Narrow-gap Semiconductors Research Articles

Ferroelectric polarization effect on the photocatalytic activity of Bi0.9Ca0.1FeO3/CdS S-scheme nanocomposites

BiFeO3 (BFO), as a kind of narrow band-gap semiconductor material, has gradually emerged advantages in the application of photocatalysis. In this paper, Ca doped BFO nanoparticles Bi0.9Ca0.1FeO3 (BCFO) were prepared by sol-gel method. And BCFO and CdS nanocomposites with two morphologies were obtained by controlling the time of loading CdS under a low temperature liquid phase process. It is found that the band gap becomes narrower after doping Ca into BFO, which is conducive to the absorption of visible light. Among all the samples, the composite of CdS nanowires and BCFO nanoparticles obtained by reaction time of 10 min has the best photocatalytic performance. The degradation rate of Methyl Orange solution was 94% after 90 min under visible light irradiation, which was much higher than that of pure BCFO and CdS. Furthermore, significant enhancement in the degradation rate (100% degradation in 60 min) can be achieved in poled samples after electric polarization process. The highest degradation rate is due to the promoted separation of photogenerated carriers induced by the internal polarization field and the formation of S-scheme heterostructure between BCFO and CdS. Such BCFO-CdS nanocomposites may bring new insights into designing highly efficient photocatalyst.

Synthesis, physical and electrochemical properties of the spinel CoFe2O4: Application to the photocatalytic hydrogen production

This research is achieved for the preparation of CoFe 2 O 4 with a high purity and crystanillity by nitrate route followed by a calcination at 850 °C. The material exhibits a single phase with a cubic symmetry. The electrochemistry performed in Na 2 SO 4 solution shows a stable semiconductor with a small exchange current density (112 μA cm −2 ). The capacitance-potential measurement indicates p -type behavior, typically related to the metal insertion in octahedral sites. The narrow band gap semiconductor with a direct optical transition of 1.27 eV is due to the lifting of degeneracy of Fe 3+ : 3d orbital in 6-fold coordination, making it attractive for the photo-electrochemical (PEC) conversion. Moreover, its cathodic comportment evidenced from the negative conduction band compared to the potential of the water reduction makes it an ideal candidate for the hydrogen production. The experimental data shows a great activity for the evolution of hydrogen with an average rate of 74 μmol min −1 g cat −1 and a quantum efficiency of 0.34% under visible illumination (23 mW cm −2 ). The use of C 2 O 4 2− as reducing agent increased substantially the performances by a factor of 57%. • The spinel CoFe 2 O 4 prepared by nitrate route is a narrow band gap semiconductor with a crystallite size of 27 nm. • CoFe 2 O 4 with an optical band gap of 1.27 eV is sensitive over the whole visible region. • The oxalate C 2 O 4 2− acting as sacrificial agent enhances the H 2 production in NaOH electrolyte. • The conduction band (−0.5 V SCE ) derived from Fe 3+ : e g orbital, is more cathodic than the H 2 O/H 2 level (∼-0.4 V SCE ). • H 2 evolution rate of 74 μmol min −1 (g catalyst) −1 and quantum yield of 0.34% were obtained under visible light (23 mW cm −2 ).

Achieving a high dielectric constant and low dielectric loss of polymer composites filled with an interface-bonded g-C3N4@PbS narrow-bandgap semiconductor

Scattering medium-bandgap semiconductors into polymers cannot remarkably improve the dielectric constant in composites. Nanoscale ceramic leads to a high interface leakage current. To obtain a high dielectric constant and low loss, in this work, graphitic carbon nitride (g-C3N4)@PbS microparticles were prepared for fabricating polymer-based ternary composites. Polymer/PbS binary composites were also fabricated. In terms of electric properties, ternary composites show more advantages than binary composites. In ternary composites, the improved dielectric constant is ascribed to SN bonding at the g-C3N4/PbS interface. Decreased dielectric loss is rooted in the reduced interface leakage current (from the large size of the hybrid filler) between the polymer and PbS. Under high fields, the reduced breakdown strength is attributed to the reduced bandgap in the hybrid filler compared with PbS and g-C3N4. The innovation of this work is the high-efficiency collaboration between microsized and interfacial SN bonding in g-C3N4@PbS filler. The ternary composite with 15 wt% hybrid filler has a high dielectric constant of ~ 39 and a low dielectric loss of ~ 0.04 at 100 Hz (low field) as well as a high breakdown strength of ~ 202 MV m−1 under a direct-current field. This work might enlighten the fabrication of polymer composites with great promise in energy storage.

