Narrow-gap Semiconductors Research Articles

Structural and electronic properties of the Te-Si(111) surface from first principles

The structural and electronic properties of an atomically-flat Te-Si(111) surface have been investigated by means of density-functional theory calculations. This system is interesting because it provides a template for the epitaxial growth of inherently 2D materials. A structural model of the surface is devised that is both energetically more favorable than the ideal on-top model proposed in the literature and dynamically stable. The model, characterized by a staggered arrangement of Te-Te dimers in the passivation layer, is a semiconductor with a narrow band gap resulting from the misalignment of the ${\mathrm{Te}}_{2}$ units. As for the on-top case, however, this structure does not fully conforms to the available experimental observations. A finite-temperature model is hence prepared by means of molecular dynamics simulations at 300 K. It turns out that such a model is characterized by a disordered passivation layer consisting of randomly oriented ${\mathrm{Te}}_{2}$ units and Te chains, which makes it effectively compliant with all the experimental structural data at hand. In addition, it is also a narrow-gap semiconductor compatible with the electrical conductance measurements. These findings suggest that this model is a good candidate for representing the Te-Si(111) surface.

Enhancement of thermoelectric power factor in Si-doped Mg3Sb2 compoundfor thermoelectric converting applications

Mg3Sb2 is a narrow band gap semiconductor with electrical conductivity and thermoelectric properties dependent on structure and doping elements. In this study, to improve the thermoelectric properties of Mg3Sb2, the authors doped Si at different ratios (x=0.05, 0.1, 0.15, 0.25, and 0.3) into Sb sites as the formula Mg3Sb2-xSix to improve the electrical conductivity of doping samples while maintaining a high value of the Seebeck coefficient. The results exhibited that Mg3Sb2 and Mg3Sb2-xSixcompounds prepared by solid phase reaction method (high energy grinding combined with hot pressing and sintering) showed hexagonal structure. The electrical conductivity of the Si-doped samples increased significantly compared to the undoped Mg3Sb2 sample, while the Seebeck coefficient slightly decreased in the low doping contents samples and highest increase in the Mg3Sb1.75Si0.25 sample. The power factor of all doping samples increased compared to undoped samples. The power factor values at 673K of Mg3Sb1.9Si0.1, Mg3Sb1.75Si0.25, and Mg3Sb1.7Si0.3 increased around 1.7 times compared to the Mg3Sb2

First-principles calculations to investigate structural, elastic, electronic and thermoelectric properties of narrow-band gap half-Heusler RhVX (X = Si, Ge) compounds

This research work aims to understand the structural, electronic, elastic and thermoelectric properties of half-Heusler RhVX (X = Si, Ge) compounds. The included properties are explored by using the full-potential linearized augmented plane wave method under the support of density functional theory by employing Wien2k computational code. Within the calculations, the lattice constant of RhVSi is found as 5.69 Å and for RhVGe as 5.74 Å. These values are in admirable agreement with the existing literature. Further, both the half-Heusler compounds are explored as ductile in ground state from the calculation of elastic and mechanical properties. The narrow indirect bandgap (0.21 eV for RhVSi and 0.33 eV for RhVGe) semiconductor character of both the RhVX compounds is revealed from the results of electronic properties. The thermoelectric performance of RhVX compounds is analyzed by calculating the important parameters such as thermal conductivity (both electronic and lattice), Seebeck coefficients, carrier concentration and figure of merit. Both the RhVX compounds are suggested as suitable candidates for thermoelectric power generators at high temperatures.

Phase Formation Behavior and Thermoelectric Transport Properties of S-Doped FeSe2-xSx Polycrystalline Alloys.

