Narrow-gap Semiconductors Research Articles

Ce2S3/PMo3W9/polypyrrole ternary nanocomposit: Facile synthesis, photoelectric characteristics and photocatalytic applications

A ternary nanocomposite Ce 2 S/PMo 3 W 9 /PPy were prepared to exhibit superior photocurrent responses, and performance improvement in photocatalytic Cr(VI) reduction was also demonstrated. • A ternary nanocomposite Ce 2 S/PMo 3 W 9 /PPy were synthesized. • Polyoxometalate PMo 3 W 9 is used as the electron acceptor. • PPy can prevent Ce 2 S 3 from photocorrosion. • Performance improvement in photocatalytic Cr(VI) reduction was demonstrated. Inorganic solid material Ce 2 S 3 of narrow band gap semiconductor enables an ideal visible light photocatalyst, but its disadvantages of both the photocorrosion and the faster electron-hole recombination substantially restrict its photocatalytic performance. In this work, we prepared a ternary nanocomposite Ce 2 S 3 /PMo 3 W 9 /PPy (PPy = polypyrrole), in which Ce 2 S 3 is used as the visible-light-active photocatalyst and polyoxometalate PMo 3 W 9 is used as the electron acceptor to facilitate electron-hole separation, and PPy can improve the electrical conductivity of the composite and prevent Ce 2 S 3 from photocorrosion. The photoelectric properties of Ce 2 S 3 /PMo 3 W 9 /PPy nanocomposite were experimentally characterized by the measurements of transient photocurrent responses, electrochemical impedance spectroscopy and surface photovoltage spectra. Furthermore, we investigate on photocatalytic performances of Ce 2 S 3 , Ce 2 S 3 /PMo 3 W 9 and Ce 2 S 3 /PMo 3 W 9 /PPy for reduction of Cr(VI), and Ce 2 S 3 /PMo 3 W 9 /PPy exhibited higher photocatalytic performance than the pure Ce 2 S 3 due to the energy level matching, achieving 28% increase in photocatalytic activity of Cr(VI) reduction.

Charge-Transfer Complexes of Linear and Ladder-Type Conjugated Polymers as Promising Organic Narrow-Gap Semiconductors

Charge-Transfer Complexes of Linear and Ladder-Type Conjugated Polymers as Promising Organic Narrow-Gap Semiconductors

Phenyl- and naphthyl-type heteroatom substitution blocks in naphthylene-[formula omitted]: A DFT study

Two-dimensional naphthylene-γ is a recently proposed carbon allotrope based on the concatenation of phenyl- and naphthyl-like blocks through tetragonal rings. This system is a narrow-gap semiconductor, which is also suitable to be cast into a boron-nitride configuration since it is a bipartite structure featuring only even-membered rings. It turns out BN configurations of naphthylene-γ can potentially result in systems suitable to be embedded into nanoelectronics applications, since they are expected to exhibit robust band gaps. With this motivation, we investigate the electronic properties of several hybrid systems hypothetically obtained by replacing part of naphthylene-γ’s C atoms by BN sectors with different concentrations. Our first principles simulations are based on density functional theory and show that the energy gap for these systems does not vary monotonically with increasing BN concentration. In fact, strategical choices for the sites undergoing C-to-BN substitution result in specific modifications in the electronic signature of the systems. We also investigated full-BN naphthylene-γ sheets which show band gaps significantly different from those of hexagonal-BN. We further investigated nanoribbons based on these BN sheets, showing that they closely follow the properties of their parent 2D structure, as they feature frontier states internally located over the ribbon structures.

Two-dimensional polymerized carbon nitride coupled with (0 0 1)-facets-exposed titanium dioxide S-scheme heterojunction for photocatalytic degradation of norfloxacin

Poor visible light response and low separation efficiency of photogenerated carriers restricted the availability of titanium dioxide for antibiotic photodegradation, and the fabrication of two-dimensional/two-dimensional (2D/2D) heterojunction by coupling to a narrower bandgap semiconductor was an effective approach. Herein, a simple solvothermal method was used to grow 2D (001)-facets-exposed titanium dioxide ((001)-TiO2) nanosheets in situ on the surface of 2D polymeric carbon nitride (PCN) nanosheets to obtain PCN/(001)-TiO2 heterojunction for the photodegradation of norfloxacin. The synthesized heterojunction composites exhibited superior photodegradation performance, and the norfloxacin degradation rate reached up to 99% under simulated solar for 90 min. The efficient photocatalytic performance was attributed to the reduction of the bandgap and the formation of S-scheme heterojunctions, which enhanced the absorption of visible light and the carrier migration separation efficiency. Futhermore, the hierarchical 2D/2D nanosheet structure and the built-in electric field formed also sped up the charge transfer at the interface. This work presented a new viable solution to the problem of antibiotic contamination.