Two-dimensional layered architecture constructing energy and phonon blocks for enhancing thermoelectric performance of InSb

InSb is a narrow-bandgap semiconductor with a zinc blende structure and has been wildly applied in photodetectors, infrared thermal imaging, and Hall devices. The facts of decent band structure, ultrahigh electron mobility, and nontoxic nature indicate that InSb may be a potential mid-temperature thermoelectric material. The critical challenges of InSb, such as high thermal conductivity and small Seebeck coefficient, have induced its ultrahigh lattice thermal conductivity, and thus low ZT values. In view of this, we have developed a competitive strategy typified by the cost-efficient nanocompositing of z wt% QSe2 (Q = Sn, W). Specifically, the QIn+ and SeSb+ point defects were introduced in the InSb system by nanocompositing the vested two-dimensional layered QSe2. In addition, the enlarged valence band maximum of intrinsic WSe2 acted as ladders can scatter a fair number of hole carriers, resulting in the relatively enhanced Seebeck coefficient of high temperature. Moreover, the disorderly distributed nanosheets/particles, and dislocations acting as obstacles can effectively delay the heat flow diffusion, inducing the strong scattering of thermal phonons. Consequently, an enhanced power factor of ∼33.3 µW cm−1 K−2 and ZT value of ∼0.82 at 733 K have been achieved in the 3% WSe2 sample, companied with the engineering output power density ωmax ∼233 µW cm−2 and thermoelectric conversion efficiency η ∼5.2%. This artificially designed approach indicated by suited nanocompositing can integrate several engineering strategies such as point defects, nanoengineering, and energy filtering into one, providing a reference to optimize the thermoelectric performance of other thermoelectric systems.

A novel two-step strategy to fabricate phase-pure SnS photoelectrodes with tunable conductivity: formation mechanism and photoelectrochemical properties

Tin monosulfide (SnS) is a narrow band gap semiconductor for visible-light harvesting, however the easy formation of secondary phases such as Sn2S3 and SnS2 severely restricts its photoelectrochemical (PEC) properties. Herein, we propose a novel two-step strategy to fabricate phase-pure SnS photoelectrodes with tunable conductivity on Ti foil substrate and carefully investigated the formation mechanism and PEC properties. The tunable conductivity is determined by Na2SO4 pretreatment before annealing, which is supported by energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, and electron paramagnetic resonance characterizations. Na+ adsorbed to the edge of the precursor SnS2 nanosheets forming a dangling bond adsorption will protect S2− against reaction with the trace oxygen in the chemical vapor deposition system within a certain temperature range (<525 °C), thereby reducing the generation of S vacancies to adjust the S/Sn ratio and further regulate the conductivity type. Moreover, the anodic photocurrent density of SnS thin films was about 0.32 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE) with the separation and injection efficiency of 1.22% and 72.78% and a maximum cathodic photocurrent density reaching approximately −0.36 mA cm−2 at 0 V vs RHE with the separation and injection efficiency 1.15% and 5.44% respectively. The method shown in this work provides an effective approach to control the electrical conductivity of SnS thin films with considerable photocurrent response for phase-pure SnS.

Wavelength dependent photoinduced charge carrier dynamics of heterojunction materials: The case of CuO/ZnO

Heterojunction materials composed of narrow and wide band-gap semiconductors possess different photoelectric responses under different wavelength incident light, and show distinct photogenerated charge carrier dynamics. In this work, taking the hydrothermal method fabricated CuO nanosheets / ZnO nanorods heterojunction materials as example, the wavelength dependent photoinduced charge carrier dynamics in this heterojunction material have been carefully studied. Transient surface photovoltage (TSPV) measurements reveal that the photogenerated holes gather at the surface of the fabricated CuO / ZnO with 355 nm light illumination, but photogenerated electrons gather at the surface under 532 nm light. Photocatalytic degradation experiments suggest that CuO / ZnO heterojunction material has better performance under ultraviolet light than that under visible light.