Some transition-metal dichalcogenides have been actively studied recently owing to their potential for use as thermoelectric materials due to their superior electronic transport properties. Iron-based chalcogenides, FeTe2, FeSe2 and FeS2, are narrow bandgap (~1 eV) semiconductors that could be considered as cost-effective thermoelectric materials. Herein, the thermoelectric and electrical transport properties FeSe2-FeS2 system are investigated. A series of polycrystalline samples of the nominal composition of FeSe2-xSx (x = 0, 0.2, 0.4, 0.6, and 0.8) samples are synthesized by a conventional solid-state reaction. A single orthorhombic phase of FeSe2 is successfully synthesized for x = 0, 0.2, and 0.4, while secondary phases (Fe7S8 or FeS2) are identified as well for x = 0.6 and 0.8. The lattice parameters gradually decrease gradually with S content increase to x = 0.6, suggesting that S atoms are successfully substituted at the Se sites in the FeSe2 orthorhombic crystal structure. The electrical conductivity increases gradually with the S content, whereas the positive Seebeck coefficient decreases gradually with the S content at 300 K. The maximum power factor of 0.55 mW/mK2 at 600 K was seen for x = 0.2, which is a 10% increase compared to the pristine FeSe2 sample. Interestingly, the total thermal conductivity at 300 K of 7.96 W/mK (x = 0) decreases gradually and significantly to 2.58 W/mK for x = 0.6 owing to the point-defect phonon scattering by the partial substitution of S atoms at the Se site. As a result, a maximum thermoelectric figure of merit of 0.079 is obtained for the FeSe1.8S0.2 (x = 0.2) sample at 600 K, which is 18% higher than that of the pristine FeSe2 sample.

Formation of In-Plane Semiconductor-Metal Contacts in 2D Platinum Telluride by Converting PtTe2 to Pt2Te2.

Monolayer PtTe2 is a narrow gap semiconductor while Pt2Te2 is a metal. Here we show that the former can be transformed into the latter by reaction with vapor-deposited Pt atoms. The transformation occurs by nucleating the Pt2Te2 phase within PtTe2 islands, so that a metal-semiconductor junction is formed. A flat band structure is found with the Fermi level of the metal aligning with that of the intrinsically p-doped PtTe2. This is achieved by an interface dipole that accommodates the ∼0.2 eV shift in the work functions of the two materials. First-principles calculations indicate that the origin of the interface dipole is the atomic scale charge redistributions at the heterojunction. The demonstrated compositional phase transformation of a 2D semiconductor into a 2D metal is a promising approach for making in-plane metal contacts that are required for efficient charge injection and is of particular interest for semiconductors with large spin-orbit coupling, like PtTe2.

The Stacking Faulted Nature of the Narrow Gap Semiconductor Sc2Si2Te6

AbstractCrystals of Sc2Si2Te6 have been grown and its crystal, micro‐ and electronic structures were investigated. The layered character of the title compound exhibits stacking faults that impede a full structural characterization by single crystal X‐ray diffraction due to diffuse scattering. Based on high resolution transmission electron micrographs and diffraction patterns, the stacking faulted nature of the real structure of Sc2Si2Te6 has been revealed. Different stacking models were derived from the idealized, faultless structure and the stacking disorder was quantitatively analyzed by Rietveld refinement of powder X‐ray diffraction patterns. An energetic comparison of the stacking models by density functional theory is in line with the experimental observations. Further, the bonding situation was investigated by electronic structure calculations. Sc2Si2Te6 is a narrow gap semiconductor with an indirect band gap of 0.65 eV.

Hopping thermopower in FEGA3

A narrow-gap semiconductor FeGa3 has attracted much interest for its unusual physical properties, in particular, for its colossal thermopower at low temperatures for which the origin has not been elucidated yet. In this study, the temperature-dependence data of the conductivity, the Hall coefficient, and the Seebeck coefficient on FeGa3 samples reported by Wagner-Reetz et al. [Phys. Rev. B 90, 195206 (2014)] have been analyzed with including the effects of impurity Hubbard bands. As a result, it is shown that, contrary to the claim by Wagner-Reetz et al., the large negative peak of the Seebeck coefficient is not due to the phonon-drag effect but due to hopping conduction in the impurity Hubbard bands. The significant effect of nonpolar-optical-phonon scattering on free-electron conduction is also pointed out.

