Bulk Band Gap Research Articles

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.

Evidence of the fractional quantum spin Hall effect in moiré MoTe2.

Quantum spin Hall (QSH) insulators are two-dimensional electronic materials that have a bulk band gap similar to an ordinary insulator but have topologically protected pairs of edge modes of opposite chiralities1-6. So far, experimental studies have found only integer QSH insulators with counter-propagating up-spins and down-spins at each edge leading to a quantized conductance G0 = e2/h (with e and h denoting the electron charge and Planck's constant, respectively)7-14. Here we report transport evidence of a fractional QSH insulator in 2.1° twisted bilayer MoTe2, which supports spin-Sz conservation and flat spin-contrasting Chern bands15,16. At filling factor ν = 3 of the moiré valence bands, each edge contributes a conductance with zero anomalous Hall conductivity. The state is probably a time-reversal pair of the even-denominator 3/2-fractional Chern insulators. Furthermore, at ν = 2, 4 and 6, we observe a single, double and triple QSH insulator with each edge contributing a conductance G0, 2G0 and 3G0, respectively. Our results open up the possibility of realizing time-reversal symmetric non-abelian anyons and other unexpected topological phases in highly tunable moiré materials17-19.

Topological laser on square lattice with gain–loss-induced higher-order corner modes

We investigate the higher-order topological laser in the two-dimensional (2D) coupled-cavity array. By adding staggered on-site gain and loss to the 2D Hermitian array with a trivial phase, the system will emerge degenerate topological corner modes, which are protected by bulk band gap. For such a non-Hermitian model, by adjusting the parameters of the system and introducing the pumping into the cavity at the corner, a single-mode lasing with topological protection emerges. Furthermore, single-mode lasing exists over a wide range of pumping strengths. No matter where the cavity is initially stimulated, after enough time evolution, all the cavities belonging to the topological corner mode can emit a stable laser.

Finite barrier bound state

A boundary mode localized on one side of a finite-size lattice can tunnel to the opposite side which results in unwanted couplings. Conventional wisdom tells that the tunneling probability decays exponentially with the size of the system which thus requires many lattice sites before eventually becoming negligibly small. Here we show that the tunneling probability for some boundary modes can apparently vanish at specific wavevectors. Thus, similar to bound states in the continuum, a boundary mode can be completely trapped within very few lattice sites where the bulk bandgap is not even well-defined. More intriguingly, the number of trapped states equals the number of lattice sites along the normal direction of the boundary. We provide two configurations and validate the existence of this peculiar finite barrier-bound state experimentally in a dielectric photonic crystal at microwave frequencies. Our work offers extreme flexibility in tuning the coupling between localized states and channels as well as a new mechanism that facilitates unprecedented manipulation of light.

Understanding the Adiabatic Evolution of Surface States in Tetradymite Topological Insulators under Electrochemical Conditions.

Nontrivial surface states in topological materials have emerged as exciting targets for surface chemistry research. In particular, topological insulators have been used as electrodes in electrocatalytic reactions. Herein, we investigate the robustness of the topological surface states and band topology under electrochemical conditions, specifically in the presence of an electric double layer. First-principles band structure calculations are performed on the electrified (111) surfaces of Bi2Te3, Bi2Se3, and Sb2Te3 using an implicit electrolyte model. Our results demonstrate the adiabatic evolution of the surface states upon surface charging. Under oxidizing potentials, the surface states are shifted upward in energy, preserving the Dirac point on the surface and the band inversion in the bulk. Conversely, under reduced potentials, hybridization is observed between the surface and bulk states, suggesting a likely breakdown of topological protection. The position of the Fermi level, which dictates the working states in catalytic reactions, should ideally be confined within the bulk bandgap. This requirement defines a potential window for the effective application of topological electrocatalysis.

Corner states and particle trapping in waterborne acoustic crystals

Higher-order topological insulators exhibit intriguing capacity to confine energy on lower-dimensional boundaries owing to the unique bulk-boundary correspondence. Many potential practical applications of the higher-order states in classical wave systems have been proposed and achieved. In this work, we implement a second-order topological insulator in a waterborne acoustic crystal by drilling and grooving a copper plate based on a two-dimensional Su–Schrieffer–Heeger lattice with different intracell and intercell couplings. The far-field transmission spectrum and near-field pressure field distributions verify the existence of the one-dimensional edge states and zero-dimensional corner states in the bulk bandgap. Due to the highly localized edge and corner states, the polystyrene particles are trapped at the edges and corners by the acoustic gradient force. Our findings provide a good platform to manipulate underwater acoustic wave and may inspire topological acoustic applications.