Direct Z-Scheme Oxygen-vacancy-rich TiO2/Ta3N5 heterojunction for degradation of ciprofloxacin under visible light: Degradation pathways and mechanism insight

An oxygen-vacancy-rich TiO2/Ta3N5 (Vo-TiO2/Ta3N5) composite material was constructed via a solvothermal method, and the photocatalytic degradation of ciprofloxacin (CIP) performance was investigated in-depth. The solvothermal synthesis method introduces oxygen vacancies in TiO2, and successfully combines Vo-TiO2 and Ta3N5 to form a Z-Scheme structure. As a narrow bandgap semiconductor, Ta3N5 increases the absorption of the catalyst in the visible light region. Oxygen vacancies form an intermediate energy level in TiO2, which promotes the separation of photogenerated electrons and holes. In addition, the Z-Scheme energy band structure formed by Vo-TiO2 and Ta3N5 enables more photogenerated carriers to participate in the photocatalytic reaction, so that the Vo-TiO2/Ta3N5 heterojunction structure has excellent photocatalytic degradation of ciprofloxacin (CIP) performance. Furthermore, Vo-TiO2/Ta3N5 also can degrade rhodamine B, Methylene Blue and kill Staphylococcus aureus. It demonstrates that the sample is a multifunctional catalyst. This research provides a basis for the development of new nanocomposite photocatalysts by combining defect engineering and heterostructure construction of two modified semiconductor methods.

Helical conducting edge states in narrow-gap semiconductors without band inversion

Band inversion accompanied by the emergence of gapless and helical edge states at the boundary is a hallmark of 2D topological insulators. However, a recent experiment reports the existence of edge states before band inversion in InAs/GaSb quantum wells [F. Nichele et al., New J. Phys. 18, 083005 (2016)]. The underlying physics remains an open problem. Here we provide a possible solution by showing that helical edge states emerge before band inversion as long as the bulk gap falls below a positive threshold determined by the particle-hole asymmetry and interband coupling. In the presence of quantum confinement or local electrostatic potential near the boundary, these edge states may enter the gap of bulk states and be detected as edge conductance in transport measurements. Our work reveals the possible existence of the helical conducting edge states in a large class of narrow-gap semiconductors.

Electronic structure, optical, thermoelectric and magnetic properties of A2CdP2 (A = Ca, Sr and Ba) n-type narrow band gap semiconductors

Structural, Optoelectronic, thermoelectric and magnetic properties of A2CdP2 (A = Ca, Sr and Ba) Zentl phase compounds in Cmc21 symmetry are investigated using FP-LAPW method with GGA-PBE, GGA-PBEsol, HSE06, G0W0 and GGA-mBJ potentials based on DFT. Estimated structure parameters are well agreement with the experiment. Cohesive energy shows that Ba2CdP2 is more stale then the rest compounds and posses high melting point. Electronic properties shows that all the compounds are direct band gap semiconductors at central symmetry while GGA-mBJ transit the direct band gap nature to indirect nature at ᴦ-H symmetry. The direct band gap values are ranging from 0.74 to 1.47 eV. The calculated band gaps are decreasing with the cation replacement and increasing by hybrid functionals. The result revel that all the compounds are optically active in infrared region of electromagnetic spectrum. Optical properties reveals that these compounds can be used as potential candidate for optoelectronic devices. Due to the narrow band gap semiconducting nature of these compounds their thermoelectric parameters are also calculated which shows that these compounds are efficient to use as the active thermoelectric materials. DFT and Post-DFT calculation indicate the paramagnetic nature of all compounds.

Add a Pinch of Tetrel: The Transformation of a Centrosymmetric Metal into a Nonsymmorphic and Chiral Semiconductor.