Analysis of Specific Features of the QD Electronic Spectrum for Narrow-Gap Semiconductors

Analysis of Specific Features of the QD Electronic Spectrum for Narrow-Gap Semiconductors

Quantitative analysis of free-electron dynamics in InSb by terahertz shockwave spectroscopy

The contribution of free electrons to the dielectric function is largely determined by their plasmonic resonance, a collective density oscillation. We studied this broadband and carrier-density-dependent response in the narrow-gap semiconductor InSb with THz shockwave spectroscopy. A synthesized waveform, with a steep onset followed by a short electric-field plateau, gives access to a multioctave spectrum of frequencies from 100 GHz up to the mid infrared. By measuring the entire spectral characteristics of the plasma, we analyze the dynamics of photogenerated free electrons for a wide range of excitation fluences. Thanks to this analysis, we were able to quantify the coefficients of both electron trapping and the Auger process from cryogenic to room temperature.

Effect of additives and optimized Cyclic voltammetry parameters on the morphology of electrodeposited Tellurium thin film

• The CV curve of Tellurium shifts with increasing deposition time. • Mechanism of Tellurium displacement and effect of additives. • Dense Tellurium film was fabricated under optimized deposition potentials. • Various dendritic morphologies of Tellurium were obtained by different additives. Thin film technologies in terms of preparation and fabrication are of great importance in various fields, such as integrated circuits (IC), LCD monitors, photovoltaics, solar cells and sensors. Tellurium, as a narrow bandgap semiconductor, has attracted much attention due to its rich optical and electrical properties. Taking advantage of these properties, its films have been explored in recent years as saturable absorbers, gas sensors, and thermoelectric materials. However, it is challenging to control the growth of the specific morphology of tellurium films. In this work, based on the electrodeposition challenge, we systematically investigated the electrodeposition process of tellurium films. It was found that the CV curves of electrodeposited tellurium shift in the positive direction with increasing deposition time. In addition, the stabilized potential was determined based on the stable deposition peak potential after a certain deposition time. Moreover, three different additives, namely polyvinyl alcohol (PVA), tartaric acid (TA) and sodium lignosulfonate (SLS), were adopted to manipulate the morphology of as-deposited tellurium films. A rational route to deposit tellurium films with various morphologies have been established.

Enhanced thermoelectric performance of p-type Bi2Te3-based materials by suppressing bipolar thermal conductivity

Thermal excitation causes a surge in carrier concentration in narrow bandgap semiconductors that seriously limit its application in the high-temperature zone. Therefore, suppressing bipolar thermal conductivity and broadening the application temperature zone is vitally important for commercial Bi2Te3-based thermoelectric materials. Here, Cu0.6Ni0.4 (CN) nanoparticles synthesized via hydrothermal method are introduced into Bi0.4Sb1.6Te3 (BST) commercial materials. Then, the electrical and thermal properties of BST-CN (with x wt%, x = 0, 0.1, 0.2, 0.4, BST-x-CN) materials are systematically investigated. The results exhibited that the weighted mobility could be improved and the bipolar thermal conductivity could be suppressed for all dealt samples. The maximum and average ZT (figure of merit) values were 1.3 at 393 K and 1.17 in the temperature range of 303–483 K for BST-0.1-CN sample, which exhibited an enhancement by 28.7% and 28.6% than that of the pure sample, respectively. And the optimal ZT values are attained at higher temperatures with the CN nanoparticles contents increasing. Meanwhile, the sintered technique was optimized to enhance electron transport properties, the highest room-temperature power factor of 5.08 mW/m/K2 was attained for the BST-0.1-CN cycle-2 sintered sample. And the optimal ZT value is 1.35 at 393 K for the BST-0.1-CN cycle-1 sintered sample. Finally, the results indicated that the thermoelectric performance could be improved and the application temperature zone could be broadened by suppressing bipolar thermal conductivity owing to the existence of CN nanoparticles.