Supramolecular Diiodine-Bromostannate(IV) Complexes: Narrow Bandgap Semiconductors

Three supramolecular bromostannates(IV) with “trapped” diiodine molecules, Cat2{[SnBr6](I2)} (Cat = Me4N+ (1), 1-MePy+ (2) and 4-MePyH (3)), were synthesized. In all cases, I2 linkers are connected with bromide ligands via halogen···halogen non-covalent interactions. Articles 1–3 were studied using Raman spectroscopy, thermogravimetric analysis, and diffuse reflectance spectroscopy. The latter indicates that 1–3 are narrow band gap semiconductors.

First-principles study on the electronic structure, magnetic and optical properties of strain regulated (V, Cr) co-doped SnSe2

The effects of strain and doped on the magnetic and optical properties of SnSe2 were studied by density functional theory. The results shown that intrinsic SnSe2 is a narrow indirect band gap semiconductor (0.781 eV). And shown that the ferromagnetism is enhanced in tensile strain, in which the ΔE increases nearly three times with the X6%, the ΔE increases nearly two times with the Y4% and the ΔE increases nearly two times with the B2%. Therefore, the effect of tensile strain on the ferromagnetism of SnSe2 is more obvious. In terms of charge transfer, the amount of charge transfer under tensile strain is more than that under compressive strain. More interestingly, with the regulations of doping and strain, the absorption spectrum in the visible part is weakened, accompanied by phenomena of the red shift. The paper provides a feasible scheme for regulating the ferromagnetic state of SnSe2 and developing spin electronics and optoelectronic devices.

Deciphering the Role of Substitution in Transition-Metal Phosphorous Trisulfide (100) Surface: A Highly Efficient and Durable Pt-free ORR Electrocatalyst.

The rational design and development of earth-abundant, cost-effective, environmentally benign, and highly robust oxygen reduction reaction (ORR) electrocatalysts can circumvent the obstacles associated with the large-scale commercialization of fuel cells. Here, using first-principles-based density functional theory (DFT), we have computationally screened the potential and feasibility of transition-metal phosphorous trisulfides (TMPS3 ) (100) surfaces as efficient ORR electrocatalyst in acidic fuel cell application. MnPS3 (100) surface emerges to be the best among TMPS3 surfaces with optimal O2 activation resulting in very low overpotential. The study reveals that ORR occurs on the MnPS3 surface via 4e reduction associative pathway where the kinetically rate-determining step (RDS) is the formation of O*+H2 O with an activation barrier of 0.66 eV. Additionally, high CO tolerance and easy desorption of H2 O make MnPS3 a robust catalyst. Substitution in half of the Mn sites of MnPS3 (100) surface with Co considerably enhances the ORR activity. Mn0.5 Co0.5 PS3 (100) surface exhibits an ultralow overpotential of 0.39 V vs RHE switching ORR pathway from associative to dissociative. Spontaneous dissociation of H2 O2 on Mn0.5 Co0.5 PS3 proves 4e reduction pathway excluding 2e one. Electronic structure analysis reveals that pristine MnPS3 (100) surface is a narrow band gap semiconductor which upon Co substitution transforms into a conducting metallic surface enhancing ORR activity. Besides, Mn0.5 Co0.5 PS3 (100) surface obtains the apex of the volcano plot due to its optimal position of the d-band center which further justifies the improved ORR activity. With Pt-like onset potential, facile H2 O desorption ability, and robust dynamic and thermal stability, this CO tolerant Mn0.5 Co0.5 PS3 catalyst can be a potential alternative to Pt with encouraging practical viability.