Centrosymmetric skutterudite RhP3 was converted to a nonsymmorphic and chiral compound RhSi0.3P2.7 (space group P212121) by means of partial replacement of Si for P. The structure, determined by a combination of X‐ray crystallography and solid state 31P NMR, exhibits branched polyanionic P/Si chains that are unique among metal phosphides. A driving force to stabilize the locally noncentrosymmetric cis‐RhSi2P4 and fac‐RhSi3P3 fragments is π‐electron back‐donation between the Rh t2g‐type orbitals and the unoccupied antibonding Si/P orbitals, which is more effective for Si than for P. In situ studies and total energy calculations revealed the metastable nature of RhSi0.3P2.7. Electronic structure calculations predicted centrosymmetric cubic RhP3 to be metallic which was confirmed by transport properties measurements. In contrast, the electronic structure for chiral orthorhombic RhSi0.3P2.7 contained a bandgap, and this compound was shown to be a narrow gap semiconductor.

Evidence of thermionic emission in forward biased β-Ga2O3 Schottky diodes at Boltzmann doping limit

A near-ideal and homogeneous β-Ga2O3 Schottky diode with Co contact for a doping level of ∼4.2 × 1017 cm−3 in the drift layer where the Boltzmann approximation is valid is reported. Unlike Si or GaN, thermionic emission is shown to be the dominant current conduction mechanism in the β-Ga2O3 Schottky diode at this doping level. A wide depletion region appended with a large built-in potential is observed to limit the thermionic field emission current, which is otherwise evident in narrower bandgap semiconductor (such as Si or GaN) Schottky diodes having a similar carrier concentration in the drift region. The results shown in this study can be used to identify the theoretical limits of drift layer doping beyond which the ideality factor and reverse leakage current should start deteriorating in ultra-wide bandgap semiconductor based Schottky diodes.

Preparation, characterization, and photoluminescent and semiconductive properties of an iron compound

One iron compound {[Fe(bipy)3]2(SO3)(OH)4}·14H2O (1) (bipy = 2,2'-bipyridine) has been obtained from a hydrothermal reaction and structurally characterized by single-crystal X-ray diffraction. Compound 1 is characterized by an isolated (0D) structure and the iron center is coordinated by six nitrogen atoms of three bipy molecules to yield an octahedron. Photoluminescence investigation shows that compound 1 displays an emission in the green region. Time-dependent density functional theory (TDDFT) calculation reveals that the photoluminescent emission can be assigned to the ligand-to-metal charge transfer (LMCT) (from the HOMO of the bipy to the LUMO of the iron). Solid-state diffuse reflectance spectra discover that compound 1 features a narrow optical band gap of 1.82 eV, indicating that it is a potential narrow band gap semiconductor.

Continuous-wave operation of electrically driven single mode microlaser

Developing current-driven single-mode micro-/nanolasers is highly desirable for various practical applications, but still faces severe challenges. Herein, a continuous-wave operation of an electrically driven laser device using a Ga-incorporated n-type ZnO microwire, MgO nanofilm, and p-type GaAs substrate is demonstrated. The device can enable a single-mode lasing peaking at 820 nm and a narrow linewidth of about 0.4 nm, and the quality factor Q is evaluated to 2000. The presence of a distinct threshold, sharp linewidth reduction, and polarized coherent illumination provides conclusive evidence for achieving lasing oscillation. Relative polaritonic features are further proofed; thus, single-mode lasing feature should be ascribed to the exciton–polariton. The results can enable a workable avenue to realize near-infrared micro-/nanolaser diodes for high-efficiency coherent light sources, which are no longer limited by conventional narrow-bandgap semiconductors.