Ag-catalyzed GaSb nanowires for flexible near-infrared photodetectors

High-quality narrow bandgap semiconductors nanowires (NWs) challenge the flexible near-infrared (NIR) photodetectors in next-generation imaging, data communication, environmental monitoring, and bioimaging applications. In this work, complementary metal oxide semiconductor-compatible metal of Ag is deposited on glass as the growth catalyst for the surfactant-assisted chemical vapor deposition of GaSb NWs. The uniform morphology, balance stoichiometry, high-quality crystallinity, and phase purity of as-prepared NWs are checked by scanning electron microscopy, energy dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, and X-ray diffraction. The electrical properties of as-prepared NWs are studied by constructing back-gated field-effect-transistors, displaying a high I on/I off ratio of 104 and high peak hole mobility of 400 cm2/(V·s). Benefiting from the excellent electrical and mechanical flexibility properties, the as-fabricated NW flexible NIR photodetector exhibits high sensitivity and excellent photoresponse, with responsivity as high as 618 A/W and detectivity as high as 6.7 × 1010 Jones. Furthermore, there is no obvious decline in NIR photodetection behavior, even after parallel and perpendicular folding with 1200 cycles.

First-principles calculations to investigate structural, elastic, optical, and thermoelectric properties of narrow band gap semiconducting cubic ternary fluoroperovskites barium based BaMF3 (M = Ag and Cu) compounds

Herein the first-principle modeling within the DFT framework is used to investigate the structural, optoelectronic, elastic, and thermoelectric properties of BaMF3 (M = Ag and Cu) ternary halide Perovskites compounds. The computed tolerance factors for BaAgF3 and BaCuF3 are 0.919 and 0.991 respectively, indicating that the cubic crystal phase of these selected compounds is thermodynamically and mechanically stable. It is found that both the compounds are structurally stable according to the Birch-Murnaghan fit curve for optimization. The IRelast package is used for investigating the elastic properties of both cubic compounds and the findings show that these compounds are mechanically stable, scratch-resistant, ductile, anisotropic, and resistant to plastic deformation. The Trans-Blaha modified Becke-Johnson (TB-mBJ) approximation is employed to determine electronic properties with accuracy and precision. The electronic-band structure analysis depicts a narrow band gap semiconductor nature of BaAgF3 and BaCuF3 compounds which displays indirect band gaps of 0.2 eV for BaAgF3 and 0.3 eV for BaCuF3 at the high symmetrical points from (Γ–M). Furthermore, the computations of total densities of states (TDOS) and partial densities of states (PDOS) are carried out to determine how different states contribute to the various band structures. The optical characteristics of BaAgF3 and BaCuF3 are comprehensively examined in 0–40 eV energy ranges. The selected materials possess high optical conduction and absorption coefficients at the high level of photon energies and show transparency at low incident photon energy ranges. The investigations of optical properties led us to the conclusion that BaAgF3 and BaCuF3 are suitable to use in high-frequency UV devices. The thermoelectric properties show that both materials hold high-power factors, electrical conductivity, and figures of merit (ZT), indicating that they are good thermoelectric materials. This is the first theoretical computational systematic analysis of structural, optoelectronic, elastic, and thermoelectric properties of BaMF3 (M = Ag and Cu) that will be experimentally validated to our knowledge.

Fabrication of a Highly Efficient Wood-Based Solar Interfacial Evaporator with Self-Desalting and Sterilization Performance.