Metal-Rich Phosphides Obtained from the Lead Flux: Synthesis, Crystal, and Electronic Structure of Sr5Pt12P9 and BaPt3P2.

Using a high-temperature ampoule technique and lead metal as a flux, we have grown single crystals and determined crystal structures from single-crystal X-ray diffraction data of two metal-rich phosphides, Sr5Pt12P9 (P 21/m, a = 6.1472(3) Å, b = 25.1713(13) Å, c = 6.4635(3) Å, β = 99.604(2)°, Z = 2, R1 = 0.0326, wR2 = 0.0786) and BaPt3P2 (P 212121, a = 6.3605(6) Å, b = 6.8541(7) Å, c = 11.3493(12) Å, Z = 4, R1 = 0.0231, wR2 = 0.0501). Both compounds belong to their own structure types and feature 3D networks of Pt and P atoms, with the channels occupied by alkaline earth metal cations. Density functional theory calculations reveal Sr5Pt12P9 to be a metal, while BaPt3P2 is a narrow-gap semiconductor with a band gap of 0.24 eV. Bonding analysis shows that both compounds feature networks of prominent covalent localized Pt-P bonds, responsible for their structural stability, as well as additional weaker and, likely, less localized Pt-Pt interactions.

Electron transport properties of a narrow-bandgap semiconductor Bi2O2Te nanosheet

A thin, narrow-bandgap semiconductor Bi2O2Te nanosheet is obtained via mechanical exfoliation, and a Hall-bar device is fabricated from it on a heavily doped Si/SiO2 substrate and studied at low temperatures. Gate transfer characteristic measurements show that the transport carriers in the nanosheet are of n-type. The carrier density, mobility, and mean free path in the nanosheet are determined by measurements of the Hall resistance and the longitudinal resistance of the Hall-bar device, and it is found that the electron transport in the nanosheet is in a quasi-two-dimensional (2D), strongly disordered regime. Magnetotransport measurements for the device at magnetic fields applied perpendicular to the nanosheet plane show dominantly weak antilocalization (WAL) characteristics at low fields and a linear magnetoresistance (LMR) behavior at high fields. We attribute the WAL characteristics to strong spin–orbit interaction (SOI) and the LMR to the classical origin of strong disorder in the nanosheet. Low-field magnetoconductivity measurements are also performed and are analyzed based on the multi-channel Hikami–Larkin–Nagaoka theory with the LMR correction being taken into account. The phase coherence length, spin relaxation length, effective 2D conduction channel number, and coefficient in the linear term due to the LMR in the nanosheet are extracted. It is found that the spin relaxation length in the Bi2O2Te nanosheet is several times smaller than that in its counterpart Bi2O2Se nanosheet, and thus, an ultra-strong SOI is present in the Bi2O2Te nanosheet. Our results reported in this study would greatly encourage further studies and applications of this emerging narrow-bandgap semiconductor 2D material.

Mixed Anion Semiconductor In8S2.82Te6.18(Te2)3.

The new heteroanionic compound In8S2.82Te6.18(Te2)3 crystallizes in the monoclinic space group C2/c with lattice parameters a = 14.2940(6) Å, b = 14.3092(4) Å, c = 14.1552(6) Å, and β = 90.845(3)°. The three-dimensional (3D) framework of In8S2.82Te6.18(Te2)3 is composed of a complex 3D network of corner-connected InQ4 tetrahedra with three Te22- dumbbell dimers per formula unit. The optical bandgap is 1.12(2) eV and the work function is 5.15(5) eV. First-principles electronic structure calculations using density functional theory (DFT) indicate that this material has potential as a p-type thermoelectric material as it is a narrow bandgap semiconductor, incorporates several heavy elements, and has multiple overlapping bands near the valence band maximum.