Electric-Field-Tunable Edge Transport in Bernal-Stacked Trilayer Graphene.

This Letter presents a nonlocal study on the electric-field-tunable edge transport in h-BN-encapsulated dual-gated Bernal-stacked (ABA) trilayer graphene across various displacement fields (D) and temperatures (T). Our measurements revealed that the nonlocal resistance (R_{NL}) surpassed the expected classical Ohmic contribution by a factor of at least 2 orders of magnitude. Through scaling analysis, we found that the nonlocal resistance scales linearly with the local resistance (R_{L}) only when the D exceeds a critical value of ∼0.2 V/nm. Additionally, we observed that the scaling exponent remains constant at unity for temperatures below the bulk-band gap energy threshold (T<25 K). Further, the value of R_{NL} decreases in a linear fashion as the channel length (L) increases. These experimental findings provide evidence for edge-mediated charge transport in ABA trilayer graphene under the influence of a finite displacement field. Furthermore, our theoretical calculations support these results by demonstrating the emergence of dispersive edge modes within the bulk-band gap energy range when a sufficient displacement field is applied.

Influence of temperature on bandgap shifts, optical properties and photovoltaic parameters of GaAs/AlAs and GaAs/AlSb p–n heterojunctions: insights from ab-initio DFT + NEGF studies

The III–V group semiconductors are highly promising absorbers for heterojunctions based solar cell devices due to their high conversion efficiency. In this work, we explore the solar cell properties and the role of electron–phonon coupling (EPC) on the solar cell parameters of GaAs/AlSb and GaAs/AlAs p–n heterojunctions using non-equilibrium Green function method (NEGF) in combination of ab-initio density functional theory (DFT). In addition, the band offsets at the heterointerfaces, optical absorption and bandgap shifts (BGSs) due to temperature are estimated using DFT + NEGF approach. The interface band gaps in heterostructures are found to be lower than bulk band gaps leading to a shift in optical absorption coefficient towards lower energy side that results in stronger photocurrent. The temperature dependent electronic BGS is significantly influenced by the phonon density and phonon energy via EPC. The phonon influenced BGS is found to change the optical absorption, photocurrent density and open-circuit voltage. In case of GaAs/AlSb junction, the interface phonons are found to have significantly higher energies as compared to the bulk phonons and thereby may have important implications for photovoltaic (PV) properties. Overall, the present study reveals the influence of EPC on the optical absorption and PV properties of GaAs/AlSb and GaAs/AlSb p–n heterojunctions. Furthermore, the study shows that the DFT + NEGF method can be successfully used to obtain the reasonable quantitative estimates of temperature dependent BGSs, optical absorption and PV properties of p–n heterojunctions.

Quantum spin Hall effect and emergence of conducting edge states in silicene supported by MX (M=Ga, In; X=S, Se, Te) monolayer

The electronic structure of silicene supported by monolayer of different monochalcogenide MX (GaS, GaSe, GaTe and InSe) substrates has been investigated by first principle density functional theory. By calculating the formation energies and phonons, it has been seen that silicene supported by monolayer of MX remains stable. The systems retain their almost [Formula: see text] planner configurations with small buckling as that of the free standing silicene and also the Bader charge analysis shows that silicene hardly interacts with any of the MX substrates. The Dirac cone with a small gap ([Formula: see text] [Formula: see text]meV) has been observed in each of the cases. All the systems show quantum spin Hall effect and the quantum spin Hall conductivities have been estimated to be within the range [Formula: see text]S/cm, which are larger than that of the free standing silicene. Our calculations show that even if the systems have bulk band gaps but the edge states are conducting in nature.

Nanotubular TiO2 films sensitized with CdTe quantum dots: Stability and adsorption distribution

Quantum dots (QDs) are zero-dimensional nanostructured semiconductors that are widely studied for their size and shape dependent optical behavior, making them attractive for numerous applications, including photovoltaics. Within this group of nanomaterials, CdTe QDs represent an interesting material as a sensitizer for TiO2 (Nt-TiO2) nanotubular matrices due to their bulk band gap of 1.45 eV and their high extinction coefficient. However, its functionality may be limited, being unstable in the presence of some redox probes due to photocorrosion processes. In this work, the composition and internal distribution of CdTe QDs adsorbed on Nt-TiO2 from toluene and toluene/acetone (70:30) and (50:50) mixtures were studied by X-ray photoelectron spectroscopy (XPS), before and after photocurrent measurements.XPS was used to characterize the coverage and distribution of QDs along the Nt-TiO2. It is shown that the increased polarity of the solvent causes a higher coverage and an extended distribution of QDs toward the interior of Nt-TiO2, which causes the photogenerated electrons to travel shorter distances to reach the underneath electrical contact, thus achieving a higher photocurrent. Additionally, the photoanode shows a progressive decrease in the Te signals inside the nanotubes after the photocurrent recording, presumably due to a photocorrosion process in the presence of Na2S and illumination.