Solar interfacial evaporation based on wood-derived materials has been considered a promising strategy for desalination and wastewater purification. Herein, we adopted delignified wood (DW) as the water transport substrate and lignosulfonate (LS)-modified narrow-band gap semiconductor nickel disulfide (NiS2) as the light-absorbing agent (LS-NiS2) to fabricate a high-efficiency evaporator (LS-NiS2@DW). On the one hand, the high absorbance (>95%) within a broad wavelength range and excellent photothermal conversion efficiency of LS-NiS2 endow efficient solar energy utilization. On the other hand, the hydrophilicity of DW facilitates water activation, which results in a lower evaporation enthalpy of LS-NiS2@DW (1274.4 kJ kg-1) than that of pure water. By combining LS-NiS2 and DW, LS-NiS2@DW achieved an evaporation rate as high as 2.80 kg m-2 h-1 under one sun irradiation (1 kW m-2), and the evaporation efficiency reached 87.4%. Notably, LS-NiS2@DW exhibits a high evaporation rate (2.42-2.69 kg m-2 h-1) in simulated seawater for 24 h with no salt crystals formed on the surface. Moreover, LS-NiS2@DW shows high antibacterial activity with about 90% reduction in bacterial survival rate. This work could provide new perspectives for the design of a high-efficiency wood-based photothermal evaporator.

Z-scheme CuO/Fe3O4/GO heterojunction photocatalyst: Enhanced photocatalytic performance for elimination of tetracycline

CuO/Fe3O4/GO, as a Z-scheme heterojunction catalyst, was successfully synthesized and used as a photocatalyst for removing tetracycline from aqueous solution. The CuO/Fe3O4/GO heterogeneous catalyst combines the narrow bandgap semiconductor CuO, oxygen vacancies of Fe3O4, and oxygen-containing reaction sites of GO. Without the addition of activators (persulfate or H2O2), the photocatalytic performance on decomposing tetracycline is very excellent. Compared with GO, Fe3O4, and CuO, CuO/Fe3O4/GO exhibits superior photocatalytic performance. Under visible light radiation, CuO/Fe3O4/GO generates h+ and ⋅O2−, which are the mainly responsible active groups for TC degradation. The effects of various pH, catalysts, and reuse on the degradation performance are evaluated, and the optimal conditions for CuO/Fe3O4/GO removal of tetracycline are obtained at pH 7, catalyst dosage 20 mg⋅L−1, TC at a concentration of 30 mg/L, nearly 97.3% of tetracycline is decomposed. This study has great potential in the treatment of wastewater containing various antibiotics.

MgIn2S4@In2O3 hierarchical tubular heterostructures with expedited photocarrier separation for efficient visible-light-driven antimicrobial activity

Semiconductor-based photocatalytic techniques provide a durable and environmentally benign route to disinfect microbes. However, their disinfection activities are often limited by inadequate use of photocarriers (e− and h+), particularly for narrow bandgap semiconductors. Here, we directly grow narrow bandgap semiconductor MgIn2S4 onto In2O3 to fabricate MgIn2S4@In2O3 hierarchical tubular heterostructures. Using the model microbes Escherichia coli bacteria, we demonstrate that MgIn2S4@In2O3 can achieve an exceptionally high antimicrobial activity (7 log reduction in viable cells count for 30 min illumination) although MgIn2S4 and In2O3 alone are almost inactive. Owing to the intimate contact between MgIn2S4 and In2O3, MgIn2S4@In2O3 enables efficient photocarrier separation which supports continuous production of copious reactive species (i.e. h+ and O2−) for disinfection. Numerical simulation indicates that photocarriers are spatially well-separated in MgIn2S4@In2O3 due to the type-II heterojunction formed. These findings signify the possibility to achieve high antimicrobial activity from otherwise almost inactive semiconductors by properly constructing the heterogeneous interfaces.