CdS-Decorated Porous Anodic SnOx Photoanodes with Enhanced Performance under Visible Light

Electrochemically generated nanoporous tin oxide films have already been studied as photoanodes in photoelectrochemical water splitting systems. However, up to now, the most significant drawback of such materials was their relatively wide band gap (ca. 3.0 eV), which limits their effective performance in the UV light range. Therefore, here, we present for the first time an effective strategy for sensitization of porous anodic SnOx films with another narrow band gap semiconductor. Nanoporous tin oxide layers were obtained by simple one-step anodic oxidation of metallic Sn in 1 M NaOH followed by further surface decoration with CdS by the successive ionic layer adsorption and reaction (SILAR) method. It was found that the nanoporous morphology of as-anodized SnOx is still preserved after CdS deposition. Such SnOx/CdS photoanodes exhibited enhanced photoelectrochemical activity in the visible range compared to unmodified SnOx. However, the thermal treatment at 200 °C before the SILAR process was found to be a key factor responsible for the optimal photoresponse of the material.

Ultrasensitive photoelectrochemical immunosensor based on Dual-Photosensitive electrodes

In the study, a photoelectrochemical (PEC) immunosensor based on dual-photosensitive electrodes was developed for cardiac troponin I (cTnI) detection. The sensing photocathode with biometric functions was prepared by CuInS2 and narrow band gap semiconductor In2S3 as the counter electrode. In this way, the separation of photoanode and biometric events was realized, and the ability of stability of the immunosensor could be effectively improved. Moreover, the attraction to the photogenerated electrons (e-) from photoanode would be increased by the abundant holes (h+) of photocathode, under the radiation of light. This tremendously improves the photoelectric response, which further improves the sensitivity of the immunosensor. The controllable-synthesis uncomplicated photoelectric material not only accords with the principle of simplicity of electrode modification but also makes the immunosensor more conducive to the practical application. Additionally, even in the case of zero bias voltage, the constructed PEC immunosensor can operate with high efficiency, namely, self-powered. The immunosensor could provide the quantitative readout photocurrent to a concentration of cTnI in the range of 0.10 pg/mL to 1.00 μg/mL and the detection limit was 0.0113 pg/mL under the optimal experimental conditions. With favorable performance in terms of anti-interference, stability, specificity and reproducibility, this immunosensor will provide new prospects for general PEC bioanalysis development.

Optoelectronic, elastic and thermoelectric properties of the perovskites (Sr3N)Sb and (Sr3N)Bi

Structural, optoelectronic, elastic and thermoelectric properties of (Sr3N)Sb and (Sr3N)Bi perovskites are investigated by density functional theory. The calculated structural properties and geometry are found consistent with experiments. The electronic band profiles show that these perovskites are narrow and direct bandgap semiconductors with bandgap of 0.99 and 1.0 eV respectively. Optical properties show that both compounds are optically active and suitable for optoelectronic devices in the infrared region of the electromagnetic spectrum. Elastic properties demonstrate that both compounds are mechanically stable, stiffer, brittle and mechanically anisotropic. Seebeck coefficient and effective masses reveal the p-type semiconductor nature of both compounds. Thermoelectric properties; thermal conductivity and power factor demonstrate that these perovskites could be used as potential candidates for thermoelectric devices.

Heteroepitaxial and homoepitaxial nucleation strategies to grow Sb2S3 nanorod arrays and therefrom a derived gain of 7.18%-efficient Sb2(S,Se)3 quasi-nanoarray heterojunction solar cells

Narrow-bandgap semiconductor nanoarrays are the promising photon-harvesters for photovoltaics because of their unique optoelectronic properties. To prepare the solution-processed nanoarrays of antimony sulfide (Sb2S3) and antimony selenosulfide (Sb2(S,Se)3) that are of a great potential in photovoltaics are still challenging. Here, a tiny-Sb2Se3-seed assisted solution-processing method for growing Sb2S3 nanorod array (NA) on non-epitaxial-for-seed-growth substrates is developed for the first time, complementing the tiny-Sb2S3-seed-derived Sb2S3-NA growth on TiO2 nanoparticle film that is actually epitaxial for the Sb2S3 seed growth. The orientation-competing-epitaxial nucleation/growth mechanisms based on the heteroepitaxial and homoepitaxial nucleation effects of randomly formed tiny seeds are proposed for, respectively, understanding the tiny-Sb2Se3-seed-derived Sb2S3-NA growth and the unavailable tiny-Sb2S3-seed-derived Sb2S3-NA or tiny-Sb2Se3-seed-derived Sb2S3-NA growth on non-epitaxial-for-seed-growth substrates. Moreover, the homoepitaxial nucleation effect of epitaxially formed tiny seeds is specified to the tiny-Sb2S3-seed-derived Sb2S3-NA growth on TiO2 nanoparticle film. Selenization of the tiny-Sb2Se3-seed-derived Sb2S3/TiO2 nanoarray heterojunction (Sb2S3/TiO2-NHJ) renders a novel Sb2(S,Se)3/TiO2 quasi-nanoarray heterojunction (Sb2(S,Se)3/TiO2-quasi-NHJ), and a champion power conversion efficiency of 7.18% is achieved in the Sb2(S,Se)3/TiO2-quasi-NHJ solar cells with an optimized S/Se atomic ratio of ca. 1/1 obtained by the selenization at 300 °C. This paper not only demonstrates the facile solution-processing methods to prepare quality NHJ and quasi-NHJ systems for optoelectronic applications, but also is expected to guide the conceptual design for semiconductor nanorod array growth.