Visible to Mid-Infrared Photodetection Based on Flexible 3D Graphene/Organic Hybrid Photodetector with Ultrahigh Responsivity at Ambient Conditions

Near-infrared (NIR) and mid-infrared (MIR) photodetectors have wide applications in biometrics, military, industry, etc. NIR and MIR organic photodetectors (OPD) require narrow-bandgap semiconductors to achieve efficient light absorption. However, it is still a challenge to synthesize organic materials with efficient absorption region above 1000 nm. Herein, a flexible hybrid OPD has been designed and fabricated by integrating organic materials with three-dimensional graphene (3DG) film, and the photodetector can detect light from visible to MIR at room temperature with an outstanding responsivity of 108 A W–1 in the NIR region (1000 nm). Moreover, the hybrid device can detect picowatt-level light with ultrahigh responsivity of 5.8 × 105 A W–1 and specific detectivity of 3 × 1015 Jones in the visible region. Furthermore, the 3DG film/organic hybrid detector is well compatible with flexible substrates and opens up a novel approach to developing flexible photodetectors with high responsivity in a wide spectrum range, suggesting possible potential applications in flexible electronics.

Межзонный двухфотонный линейно-циркулярный дихроизм в полупроводниках в приближении Кейна

Interband two-photon optical transitions are classified and expressions are obtained for the matrix elements in a narrow-gap semiconductor depending on the band parameters, degree of polarization, and light frequency. It is shown that the main contribution to two-photon linear-circular dichroism in narrow-gap semiconductors is made by optical transitions proceeding from the subband of light holes to the conduction band. The dependences of the partial coefficients of interband two-photon absorption of light, which differ from each other by the types of optical transitions, are analyzed depending on the degree of polarization of the light, and a quantitative analysis of the coefficient of linear-circular dichroism of two-photon absorption of light is carried out. Expressions are obtained for the spectral dependence of the coefficient of interband two-photon absorption of light in narrow-gap semiconductors in the Kane model.

A piezoelectric quantum spin Hall insulator VCClBr monolayer with a pure out-of-plane piezoelectric response.

The combination of piezoelectricity with a nontrivial topological insulating phase in two-dimensional (2D) systems, namely piezoelectric quantum spin Hall insulators (PQSHI), is intriguing for exploring novel topological states toward the development of high-speed and dissipationless electronic devices. In this work, we predict a PQSHI Janus monolayer VCClBr constructed from VCCl2, which is dynamically, mechanically and thermally stable. In the absence of spin orbital coupling (SOC), VCClBr is a narrow gap semiconductor with a gap value of 57 meV, which is different from Dirac semimetal VCCl2. The gap of VCClBr is due to a built-in electric field caused by asymmetrical upper and lower atomic layers, which is further confirmed by the external-electric-field induced gap in VCCl2. When including SOC, the gap of VCClBr is increased to 76 meV, which is larger than the thermal energy of room temperature (25 meV). The VCClBr is a 2D topological insulator (TI), which is confirmed by Z2 topological invariant and nontrivial one-dimensional edge states. It is proved that the nontrivial topological properties of VCClBr are robust against strain (biaxial and uniaxial cases) and external electric fields. Due to broken horizontal mirror symmetry, only an out-of-plane piezoelectric response can be observed, when a biaxial or uniaxial in-plane strain is applied. The predicted piezoelectric strain coefficients d31 and d32 are -0.425 pm V-1 and -0.219 pm V-1, respectively, and they are higher than or compared with those of many 2D materials. Finally, Janus monolayer VCFBr and VCFCl (dynamically unstable) are also constructed, and they are still PQSHIs. Moreover, the d31 and d32 of VCFBr and VCFCl are higher than those of VCClBr, and the d31 (absolute value) of VCFBr is larger than one. According to out-of-plane piezoelectric coefficients of VCXY (X ≠ Y = F, Cl and Br), CrX1.5Y1.5 (X = F, Cl and Br; Y = I) and NiXY (X ≠ Y = Cl, Br and I), it is concluded that the size of the out-of-plane piezoelectric coefficient has a positive relation with the electronegativity difference of X and Y atoms. Our studies enrich the diversity of Janus 2D materials, and open a new avenue in the search for PQSHI with a large out-of-plane piezoelectric response, which provides a potential platform in nanoelectronics.