Surface electronic structure and photo activity of double perovskite Ba[formula omitted]PrBiO[formula omitted]: First-principles investigations

Double perovskite Ba2PrBiO6 exhibits photo catalytic activity in the visible light range with the observed optical band gap of ∼1.0 eV. On the other hand, density functional theory (DFT) calculations predict a larger bulk band gap of 2.0 eV even with the standard Perdew–Burke–Ernzerhof (PBE) functional, which is known to underestimate the band gap of semiconductors. Much larger bulk band gap of 3.13 eV is predicted with a hybrid functional treatment for the exchange–correlation, which generally gives better descriptions for them. This discrepancy between experiment and theory implies the presence of unknown mechanism which effectively reduces the band gap of Ba2PrBiO6 and thereby gains the sensitivity to the visible light. The first finding is that the surface band gaps of Ba2PrBiO6 calculated using an appropriate hybrid functional are smaller than the bulk band gaps and are 2.49 eV and 2.85 eV for the Pr–Bi and Ba–O polar surfaces, respectively. We also find that work function is quite sensitive to the surface structures and its accurate calculations are essential to show that the reduction and oxidation levels are fit to the gap region. When a substitutional defect PrBi, in which Bi is substituted by Pr, is introduced into the Pr–Bi polar surface, surface band gap is further reduced to 0.93 eV, being very close to the experimental value. For the Ba–O polar surface, similar reduction to 1.59 eV is found but not substantial as compared to the Pr–Bi polar surface. These narrow surface band gaps may be the origin of the photo activity of Ba2PrBiO6 in the visible light region. We also performed Born–Oppenheimer molecular dynamics simulations to clarify the reactivity of the polar surfaces to water. We find that two hydroxyl groups were created in the adsorption of a single H2O molecule. The decomposition of water molecule in the adsorption indicates that the polar surfaces provide a promising stage for the photo catalytic activity. Further adsorption of H2O molecules leads to their diffusion on the surface.

Growth and Characterization of p-Type and n-Type Sb2Se3 for Use in Thin-Film Photovoltaic Solar Cell Devices

In this study, a two-electrode electrodeposition technique was employed to grow thin films of antimony selenide (Sb2Se3) on glass/fluorine-doped tin oxide (FTO) substrates. The highest quality thin films were consistently obtained within the range of 1600 mV to 1950 mV. Subsequent electrodeposition experiments were conducted at discrete voltages to produce various layers of thin films. Photoelectrochemical cell (PEC) measurements were performed to characterize the semiconductor material layers, leading to the identification of both p-Type and n-Type conductivity types. Optical absorption spectroscopic analysis revealed energy bandgap values ranging from 1.10 eV to 1.90 eV for AD-deposited Sb2Se3 samples and 1.08 eV to 1.68 eV for heat-treated Sb2Se3 samples, confirming the semiconducting nature of the Sb2Se3 material. Additionally, other characterization techniques, including X-ray diffraction analysis, reveal that the AD-deposited layers are almost amorphous, and heat treatment shows that the material is within the orthorhombic crystalline system. Heat-treated layers grown at ~1740 mV showed highly crystalline material with a bandgap nearing the bulk bandgap of Sb2Se3. Raman spectroscopy identified vibrational modes specific to the Sb2Se3 phase, further confirming its crystallinity. To explore the thin-film morphology, Scanning Electron Microscopy (SEM) was employed, revealing uniformly deposited material composed of grains of varying sizes at different voltages. Energy Dispersive X-ray analysis (EDX) confirmed the presence of antimony and selenium in the material layers.

Investigating the Effect of Ph Variation on the Optoelectronic and Structural Properties of Electrodeposited Copper Selenide Thin Films.