Highly Sensitive Mid-Infrared Photodetector Enabled by Plasmonic Hot Carriers in the First Atmospheric Window

The first atmospheric window of 3–5 μm in the mid-infrared (MIR) spectral range pertains to crucial application fields, with particular scientific and technological importance. However, conventional narrow-bandgap semiconductors operating at this band, represented by mercury cadmium telluride and indium antimonide, suffer from limited specific detectivity at room temperature and hindered optoelectronic integration. In this study, a plasmonic hot electron-empowered MIR photodetector based on Al-doped ZnO (AZO)/bi-layer graphene heterostructure is demonstrated. Free electrons oscillate coherently in AZO disk arrays, resulting in strong localized surface plasmon resonance (LSPR) in the MIR region. The photoelectric conversion efficiency at 3–5 μm is significantly improved due to plasmon-induced hot-electron extraction and LSPR-enhanced light absorption. The specific detectivity reaches about 1.4 × 1011 Jones and responsivity is up to 4712.3 A/W at wavelength of 3 μm at room temperature. The device’s specific detectivity is among the highest performance of commercial state-of-the-art photodetectors and superior to most of the other 2D materials based photodetectors in the MIR region. These results demonstrate that a plasmonic heavily doped metal oxides/2D material heterostructure is a suitable architecture for constructing highly sensitive room-temperature MIR photodetectors.

Synthesis of Hydride-Doped Perovskite Stannate with Visible Light Absorption Capability.

Narrow-gap semiconductors with visible light absorption capability have attracted attention as photofunctional materials. H--doped BaSn0.7Y0.3O3-δ containing Sn(II) species was recently reported to absorb visible light up to 600 nm, which represents the first demonstration of oxyhydride-based visible-light-absorbers. In the present study, a more detailed investigation was made to obtain information on the synthesis and properties of H--doped perovskite-type stannate with respect to the A-site cation of the material and the preparation conditions. H--doped ASn0.7Y0.3O3-δ (A = Ba, Ba0.5Sr0.5, and Sr) obtained by the reaction of ASn0.7Y0.3O3-δ precursors with CaH2 at 773 K under vacuum conditions was shown to have almost the same bandgap (ca. 2.1 eV), regardless of the A-site cation. Physicochemical measurements and theoretical calculations revealed that the identical bandgaps of H--doped ASn0.7Y0.3O3-δ are due to the simultaneous shift of the midgap states composed of Sn2+ with the conduction band minimum. Experimental results also indicated that the appropriate preparation conditions with respect to Y3+-substitution and the temperature for the synthesis of the ASn0.7Y0.3O3-δ precursors were essential to obtain H--doped products that have a low density of defects.

Harvesting the infrared part of solar light to promote charge transfer in Bi2S3/WO3 photoanode for enhanced photoelectrochemical water splitting

Infrared light absorbed by semiconductors hardly contributes to the solar energy conversion due to its low photon energy. Herein, photothermal effect activated by infrared part of solar light is introduced to promote the photoelectrochemical (PEC) water splitting of photoanodes. Narrow band-gap semiconductor Bi2S3 is deposited on the surface of WO3 nanosheets, exhibiting a broad-spectral response. In addition to the enhanced density of photo-generated electrons, significant temperature elevation is observed for the Bi2S3/WO3 composite photoanode under the illumination of infrared part of solar light because of the photothermal conversion property of Bi2S3. The moderately enhanced temperature accelerates charge carrier migration and finally increases the efficiency of solar energy conversion. With the assistance of photothermal effect, a remarkable photocurrent density of 4.05 mA cm−2 at 1.23 V vs. reversible reference electrode (VRHE) is achieved by Bi2S3/WO3 composite photoanode, over 880% higher than that of the pristine WO3. The introduction of photothermal effect activated by infrared light provides general and robust strategy to promote the PEC performance of photoanodes.