Growth and characterization of Ag2S semiconductor nanoparticles

In past decades, the semiconducting chalcogenide nanocrystals has attracted much attention because of their uniqueness in terms of shape and size-dependent optical, physical, chemical and thermal properties resulting from quantum confinement effect. Among these semiconductors nanoparticles silver sulfide (Ag2S) is a narrow-band gap semiconductor and promising applications in the field of optoelectronics, photodetectors, photo-rechargeable, switching devices, electrochemical potential memory devices, etc. Different synthesis methods such as sol–gel method, hydrothermal method, sonochemical method are available to prepare Ag2S nanoparticles. Among these methods, aqueous method is cost effective, easy to synthesis and low energy consumption. Here, we report the synthesis of Ag2S Semiconductor Nanoparticles (SC NPs) by co-precipitation method using 3-mercaptopropionic acid (MPA) as passivating molecules. Hydrazine hydrate (N2H4·H2O) – sulfur complex is used as source of sulfur for the preparation of Ag2S nanoparticles.Growth of Ag2S nanoparticles are controlled by concentration of the precursors, reaction time and concentration of the capping molecule. Ag2S SC NPs are characterized by X-ray diffraction, FTIR, optical absorption, photoluminescence and transmission electron microcopy. The particle size is estimated using Debye-Schrrer’s formula is 12 nm. TEM image shows the good homogeneity of particle size distribution and estimated particles is about 14 nm. The as prepared Ag2S nanoparticles exhibit monoclinic phase. The present method of synthesis of Ag2S nanoparticles is simple, cost effective and useful for biomedical, textiles and water treatment applications.

Effects of Sn dopant on power factor of Tl8.67Sb1.33-xSnxTe6 nanoparticles

In this research work the thermoelectric and electrical performance of Sn doped Tellurium Telluride Tl8.67SnxSb1.33-xTe6 (x=0.61, 0.63, 0.65, 0.67, 0.68, 0.70), was studied. The samples were synthesized by melting method. By XRD and EDX analysis it has been confirmed that single phase tetragonal crystal structure with no any other impurities. The See beck co-efficient (thermo power) (S) shows increasing positive trend showing p-type semiconductor characteristics with whole temperature range from 300 to 550 K. At ambient temperature complex behavior of See beck coefficient for Sn doped compounds has been observed initially S decreasing with increase in Sn concentration i.e. x=0.65, and then S increases with increase in Sn contents up to x=0.70. In additional the electrical conductivity (σ) and the power factors have also shows the same complex behavior with the increase of Sn concentrations.. Telluride’s are narrow band-gap semiconductors, with all elements in common oxidation states, according to (Tl+ )9(Sb3+) (Te2- )6.It is found that the power factor is increases with Sn concentrations as well as increase in temperature range from 300-500 K.

Microstructure, optical and electrical characterizations of Bi-incorporated Sb2Te3 thermoelectric compound synthesized by mechanical alloying: A comparative study with undoped Sb2Te3

Bismuth incorporated antimony telluride has been synthesized within 3 h via the facile mechanical alloying method at room temperature. The rhombohedral crystal structure, microstructure (i.e, crystallite size, lattice strain etc.), and morphology of the sample have been revealed from Rietveld's refined XRD patterns and FESEM images. FTIR study confirms that the sample is a narrow bandgap semiconductor. The detailed study of impedance spectra, ac conductivity reveals the frequency and temperature-activated non-Debye type relaxation process of this semiconducting sample. The lower dielectric loss value at high frequencies shows the better applicability of the material in the high-frequency zone of optoelectronics. All the properties are compared with the previously reported undoped 3 h milled Sb2Te3 sample.

Photoluminescence spectroscopy of HgCdTe quantum well heterostructures in the 15-30 μm wavelength range

In recent years, narrow gap heterostructures with Hg1-xCdxTe/CdyHg1-yTe quantum wells have been actively studied both in connection with the topic of topological insulators and from the point of view of their application in optoelectronics in the terahertz domain. In this work, we study the photoconductivity and photoluminescence spectra of structure with a band gap of 40 meV. In addition to interband transitions, the features of the spectra associated with the resonance states of acceptors have been identified. The possibilities of using the structures under study to develop interband emitters at a wavelength of ~30 μm, which is inaccessible to existing quantum-cascade lasers, are discussed. Keywords: narrow-gap semiconductors, HgCdTe, resonance states, acceptors, photoluminescence.