Copper Selenide (CuSe) thin film compound semiconductors have been deposited onto fluorine doped tin oxide (FTO) conducting substrates using electrodeposition technique through various electrolytic bath pH values of 2.00, 2.20, and 2.40. The deposition was carried out in potentiostatic mode using a 2-electrode system set-up at room temperature for 15 minutes at the cathodic voltage of 500 mV. The electrical and the optical properties of the thin films were carried out using photoelectrochemical (PEC) cells and ultraviolet visible spectrophotometer respectively. CuSe thin film possesses a p-type electrical conductivity and the sample grown at semiconductor and a pH of 2.2 was recorded to have the highest PEC signal and highest photo and dark currents that brings forth its maximum power. The energy band gap values of the electrodeposited CuSe thin films were observed to be 2.30 eV, 2.20 eV, and 2.00 eV at bath pH values of 2.0, 2.2, and 2.4 respectively. The optical band obtained at the various pH decreases as pH increases and these band gap values agree with the bulk band gap of CuSe. The preferred orientation peak of electrodeposited CuSe layers exists as klockmannite (CuSe) and the highest peak intensity for the preferred orientation peak occurred at pH of 2.20. At this pH value, the electrodeposited CuSe layers have the highest crystallite sizes.

Tunability of topological edge states in germanene at room temperature.

Germanene is a two-dimensional topological insulator with a large topological band gap. For its use in low-energy electronics, such as topological field effect transistors and interconnects, it is essential that its topological edge states remain intact at room temperature. In this study, we examine these properties in germanene using scanning tunneling microscopy and spectroscopy at 300 K and compare the results with data obtained at 77 K. Our findings show that the edge states persist at room temperature, although thermal effects cause smearing of the bulk band gap. Additionally, we demonstrate that, even at room temperature, applying an external perpendicular electric field switches the topological states of germanene off. These findings indicate that germanene's topological properties can be maintained and controlled at room temperature, making it a promising material for low-energy electronic applications.

Investigating the Effect of Ph Variation on the Optoelectronic and Structural Properties of Electrodeposited Copper Selenide Thin Films.

Copper Selenide (CuSe) thin film compound semiconductors have been deposited onto fluorine doped tin oxide (FTO) conducting substrates using electrodeposition technique through various electrolytic bath pH values of 2.00, 2.20, and 2.40. The deposition was carried out in potentiostatic mode using a 2-electrode system set-up at room temperature for 15 minutes at the cathodic voltage of 500 mV. The electrical and the optical properties of the thin films were carried out using photoelectrochemical (PEC) cells and ultraviolet visible spectrophotometer respectively. CuSe thin film possesses a p-type electrical conductivity and the sample grown at semiconductor and a pH of 2.2 was recorded to have the highest PEC signal and highest photo and dark currents that brings forth its maximum power. The energy band gap values of the electrodeposited CuSe thin films were observed to be 2.30 eV, 2.20 eV, and 2.00 eV at bath pH values of 2.0, 2.2, and 2.4 respectively. The optical band obtained at the various pH decreases as pH increases and these band gap values agree with the bulk band gap of CuSe. The preferred orientation peak of electrodeposited CuSe layers exists as klockmannite (CuSe) and the highest peak intensity for the preferred orientation peak occurred at pH of 2.20. At this pH value, the electrodeposited CuSe layers have the highest crystallite sizes.

Fractional disclination charge as a probe in acoustical topological crystalline insulators

The body–boundary correspondence refers to the relationship between the body and boundary states of topological insulators (TIs). In TIs, the presence of boundary states is connected to the bulk topological properties of the material. The topology can be identified by studying the energy of the topological modes within the bulk bandgap. However, not all topological materials exhibit boundary states within the insulating energy gap. In many cases, the presence of boundary states can be hidden or masked by the bulk energy bands, making it difficult to measure TIs. Recent experiments have shown that defects, which are commonly found in crystalline materials, can be used as probes to explore higher-order topologies that have been recently realized on various platforms. These defects can generate fractional charges and stable bound states in the dispersion region, allowing us to observe the clear body–disclination correspondence. We have performed simulations using a coupled acoustic cavity system with C3 and C5 symmetries to investigate the body–disclination correspondence in topological crystal insulators (TCIs) in the field of acoustics. Simulation and theoretical results have demonstrated that defects such as disclinations can be used to probe higher-order topologies that were previously unobservable in three-dimensional structures. This approach allows us to detect fractional mode charges and stable bound states, which are crucial for understanding the topological nature of TCIs. Our work demonstrates the potential of using disclination defects to study the intricate relationship between the body and boundary states in topological materials, particularly in the context of acoustics.