Band Structure and Lattice Vibration of Elemental Tellurium Investigated by Temperature‐Dependent Mid‐and‐Far Infrared Transmission and Raman Spectroscopy

Tellurium (Te) recently attracts much attention owing to its peculiar band structure arousing high‐efficiency thermoelectricity and layered crystal structure suitable for band engineering. Herein, the temperature evolution of band structure and lattice vibration in Te single crystal are studied by mid‐and‐far infrared transmission (7500–370 and 680–30 cm−1) and Raman spectra measurements. Mid‐infrared absorption spectra confirm Te is a narrow indirect gap semiconductor, in which the direct (indirect) bandgap is about 0.333 (0.325) eV, the two upper valence bands (H4 and H5) and the third valence band (H6) being about 0.333, 0.42, and 0.76 eV, respectively, below the conduction‐band minima at 5 K. Meanwhile, the direct and indirect gaps of Te reveal non‐monotonic variation from 5 to 290 K due to the competition between electron–phonon interaction and lattice thermal expansion, which is well described by the modified Manoogian–Leclerc model. Moreover, the temperature evolution of Raman and far‐infrared transmission spectra proves there are two kinds of phonons in Te, which originates from the different thermal expansion of Te atoms along parallel and vertical directions of spiral chain. Thus, the anisotropic thermal expansion in Te is indicated and a reference for studying thermoelectricity of Te‐based optoelectronic devices is provided.

Exploring the photovoltaic capabilities of Sc4C3 MXene using density functional theory

The purpose of this study is to explore the structural and optoelectronic properties of a two-dimensional MXene, Sc4C3 as a viable choice for charge transport layers and absorber layer additive in emerging photovoltaic technologies such as perovskite solar cells (PSCs), employing the principles of density functional theory (DFT) using the WIEN2k tool. The optimized lattice constant of the material and the volume for which the energy is minimum are estimated as 13.66 bohr and 2548.66 bohr3 respectively. The estimation of elastic properties, viz. the bulk modulus, shear modulus and Poisson’s ratio as 115 GPa, 71 GPa and 0.25 respectively, reveals that the material is resistant to deformation. The analysis of electronic properties and band structure indicates that Sc4C3 MXene has a direct bandgap of 0.747 eV at the Г point, and hence can be considered as a narrow bandgap semiconductor material. The study shows that the material possesses optimal light absorption properties with an absorption coefficient, dielectric permittivity and refractive index of 1.84 × 102 cm−1, 19.02 and 4.42 respectively, which are suitable for photovoltaic applications.

Atomic insights into the influence of Bi doping on the optical properties of two-dimensional van der Waals layered InSe

As a narrow-gap semiconductor, III–VI two-dimensional (2D) van der Waals layered indium selenide (InSe) has attracted a lot of attention due to excellent physical properties. For potential optoelectronic applications, the tunability of the optical property is challenging, e.g., the modulation of optical bandgap commonly by element doping. However, the deep understanding of the influence of element doping on the microstructure and the optical properties lacks of systematic investigation. In this work, by using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we investigate the influence of Bi doping on controlling of the microstructure and optical properties of InSe single crystal in detail. The results show that Bi doping can introduce additional stacking faults in InSe single crystal, and more importantly, the atomic spacing and lattice constant of Bi-doped InSe are changed a lot as compared to that of the undoped one. Further optical characterizations including photoluminescence and transmission spectra reveal that Bi-doping can broaden the transmission wavelength range of InSe and make its optical bandgap blue-shift, which can also be physically interpreted from the doping-induced structure change. Our work expands new ideas for the optical property modulation of 2D thin-layer materials and brings new possibilities for the development of thin-layer InSe optical devices.