Tailoring band gaps of KBiO3-NaBiO3 heterostructures by the interface actions for enhancing visible light photoelectric response

Heterostructures with wide and narrow band gap semiconductors may harvest short and long wavelength light for enhancing photoelectric response. Here we adopt narrower-band-gap KBiO 3 to combine with wider-band-gap NaBiO 3 for KBiO 3 /NaBiO 3 heterostructures by adding KOH into NaBiO 3 . The band gaps of the heterostructures change with the ratios of KOH:NaBiO 3 in raw materials and are abnormally narrower than that of KBiO 3 . The interfaces between KBiO 3 and NaBiO 3 change with their ratios and then their interactions vary. The width and intensity of surface photovoltage spectra of the heterostructures increase notably in the visible light region, which results from prolonged lifetime of photogenerated electrons transferring from KBiO 3 to NaBiO 3 . • KBiO 3 /NaBiO 3 heterostructures are constructed with different ratios. • The band gaps of KBiO 3 /NaBiO 3 heterostructures are tailored by their interactions. • The SPV range of NaBiO 3 is expanded from 475 to 600 nm by the heterostructures.

Conductivity of Two-Dimensional Small Gap Semiconductors and Topological Insulators in Strong Coulomb Disorder

We are honored to dedicate this article to Emmanuel Rashba on the occasion of his 95 birthday. In the ideal disorder-free situation, a two-dimensional band gap insulator has an activation energy for conductivity equal to half the band gap, $\Delta$. But transport experiments usually exhibit a much smaller activation energy at low temperature, and the relation between this activation energy and $\Delta$ is unclear. Here we consider the temperature-dependent conductivity of a two-dimensional narrow gap semiconductor on a substrate containing Coulomb impurities, mostly focusing on the case when amplitude of the random potential $\Gamma \gg \Delta$. We show that the conductivity generically exhibits three regimes and only the highest temperature regime exhibits an activation energy that reflects the band gap. At lower temperatures, the conduction proceeds through nearest-neighbor or variable-range hopping between electron and hole puddles created by the disorder. We show that the activation energy and characteristic temperature associated with these processes steeply collapse near a critical impurity concentration. Larger concentrations lead to an exponentially small activation energy and exponentially long localization length, which in mesoscopic samples can appear as a disorder-induced insulator-to-metal transition. We arrive at a similar disorder driven steep insulator-metal transition in thin films of three-dimensional topological insulators with very large dielectric constant, where due to confinement of electric field internal Coulomb impurities create larger disorder potential. Away from neutrality point this unconventional insulator-to-metal transition is augmented by conventional metal-insulator transition at small impurity concentrations, so that we arrive at disorder-driven re-entrant metal-insulator-metal transition.

Ab initio investigation of electronic structure and topological features in ErNiSb and ErSb compounds

This manuscript is devoted to the electronic structure, as well as magnetic properties, investigations of the ternary ErNiSb and binary ErSb intermetallic compounds using GGA+U within so called ab initio approach. Our detailed study of the band structure obtained with accounting for electronic correlations in the narrow-gap semiconductor ErNiSb and semimetal ErSb is performed in order to identify topological features.

125Te NMR study of the bulk of topological insulators Bi2Te3 and Sb2Te3

AbstractThe narrow band gap semiconductors Bi2Te3 and Sb2Te3 are well known for their room temperature thermoelectric performance. Recently, they were shown to serve as model systems of three‐dimensional topological insulators with a bulk band gap and very robust, spin‐resolved surface states due to a special band inversion in their periodic bulk. Evidently, it is of interest to investigate the special surface states with a local probe like nuclear magnetic resonance (NMR). However, especially the bulk NMR of these materials shows peculiar and rather unexpected phenomena, e. g., a magnetic field induced deformation of the local charge symmetry and excessive line broadening due to a special internuclear coupling. Here we report a comprehensive account of the room temperature NMR properties of the spin‐1/2 nucleus 125Te of single crystalline Bi2Te3 and Sb2Te3. This includes a very unusual spin‐lattice relaxation that seems not to be reflected in the NMR shift, as well as an exchange enhanced NMR line broadening.