(Invited) Thermoelectric Transport in Nanostructured 3D Topological Insulators and Stacked 2D Materials / Ferecrystals

Research in thermoelectric (TE) quantum structures was greatly propelled by the pre[1]diction in the early 1990s of a significant boost in TE efficiency by quantum size effects. Recently, research interest has shifted from quantum size effects in conventional semiconductors toward new types of quantum materials (e.g., topological insulators [TIs], Weyl and Dirac semimetals) characterized by their nontrivial electronic topology. Bi2Te3, Sb2Te3, and Bi2Se3, established TE materials, are also TIs exhibiting a bulk bandgap and highly conductive and robust gapless surface states. The signature of the nontrivial electronic band structure on the TE transport properties can be best verified in transport experiments using nanowires and thin films. However, even in nanograined bulk, the typical peculiarities in the transport properties of TIs can be seen. The signature of TI surface states on the thermoelectric properties of nanowire model systems will be discuss in depth and how these states can be modified by chemical modifications and in the vicinity of magnetic insulators.In the second part, we will report a different material class of stacked 2D materials based on metal monochalcogenides (MMCs), topological insulators and non-van der Waals (nvW) 2D Materials called ferecrystals via atomic layer deposition (ALD). The ferecrystals can be tailored to exhibit unusual properties such as low thermal conductivity or electronic or magnetic properties. The electronic properties of the 2D materials are modified by the interactions between the interwoven layered and non-layered materials. We have developed ferecrystal combinations, where the 2D material is the electronically active or in-active layer by selection of the bandgap for each layer. Figure 1

Sol-gel synthesis and photoluminescent properties of metal oxide-metal oxide coupled nanocomposites

Tin oxide (SnO2) and zinc oxide (ZnO) coupled metal-oxide nanocomposites or SnO2–ZnO were prepared using a sol-gel method. The stretching mode frequencies and crystal structures were examined by using Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), respectively. The microscopic analyses by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed a flower-like morphology of the nanocomposites. The analysis of surface chemical and electronic states was conducted by X-ray photoelectron spectroscopy (XPS). The band gap energies as confirmed from the ultraviolet–visible spectroscopy (UV–Vis) data were found to be larger than those of the bulk band gaps due to the increase between energy levels associated with smaller particle sizes. The UV (380 nm) and orange (595) photoluminescence (PL) emissions observed from the SnO2:ZnO nanocomposites were attributed to a combinatorial contribution from radiative transitions in SnO2 and ZnO components. The proposed mechanisms of these emissions are discussed.

Formation of well-ordered surfaces of Bi2-xSbxTe3-ySey topological insulators using wet chemical treatment

The surface preparation of topological insulators (TIs) is a critical task in order to realize their efficient applications. A chemical treatment in an anhydrous solution of hydrogen chloride in isopropanol (HCl-iPA) and a subsequent annealing at relatively low temperature in ultrahigh vacuum (UHV) was successfully used for the surface preparation of bulk 3D (0001) TIs Bi2Te3, Sb2Te3, Bi2Se3 and an MBE - grown Bi2-xSbxTe3-ySey (BSTS) thin films. The surface treatment showed a significant modification of the initial TI surfaces, which was free from the structural disorder, oxidation and chemical impurities determined by X-ray photoelectron spectroscopy and low-energy electron diffraction. The insulating nontrivial bulk gap and well resolved gapless surface states with a linear dispersion of a massless Dirac cone were observed by angle-resolved photoelectron spectroscopy (ARPES). In the BSTS film, the Fermi level is located within the bulk band gap. The negative magnetoconductance corresponding to weak antilocalization demonstrated the contribution of the surface states of BSTS, that promise to be protected from backscattering. The surface treatment method proposed in this work is highly efficient for both bulk and thin TI films that can be useful for the deposition of an insulators/metals to fabricate transistor and spin valve systems.

Experimental investigation of the effect of topological insulator on the magnetization dynamics of ferromagnetic metal: and heterostructure

We have studied the spin-pumping phenomenon in ferromagnetic metal (FM) ()/topological insulator (TI) () bilayer system to understand magnetization dynamics of FM in contact with a TI. TIs embody a spin–momentum-locked surface state that spans the bulk band gap. Due to this special spin texture of the topological surface state, the spin-charge interconversion efficiency of TI is even higher than that of heavy metals. We evaluated the parameters like effective damping coefficient (), spin-mixing conductance () and spin current density () to demonstrate an efficient spin transfer in heterostructure. To probe the effect of the topological surface state, a systematic low-temperature study is crucial as the surface state of TI dominates at lower temperatures. The exponential increase of for all different thickness combinations of FM/TI bilayers and the enhancement of effective damping coefficient () with lowering temperature confirms that the spin chemical potential bias generated from spin-pumping induces spin current into the TI surface state. Furthermore, low-temperature measurements of effective magnetization () and magnetic anisotropy field (Hk ) showed anomaly around the same temperature region where the resistivity of TI starts showing metallic behavior due to the dominance of conducting TI surface state. The anomaly in Hk can result from the emerging exchange coupling between the TI surface state and the local moments of the FM layer at the interface without any long-range ferromagnetic order in TI at the interface.