∼40% over the spectral range of 0.5 to 2.5 THz. In addition, the time evolution of Δσ(t) represents a long relaxation channel lasting for more than 50 ps. We model the decay dynamics of Δσ(t) using coupled second order rate equations, highlighting the contributions from surface recombination as well as from trap mediated relaxation channels of the photoinjected carriers.

Topological insulator metamaterial with giant circular photogalvanic effect

One of the most notable manifestations of electronic properties of topological insulators is the dependence of the photocurrent direction on the helicity of circularly polarized optical excitation. The helicity-dependent photocurrents, underpinned by spin-momentum locking of surface Dirac electrons, are weak and easily overshadowed by bulk contributions. Here, we show that the chiral response can be enhanced by nanostructuring. The tight confinement of electromagnetic fields in the resonant nanostructure enhances the photoexcitation of spin-polarized surface states of topological insulator Bi1.5Sb0.5Te1.8Se1.2, leading to an 11-fold increase of the circular photogalvanic effect and a previously unobserved photocurrent dichroism (ρcirc = 0.87) at room temperature. The control of spin transport in topological materials by structural design is a previously unrecognized ability of metamaterials that bridges the gap between nanophotonics and spin electronics, providing opportunities for developing polarization-sensitive photodetectors.

Reversible engineering of topological insulator surface state conductivity through optical excitation

Despite the broadband response, limited optical absorption at a particular wavelength hinders the development of optoelectronics based on Dirac fermions. Heterostructures of graphene and various semiconductors have been explored for this purpose, while non-ideal interfaces often limit the performance. The topological insulator (TI) is a natural hybrid system, with the surface states hosting high-mobility Dirac fermions and the small-bandgap semiconducting bulk state strongly absorbing light. In this work, we show a large photocurrent response from a field effect transistor device based on intrinsic TI Sn-Bi_1.1Sb_0.9Te₂S (Sn-BSTS). The photocurrent response is non-volatile and sensitively depends on the initial Fermi energy of the surface state, and it can be erased by controlling the gate voltage. Our observations can be explained with a remote photo-doping mechanism, in which the light excites the defects in the bulk and frees the localized carriers to the surface state. This photodoping modulates the surface state conductivity without compromising the mobility, and it also significantly modify the quantum Hall effect of the surface state. Our work thus illustrates a route to reversibly manipulate the surface states through optical excitation, shedding light into utilizing topological surface states for quantum optoelectronics.

Millimetre-long transport of photogenerated carriers in topological insulators

Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in Bi_2-xSb_xSe₃ nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons. Remarkably, these excitons can transport over hundreds of micrometers along the topological insulator (TI) nanoribbons before recombination at up to 40 K. The macroscopic transport distance, combined with short carrier lifetime obtained from transient photocurrent measurements, indicates an exciton diffusion coefficient at least 36 m² s⁻¹, which corresponds to a mobility of 6 × 10⁴ m² V⁻¹ s⁻¹ at 7 K and is four order of magnitude higher than the value reported for free carriers in TIs. The observation of highly dissipationless exciton transport implies the formation of superfluid-like exciton condensate at the surface of TIs.

Exciton condensation may emerge at room temperature in topological materials with strong Coulomb interactions and vanishing electron effective mass. Here, Hou et al. report the formation of excitons in Bi_2-xSb_xSe₃ nanoribbons, which can transport over hundreds of micrometres before recombination up to 40 K, further implying exciton condensation.

Quantized Conductance in Topological Insulators Revealed by the Shockley-Ramo Theorem

Crystals with symmetry-protected topological order, such as topological insulators, promise coherent spin and charge transport phenomena even in the presence of disorder at room temperature. We demonstrate how to image and read out the local conductance of helical surface modes in the prototypical topological insulators Bi_{2}Se_{3} and BiSbTe_{3}. We apply the so-called Shockley-Ramo theorem to design an optoelectronic probe circuit for the gapless surface states, and we find a well-defined conductance quantization at 1e^{2}/h within the experimental error without any external magnetic field. The unprecedented response is a clear signature of local spin-polarized transport, and it can be switched on and off via an electrostatic field effect. The macroscopic, global readout scheme is based on an electrostatic coupling from the local excitation spot to the readout electrodes, and it does not require coherent transport between electrodes, in contrast to the conventional Landauer-Büttiker description. It provides a generalizable platform for studying further nontrivial gapless systems such as Weyl semimetals and quantum spin-Hall insulators.

Graphene Interdigital Electrodes for Improving Sensitivity in a Ga2O3:Zn Deep-Ultraviolet Photoconductive Detector

Graphene (Gr) has been widely used as a transparent electrode material for photodetectors because of its high conductivity and high transmittance in recent years. However, the current low-efficiency manipulation of Gr has hindered the arraying and practical use of such detectors. We invented a multistep method of accurately tailoring graphene into interdigital electrodes for fabricating a sensitive, stable deep-ultraviolet photodetector based on Zn-doped Ga₂O₃ films. The fabricated photodetector exhibits a series of excellent performance, including extremely low dark current (∼10^-11 A), an ultrahigh photo-to-dark ratio (>10⁵), satisfactory responsivity (1.05 A/W), and excellent selectivity for the deep-ultraviolet band, compared to those with ordinary metal electrodes. The raise of photocurrent and responsivity is attributed to the increase of incident photons through Gr and separated carriers caused by the built-in electric field formed at the interface of Gr and Ga₂O₃:Zn films. The proposed ideas and methods of tailoring Gr can not only improve the performance of devices but more importantly contribute to the practical development of graphene.

Growth and quantum transport properties of vertical Bi2Se3 nanoplate films on Si substrates

Controlling the growth direction (planar versus vertical) and surface-to-bulk ratio can lead to lots of unique properties for two-dimensional layered materials. We report a simple method to fabricate continuous films of vertical Bi₂Se₃ nanoplates on Si substrate and investigate the quantum transport properties of such films. In contrast to (001) oriented planar Bi₂Se₃ nanoplate film, vertical Bi₂Se₃ nanoplate films are enclosed by (015) facets, which possess high surface-to-bulk ratio that can enhance the quantum transport property of topological surface states. And by controlling the compactness of vertical Bi₂Se₃ nanoplates, we realized an effective tuning of the weak antilocalization effect from topological surface states in Bi₂Se₃ films. Our work paves a way for exploring the unique transport properties of this unconventional structure topological insulator film.

Spin Hall photoconductance in a three-dimensional topological insulator at room temperature

Three-dimensional topological insulators are a class of Dirac materials, wherein strong spin-orbit coupling leads to two-dimensional surface states. The latter feature spin-momentum locking, i.e., each momentum vector is associated with a spin locked perpendicularly to it in the surface plane. While the principal spin generation capability of topological insulators is well established, comparatively little is known about the interaction of the spins with external stimuli like polarized light. We observe a helical, bias-dependent photoconductance at the lateral edges of topological Bi₂Te₂Se platelets for perpendicular incidence of light. The same edges exhibit also a finite bias-dependent Kerr angle, indicative of spin accumulation induced by a transversal spin Hall effect in the bulk states of the Bi₂Te₂Se platelets. A symmetry analysis shows that the helical photoconductance is distinct to common longitudinal photoconductance and photocurrent phenomena, but consistent with optically injected spins being transported in the side facets of the platelets.

While the spin generation in topological insulators is well studied, little is known about the interaction of the spins with external stimuli. Here, Seifert et al. observe a helical, bias-dependent photoconductance at the lateral edges of topological Bi₂Te₂Se platelets for perpendicular incidence of light, distinct to common longitudinal photoconductance phenomena.